getting to grips with fuel economy

Flight Operations Support - Customer Services Directorate

F U E LECONOMY

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STL 945.7190/99 November 2001Issue 2

Flight Operations Support & Line Assistance

GETTING TO GRIPS WITH FUEL ECONOMY

MANAGING FLIGHT OPERATIONS WITHRECOMMENDATIONS ON FUEL CONSERVATION

A FLIGHT OPERATIONS VIEW

STL 945.7190/99 November 2001Issue 2

Flight Operations Support & Line Assistance

GETTING TO GRIPS WITH FUEL ECONOMY 1

TABLE OF CONTENTS

1. PREAMBLE...................................................................................................................... 11

2. INTRODUCTION: BASIC PREMISES AND OUTLINE.................................................... 12

3. PRE-FLIGHT PROCEDURES ......................................................................................... 14

3.1 Center of gravity...................................................................................................... 14

3.1.1 Preliminary ................................................................................................... 14

3.1.2 Automatic center of gravity management .................................................... 14

3.1.3 Influence on fuel consumption ..................................................................... 15

3.1.4 Summary...................................................................................................... 17

3.2 Excess weight ......................................................................................................... 18

3.2.1 Aircraft weight .............................................................................................. 18

3.2.2 Overload effect............................................................................................. 19

3.2.3 Means to diminish aircraft weight ................................................................ 22

a) Zero fuel weight..................................................................................... 22

b) Embarked fuel ....................................................................................... 23

Embarked fuel minimization .................................................................. 23

Fuel transportation ................................................................................ 25

3.3 A.P.U....................................................................................................................... 26

3.3.1 Preliminary ................................................................................................... 26

3.3.2 Fuel conservation and A.P.U. ...................................................................... 26

3.3.3 Optimisation procedures .............................................................................. 27

3.3.4 Summary...................................................................................................... 28

3.4 Taxiing..................................................................................................................... 28

3.4.1 Preliminary ................................................................................................... 28


3.4.2 Engine-out taxi operation ............................................................................. 29

Taxiing out with one (or two) engine(s) shut down ...................................... 29

Taxiing in with one (or two) engine(s) shut down ........................................ 29

During taxi in and out, one (or two) engine(s) shut down ............................ 29

3.4.3 Summary...................................................................................................... 30

3.5 Conclusion on pre-flight procedures ....................................................................... 30

4. WITHIN THE FLIGHT ENVELOPE.................................................................................. 31

4.1 Climb ....................................................................................................................... 31

4.1.1 Preliminary ................................................................................................... 31

4.1.2 Managed mode ............................................................................................ 32

a) A300-600, A310, A320 family, A330 ..................................................... 32

b) A340 family............................................................................................ 34

4.1.3 Selected mode ............................................................................................. 36

4.1.4 Crossover altitude versus optimum altitude................................................. 43

4.2 Step climb ............................................................................................................... 46

4.2.1 Preliminary ................................................................................................... 46

4.2.2 Trade-off between manoeuvrability and economy....................................... 46

4.2.3 Delays in altitude follow-up .......................................................................... 48

4.3 Cruise...................................................................................................................... 49

4.3.1 Preliminary ................................................................................................... 49

4.3.2 Managed mode ............................................................................................ 50

a) Economy Mach number ........................................................................ 50

b) Time/fuel relation................................................................................... 55

4.3.3 From Managed to Selected Mode ............................................................... 57


4.3.4 Selected mode ............................................................................................. 59

a) Preliminary .........................................................................................…59

b) Flight at a given Mach number..........................................................….61

b.1 Optimum altitude. ....................................................................... 61

b.2 Optimum altitude on short stage ..................................................... 68

c) Flight at a given Flight level................................................................... 71

d) Wind influence....................................................................................... 74

4.4 Descent ................................................................................................................... 79

4.4.1 Preliminary ................................................................................................... 79

4.4.2 Managed mode ............................................................................................ 80

4.4.3 Selected mode ............................................................................................. 81

Fuel consumption with respect to speed ..................................................... 81

Premature descent....................................................................................... 87

Temperature influence................................................................................. 88

4.5 Holding .................................................................................................................... 89

4.5.1 Preliminary ................................................................................................... 89

4.5.2 Various configuration / speed combinations ................................................ 90

4.5.3 Linear holding .............................................................................................. 94

4.6 Approach................................................................................................................. 97

4.6.1 Decelerated and stabilised approach .......................................................... 97

4.6.2 Premature landing gear extension............................................................. 100

4.7 Conclusion within the flight envelope.................................................................... 102

4.7.1 With regard to the climb phase: ................................................................. 102

4.7.2 With regard to the step climb phase: ......................................................... 102

4.7.3 With regard to the cruise phase:................................................................ 102


4.7.4 With regard to the descent phase:............................................................. 102

4.7.5 With regard to holding:............................................................................... 103

4.7.6 With regard to the approach phase: .......................................................... 103

5. GENERAL CONCLUSION............................................................................................. 104

APPENDICES

APPENDIX 1............................ ECONOMY MACH NUMBER ACCORDING TO COST INDEX............................................................................................................................................. AND FL

APPENDIX 2......................................................................................... OPTIMUM ALTITUDES 116

APPENDIX 3......................................................OPTIMUM ALTITUDES ON SHORT STAGES 120

APPENDIX 4............................................................ LONG RANGE CRUISE MACH NUMBER 123

APPENDIX 5.........................WIND ALTITUDE TRADE FOR CONSTANT SPECIFIC RANGE 127

APPENDIX 6.............................................................................................................. DESCENT 128

APPENDIX 7...............................................................................................................HOLDING 132

List of Figures

Figure 1: Specific range variation for different center of gravity positions........................15

Figure 2: Specific range variation versus CG Variation – MO.82 .....................................16

Figure 3: Fuel burn penalty. Specific range variations for the caseof 1000 kg excess weight ..................................................................................20




Figure 6: Fuel burn penalty. Specific range variations for the caseof 1000 kg less excess weight...........................................................................21

Figure 7: Contingency fuel with respect to flight distance ................................................24

Figure 8: Climb profiles .....................................................................................................31

Figure 9: Climb laws .........................................................................................................32

Figure 10: Profile used for the computation........................................................................37

Figure 11: ∆ consumption between the optimum climb law and different climb laws.........37

Figure 12: ∆ time between the optimal climb law and different climb laws.........................38

Figure 13: ∆ time between the optimal climb law and different climb laws.......................398

Figure 14: ∆ consumption between the optimum climb law and different climb laws......389

Figure 15: ∆ time between the optimum climb law and different climb laws......................39

Figure 16: ∆ consumption between the optimum climb law and different climb laws.........40

Figure 17: Annual potential savings per aircraft if 10 kg fuel is saved for each climb .......42

Figure 18: Climb law ...........................................................................................................43

Figure 19: Specific range variations for different weights and altitudes .............................46

Figure 20: Optimum altitude ...............................................................................................46

Figure 21: Step climb profiles .............................................................................................47

Figure 22: Economic cruise Mach number for various flight levels and cost indices .........50

Figure 23: Economic cruise Mach number for various flight levels and cost indices ........51



Figure 26: Economic cruise Mach number for various weights and cost indices ...............52






Figure 31: Time/fuel relation for a typical stage length. Stage length: 3000 Nm, M 0.8.....55



Figure 34: Fuel consumption increment on a 2000 Nm stage length, when the pilotswitches in selected mode and increases Mach number ..................................57

Figure 35: Fuel consumption increment on a 2000 Nm stage length, when the pilotswitches in selected mode and increases Mach number .................................58

Figure 36: Fuel consumption increment on a 2000 Nm stage length, when the pilotswitches in selected mode and increases Mach number ..................................58

Figure 37: Fuel consumption and time for different flight levels and Mach numberson a 3000 Nm trip. TOW=130t. .........................................................................60

Figure 38: Fuel consumption and time for different flight levels and Mach numberson a 2000 Nm trip. TOW=60t.............................................................................60

Figure 39: Fuel consumption and time for different flight levels and Mach numberson a 3000 Nm trip. TOW=210t. .........................................................................61

Figure 40: Fuel consumption increment when flying above optimum altitude.Stage length: 1000 Nm. M 0.78.........................................................................62

Figure 41: Fuel consumption increment when flying above optimum altitude.Stage length: 1000 Nm. M 0.78.........................................................................62

Figure 42: Fuel consumption increment when flying below optimum altitude. M 0.8 .........63

Figure 43: Fuel increment when flying below optimum altitude. M 0.8...............................63

Figure 44: Fuel consumption increment when flying below optimum altitude. M 0.8 .........64


Figure 45: Optimum flight profile.........................................................................................64

Figure 46: Optimum altitude with respect to aircraft weight................................................65






Figure 52: Optimum altitude on short stage........................................................................69




Figure 56: LRC Versus MRC at Given altitude ...................................................................71

Figure 57: Long Range Cruise Mach number.....................................................................72





Figure 62: Fuel consumption and trip time for various Mach numbers and flight levelsin windy conditions. TOW=130t. ........................................................................75

Figure 63: Fuel consumption and trip time for various Mach numbers and flight levelsin windy conditions. TOW=80t. ..........................................................................75

Figure 64: Fuel consumption and trip time for various Mach numbers and flight levelsin windy conditions. TOW=250t. ........................................................................76

Figure 65: Wind influence on specific range…………………………………………………...77

Figure 66: Wind altitude trade for constant specific range .................................................78

Figure 67: Descent profiles.................................................................................................79

Figure 68: Descent consumption from the same point in cruise.........................................82






Figure 73: Annual potential money savings for a 20 KT decrease in descent speed.........85

Figure 74: Profile of a too early descent .............................................................................87

Figure 75: Green dot speed definition ................................................................................89

Figure 76: Fuel flow with respect to holding altitude for several configurations .................90




Figure 80: Description of the holding options .....................................................................95

Figure 81: The stabilized approach ....................................................................................97

Figure 82: The decelerated approach.................................................................................98

Figure 83: Money saved by the decelerated approach in comparison with thestabilized one.....................................................................................................99

Figure 84: Premature landing gear extension...................................................................100


List of Tables

Table 1: Fuel consumption variation between a 20% CG and a 35% CG.Distance: 1000 Nm. ...........................................................................................17

Table 2: Fuel consumption increment for 1000 kg excess weight...................................22

Table 3: Extra fuel for single engine use rather than the A.P.U. .....................................27

Table 4: Climb parameters for different cost indices (climb to FL330) ............................33

Table 5: Delta in fuel and in time between a high and a low cost index..........................34

Table 6: Comparison between the previous climb speed laws and the new ones..........36

Table 7: Delta in fuel and in time between the optimum climb law and the mostunfavorable one.................................................................................................40

Table 8: Annual fuel savings corresponding to 10 kg fuel savings..................................41

Table 9: Recommended climb laws.................................................................................42

Table 10: Standard climb laws ..........................................................................................44

Table 11: Crossover altitude and first optimum altitude ....................................................44

Table 12: Delta in fuel and time when flying at crossover altitude instead of flyingat optimum altitude. Stage length: 1000 Nm. Cruise Mach number..................45

Table 13: Percentage of fuel increment and of time gain when flying at crossoveraltitude instead of flying at optimum altitude. Stage length: 1000 Nm.Cruise Mach number. ........................................................................................45

Table 14: Delta between optimum FL and MAX FL...........................................................48

Table 15: Fuel increment for delayed climb to FL 370 at optimum weight ........................48

Table 16: Fuel increment in percent for a delayed climb...................................................49

Table 17: Delta in fuel and time in the case of a 0.005 Mach increase.............................59

Table 18: Fuel consumption variation with head/tailwind ..................................................76

Table 19: Relevant parameters for a descent from FL 370 at different cost indices(standard conditions) .........................................................................................80

Table 20: Delta time and fuel between a descent at a high and a low cost index(decent from FL 370, standard conditions)........................................................81


Table 21: Annual potential fuel savings for a 20 KT decrease in descent speed..............84

Table 22: Comparison of descent A319/A320/A321 .........................................................85

Table 23: Comparison of descent A330 GE/PW/RR.........................................................86

Table 24: Comparison of descent A340 CFM ...................................................................86

Table 25: Comparison of descent A300-600.....................................................................86

Table 26: Comparison of descent A310 ............................................................................87

Table 27: Fuel increment for a one minute early descent from flight level 350.Cruise Mach number .........................................................................................88

Table 28: Recommended holding configurations ..............................................................89

Table 29: Percentage of fuel flow increment when holding at S speed in conf 1 insteadof holding at green dot speed in clean conf. Flight level 100 ............................92

Table 30: Fuel increment when holding at S speed in conf 1 instead of holding atgreen dot speed in clean conf for a period of 15 minutes. Flight level 100 .......93

Table 31: Maximum recommended speeds ......................................................................94

Table 32: Fuel saved by reduction from M 0.78 to green dot speed at FL 350.................95



Table 35: Fuel saved by reduction from M 0.8 to green dot speed at FL 350...................96

Table 36: Money and fuel saved thanks to the second option ..........................................96

Table 37: Fuel increment between a stabilized approach and a decelerated approach ...98

Table 38: Potential annual savings due to the decelerated approach...............................99

Table 39: Fuel increment in case of premature extension ..............................................101

Table 40: Potential annual savings..................................................................................101


1. PREAMBLE

The energy crisis of the 70's woke airlines up to the seriousness of fuel savings.Nowadays, this fear of a sudden or even gradual fuel price rise, coupled with a verycompetitive and deregulated aviation market forces commercial airlines once again intodrastic measures to save fuel. Airlines try to reduce their operational costs in every facet oftheir business. Fuel conservation has become one of the major preoccupations for allairlines. Fuel bills indeed are representing a considerable part of overall aircraft operatingcosts. All ways and means to keep fuel costs under optimal control have to be rationallyenvisaged, safety being of course the number one priority in any airline operation. Someoperational costs cannot be cut down without degrading safety and are therefore totallyinflexible.

The purpose of this document is to examine the influence of flight operations on fuelconservation with a view towards providing recommendations to enhance fuel economy.No dedicated attempt is made to identify the trade-off of fuel saved versus the otheroperating variables, such as cost or trip time. This is the scope of another brochure called"Getting to Grips with the Cost Index: Balancing Cost of Fuel and Cost of Time".

The present brochure systematically reviews fuel conservation aspects relative to groundand flight performance. Whilst the former pertains to center of gravity position, excessweight, auxiliary power unit (A.P.U.) operations and taxiing, the latter details climb, stepclimb, cruise, descent, holding and approach. Wind/altitude trade effects are also reviewedto provide airline pilots, engineers, or managers with useful insights on operational factors.

None of the information contained herein is intended to replace procedures orrecommendations contained in the Flight Crew Operating Manuals (FCOM), the objectivebeing rather to highlight the areas where flight crews can contribute significantly to fuelsavings, circumstances permitting.

In reviewing the fuel economy theme with regard to our whole fleet (A300-600, the A310,the A319/320/321, the A330 and A340) the major tool used was PEP for Windows, theAirbus' software enabling airlines' operation engineering departments to compute aircraftperformance as a function of specified operational conditions.


Would you please send your comments and remarks to the following contact point atAirbus. The topic of fuel conservation has been the subject of a lot of debate/controversy,action/inaction in recent years and we value your contributions very much. These will betaken into account in our follow-up with you as well as in the following issues to be edited.

Flight Operations Support & Line AssistanceCustomer Services Directorate1, Rond Point Maurice Bellonte, BP 3331707 BLAGNAC Cedex - FRANCETELEX AIRBU 530526ESITA TLSBI7XTELEFAX 33/(0)5 61 93 29 68 or 33/(0)5 61 93 44 65


2. INTRODUCTION: BASIC PREMISES AND OUTLINE

This brochure considers the two flight management modes: the "managed" mode and the"selected" mode.

The managed mode corresponds to flight management by means of a dedicated tool, theflight management system (FMS). Crews interface through the multipurpose control anddisplay unit (MCDU) introducing basic flight variables such as weight, temperature, altitude,winds and the cost index. From these data, the FMS computes the various flight controlparameters such as the climb law, step climbs, economic Mach number, optimum altitude,descent law. Hence, when activated, this mode enables almost automatic flightmanagement.

When in managed mode, aircraft performance data is extracted from the FMS database.These same databases were simplified to alleviate computation density and calculationoperations in the FMS; results may therefore be less precise than reality but they constituteuseful indications for experienced guidance.

When in selected mode, crews conduct the flight and flight parameters such as speed,altitude and heading have to be manually introduced on the flight control unit (FCU). Thedatabases used to compute aircraft performance in this configuration are used onground-based mainframe computers; hence they are more complete and more precisethan those of the FMS. For this reason, straight comparisons between performance resultsstemming from these two modes necessitates adjustments beyond the scope of thisbrochure.

Calculations presented here were made taking into account average numbers of take-offsand landings for the year 1998 and for the following aircraft types:

A300-600: 1300 take-offs and landings per year per aircraft

A310: 1100 take-offs and landings per year per aircraft






Assumed price of fuel: 1 US dollar per gallon


3. PRE-FLIGHT PROCEDURES

Operation of the aircraft starts with the aircraft on ground during aircraft preparation andloading.

This section should highlight the impact of some ground operations on fuel consumption.Even if these operations enable only little savings in comparison with savings made duringthe cruise phase, ground staff have to be sensitive to these and should get adaptedprofessional practices.

This part is divided into four different sections:

− The first section is about the center of gravity position and its impact on fuelconsumption.

− The second one is about excess weight and fuel consumption.

− The third section is about Auxiliary Power Unit consumption.

− The fourth section is about fuel saving taxi practices.

3.1 Center of gravity

3.1.1 Preliminary

The gross weight is the sum of the dry operating weight, payload and fuel. The resultantforce acts through the center of gravity of the aircraft. The balance chart allows todetermine the overall center of gravity of the airplane taking into account the center ofgravity of the empty aircraft, the fuel distribution and the payload. The center of gravitymust be checked to be within the allowable range referred to as the center of gravityenvelope.

In terms of fuel consumption, forward center of gravity needs a nose up pitching moment,which adds to the one created by weight and leads to an increase in fuel consumptionbecause of induced drag. It is better to have the center of gravity as far aft as possible.However, such a center of gravity position deteriorates an aircraft's dynamic stability.

3.1.2 Automatic center of gravity management

AIRBUS has developed a trim tank transfer system, which controls the center of gravity ofthe airplane. When the airplane is in cruise, the system optimizes the center of gravityposition to save fuel by reducing the drag on the airplane. The system either transfers fuelto the trim tank (aft transfer) or from the trim tank (forward transfer). This movement of fuelchanges the center of gravity position. The crew can also manually select forward fueltransfer.


The Fuel Control and Management Computer (FCMC) calculates the center of gravity ofthe airplane according to the aerodynamic surfaces and compares the result to a targetvalue. From this calculation, the FCMC determines the quantity of fuel to be moved aft orforward in flight (usually one aft fuel-transfer is carried out during each flight).

3.1.3 Influence on fuel consumption

The following graphs show the gain or loss in fuel expressed in terms of specific range witha center of gravity of 20% and 35%, compared to the consumption for a center of gravityposition of 27% at cruise Mach. For the other aircraft, all curves have a similar shape tothese ones:

Figure 1: Specific range variation for different center of gravity positions


Figure 2: Specific range variation versus CG Variation – MO.82


For the A300-600, A310, A330 and A340 types, the further aft the center of gravity, themore significant fuel savings will be. Furthermore, when flying above optimum altitudes, athigh weights, the decrease or increase in fuel consumption as a function of the reference issignificant.

Thus, loading is very important especially for high weights and for aircraft having noautomatic center of gravity management: we notice that the specific range variation canreach 2% for high weights and FL's above 350 for the A300-600, A310, A330 and A340types.

Contrary to the other aircraft, specific range variations with respect to the center of gravityposition are random for the whole A320 family. This is due to a complex interaction ofseveral aerodynamic effects. Whatever the influence of the center of gravity position onspecific range, it can however be said on the A320 family, this influence is very small.

As the specific range characterizes fuel consumption of aircraft at a given weight, it is quitedifficult to quantify the impact of the center of gravity position on an entire stage length. Ona 1000 NM stage length, the increases in fuel consumption when the center of gravityposition is 20% with regard to the fuel consumption when the center of gravity position is35% are summed up in the following table. The results characterize the worst cases, that isto say that an aircraft with a high weight and at a high flight level is considered.

Table 1: Fuel consumption variation between a 20% CG and a35% CG. Distance: 1000 Nm.

Aircraft types Fuel increment (kg)

A319/A320/A321 Negligible

A330 220

A340 380

A310 250

A300-600 230

3.1.4 Summary

For better fuel consumption the center of gravity must be placed further aft, but aircraftstability must be the deciding factor. The aircraft must be so loaded that the center ofgravity is still within the allowable range.


3.2 Excess weight

Another way to save fuel is to avoid excess weight.

3.2.1 Aircraft weight

Airlines must first of all correctly estimate real aircraft weights: an unrealistic weight mayput aircraft at increasing risk during take-off.

Airbus already reported the market tendency of increasing passenger weights, mainlybecause of carry-on baggage and duty-free articles. The J.A.A. and the F.A.A. also noticedthis phenomenon and therefore reviewed regulations concerning passenger and baggagestandard weights.

The J.A.A. has produced JAR OPS (1.4). For the purpose of calculating the mass of anaircraft, the total masses of passengers, their hand baggage entered on the loadsheet shallbe computed using:

− either the actual mass values to be weighed case-by-case;

− or standard mass values such as:

a) passengers including hand baggage:

All flights except 84kg / 185lbHoliday charters 76kg / 168lbChildren (2-12 years) 35kg / 77lb

b) checked baggage

All flights except domestic 13kg / 25lband intercontinental flightsDomestic flights 11kg / 24lbIntercontinental flights 15kg / 33lb

A review of these weights will have to be performed every five years, and the loadsheetshould always contain references to the weighing method adopted.

Initially, the following standard mass values for males and females including hand baggagehad been agreed upon:

Males Female

Scheduled, medium/long-haul 86kg / 190lb 69kg / 152lbSchedules, European short-haul 89kg / 196lb 71 kg / 157lbNon-scheduled 84kg / 185lb 69kg / 152lb


In the USA, the F.A.A. has issued an Advisory Circular to provide methods and proceduresfor developing weight and balance control. This also involves initial and periodicre-weighing of aircraft (every 3 years) to determine average empty and actual operatingweight and CG position for a fleet group of the same model and configuration. In the pastthe following standard average weights had been adopted:

Males & Females Children

Summer (1/5 thru 31/10) 73kg / 160lb 36kg / 80lbWinter (1/11 thru 30/4) 75kg / 165lb 36kg / 80lbCarry-on baggage allowance 4.5kg / 10lb 4.5kg / 10lb

AC 120-27B features a 10 Ib increase for adult weights:

Summer (1/5 thru 31/10) 77kg / 170lb 3kg / 80lbWinter (1/11 thru 30/4) 80kg / 175lb 36kg / 80lbCarry-on baggage allowance 4.5kg / 10lb 4.5kg / 10lb

Similar to J.A.A., airlines will have to adopt standard weights unless they request differentvalues, which would have to be proven by a survey at the risk of ending up with higherstatistics.

Harmonization between F.A.A. and J.A.A. is desirable, as it would eventually prompt allairlines to undergo the same penalty with minimal competitive detriment.

3.2.2 Overload effect

The specific range, flying at given altitude, temperature and speed depends on weight. Theheavier the aircraft, the higher the fuel consumption.

In addition, fuel savings can be made during climb since the aircraft would reach its optimalflight level earlier if it were lighter.

The effect of overloading with respect to in-flight weight is shown on the following graphsfor 1000 kg excess load in cruise for four different aircraft. The characteristic curves for theother aircraft types have a similar shape.


Figure 3: Fuel burn penalty. Specific range variation for the case of 1000 kg excess weight

Figure 4: Fuel burn penalty. Specific range variationsfor the case of 1000 kg excess weight


Figure 5: Fuel burn penalty. Specific range variations for the case of 1000 kg excess weight

Figure 6: Fuel burn penalty. Specific range variations for the case of 1000 kg excess weight


The increase in fuel consumption is more important at high flight levels and heavy weights.Indeed, specific range can be increased by 2% if aircraft weight is diminished by 1000 kg.

Excess weight has a more important impact on the A310 and A300-600 as the increase inspecific range for 1000 kg less varies from 0.15% up to 3%. The impact is also significanton A319, A320 and A321 types as the increase in specific range varies between 0.4% to2%. Compared to that, the A330 and A340 types seem to be the least affected by excessweight in terms of fuel consumption. But as wide bodies fly longer segments, it representsa bigger amount of fuel: by way of example, the following table hints at fuel savings per1000 kg for typical stage lengths at optimum altitude.

Table 2: Fuel consumption increment for 1000 kg excess weight

Aircraft types Mach Stage Fuel penalty

A319 (55 000 kg) 0.78 1000 Nm 40 kg

A320 (65 000 kg) 0.78 1000 Nm 70 kg

A321 (75 000 kg) 0.78 1000 Nm 90 kg

A330 (200 000 kg) 0.82 4000 Nm 200 kg

A340 (260 000 kg) 0.82 6000 Nm 160 kg

A310 (140 000 kg) 0.8 2000 Nm 130 kg

A300-600 (160 000 kg) 0.8 2000 Nm 260 kg

This table shows that the increase in fuel consumption is significant just for the case of asingle ton's excess weight. Ground staff must try to avoid this whenever possible.

3.2.3 Means to diminish aircraft weight

In order to diminish aircraft weight, either the zero fuel weight or the embarked fuel can bereduced.

a) Zero fuel weight

One must be aware that zero fuel weight can increase substantially during an aircraft's lifebecause of an accumulation of unnecessary catering equipment, supplies etc... Any emptycargo containers, interior and exterior dirt and rubbish must be removed in order tominimize aircraft zero fuel weight. Airline staff have to be sensitive to these issues anddedicated efforts are necessary to avoid excess weight.


b) Embarked fuel

Embarked fuel minimization

In the same vein, unnecessary fuel weight must be resolutely avoided: flights must beplanned very precisely to calculate the correct amount of fuel to be embarked. Trip fuel isestimated with a certain accuracy depending on theoretical aircraft performance. However,if real aircraft performance is far below the nominal one, the pilot will take more fuel thannecessary (to ensure having enough fuel) which in itself will result in aircraft weight andfuel consumption increases. So flight planning should be based on aircraft performancemonitoring by taking into account performance factors derived from specific rangevariations.

According to the JAR OPS 1.255, contingency fuel is the higher amount of fuel between (a)and (b).

(a) Either:

• 5 % of the planned trip fuel or, in the event of in-flight replanning, 5 % of the tripfuel for the remainder of the flight; or

• Contingency fuel can be less. In this case it can be no less than 3 % of theplanned trip fuel, or in the event of in-flight replanning, 3 % of the trip fuel for theremainder of the flight provided that an en-route alternate is available. The en-route alternate should be located within a circle having a radius equal to 20 % ofthe total flight plan distance, the center of which lies on the planned route at adistance from the destination of 25 % of the total flight plan distance, or at 20 % ofthe total flight plan distance plus 50 NM, whichever is greater; or

• It can also be the fuel necessary to fly 15 minutes at holding speed at 1500 ftabove the destination airport in standard conditions, when an operator hasestablished a program, approved by the Authority, to monitor fuel consumption oneach individual route/airplane combination and uses this data for a statisticalanalysis to calculate contingency fuel for that route/airplane combination; or

• Finally, if the airline has kept track of the consumption for each aircraft, thecontingency fuel required is an amount of fuel sufficient for 20 minutes flying timebased upon the planned trip fuel consumption, provided that the operator hasestablished a fuel consumption monitoring program for individual airplanes anduses valid data determined by means of such a fuel calculation program.

(b) An amount to fly for 5 minutes at holding speed at 1500 ft above the destinationaerodrome in standard conditions.


What we can conclude is that depending on flight distance, there is a lowest contingencyfuel. The following graphs show the different contingency fuel quantities for differentdistances.

Figure 7: Contingency fuel with respect to flight distance

15 min holding5 min holding

According to the Federal Aviation Administration, contingency fuel is the amount necessaryto fly for 45 minutes at normal cruising speed. In this case, there is no way to influence theembarked fuel.

In the same vein, in order to diminish the amount of embarked fuel, alternate airportsshould be chosen as near as possible to the destination so as to minimize the diversionfuel reserve.

According to the FAA, the diversion fuel reserve is not necessary if:

− Part 97 of subchapter F prescribes a standard instrument approach procedure for thefirst airport of intended landing and

− For at least one hour before and one hour after the estimated time of arrival at theairport, the weather reports or forecasts or any combination of them indicates:

(i) the ceiling will be at least 2 000 feet above the airport elevation, and(ii) visibility will be at least 3 statute miles.


According to the JAR OPS 1.295c, the diversion fuel reserve is not necessary if:

(1) Both:

• The duration of the planned flight from take-off to landing does not exceed6 hours; and

• Two separate runways are available at destination and the meteorologicalconditions prevailing are such that, for the period from one hour before until onehour after the expected time of arrival at destination, the approach from therelevant minimum sector altitude and the landing can be made in VMC (see IEMOPS 1.295(c)(1)(ii)); or

(2) The destination is isolated and no adequate destination alternate exists.

Fuel transportation

Carrying extra fuel may be of value when a fuel price difference exists between twoairports. However, since the extra fuel on board leads to an increase in fuel consumptionthe breakeven point must be carefully determined.

K is the transport coefficient:

LWTOWK∆∆

=

The addition (or the subtraction) of one ton to landing weight, means an addition (or asubtraction) of K tons to take-off weight.

EXAMPLE: with K=1.3, if 1300 kg fuel is added at the departure, 1000 kg of this fuelamount will remain at destination. So carrying one ton fuel costs 300 kg morefuel.

At the departure, if ∆MD tons of fuel are embarked, at destination

KMDMA ∆

=∆

will remain.

The extra-cost at departure is: dPMD x∆

With Pd: fuel price at departure.

At arrival, the saving is: aPMA x∆

With Pa: fuel price at arrival.


A cost linked to a possible increase in flight time can be added:

hPT x∆

With Ph: one-hour flight price.

It is profitable to carry extra fuel if:

0PTPMDPKMD

hda ≥∆−∆−∆ xxx

That is to say:

hda PTMDKPKP xxx ∆

∆+≥

With ∆T=0, it is profitable to carry extra fuel if Pa ≥ K x Pd that is to say if the arrival fuelprice to departure fuel price ratio is higher than the transport coefficient K.

Thus carrying extra fuel may be of value when a fuel price differential exists between twoairports. Graphs such as FCOM 2.05.70 on A320/A330/A340 assist in determining theoptimum fuel quantity to be carried as a function of initial take-off weight (without fuelexcess), stage length, cruise flight level and fuel price ratio.

3.3 A.P.U.

3.3.1 Preliminary

The Auxiliary Power Unit (A.P.U.) is a self-contained unit, which makes the aircraftindependent of external pneumatic and electrical power, supply.

A.P.U. fuel consumption obviously represents very little in comparison with the amount offuel for the whole aircraft mission. Nevertheless, operators have to be aware that adoptingspecific procedures on ramp operations can help save fuel and money.

3.3.2 Fuel conservation and A.P.U.

On ground, at sea level, under ISA conditions, A.P.U. fuel consumption varies dependingon A.P.U. types. It goes from 60 to 80 kg/h in no load conditions and from 110 to 160 kg/hfor air conditioning + electric load and for main engine start operations.

A.P.U. specific procedures to save fuel have to be defined by the operators: they have tochoose between using ground equipment (Ground Power Unit, Ground Climatisation Unit,Air Start Unit) or A.P.U. It depends on several parameters: when the turn-around is quitelong, or when the aircraft does a night-stop, the use of G.P.U. is well adapted, as timeconsiderations are not prevailing. It enables to save both fuel and A.P.U. life.


Remember that one extra minute of A.P.U. operation per flight at 180 kg/hr fuel flow,means an additional 3000 kg per year per aircraft.

However, for short turn-around (45 minutes on average), the use of A.P.U. isadvantageous. Moreover, to limit A.P.U. start cycles and improve reliability, we advise tokeep the A.P.U. running, even if it is not fully used during the next 45 minutes. It is better tooperate with A.P.U. at Ready To Load (RTL) than to shut it down and perform a new startcycle.

So operators are advised to use ground equipment when this is of a good quality level, andtherefore to try to conclude agreements with airport suppliers to get preferential prices.

However, in some countries, ground operations are restricted by law. The use of the APUis limited to a defined time prior to departure time and after the arrival.

Note: It is not really correct to compare the amount of fuel burnt by A.P.U. and groundequipment, as we also have to consider A.P.U. maintenance.

3.3.3 Optimisation procedures

The disconnection of ground equipment supplies and the start of A.P.U. must becoordinated depending on A.T.C. A « one minute anticipation » in each A.P.U. start willlead to a significant amount of fuel savings at year's end (2000 to 4000 kg depending onA.P.U. types). The following table gives the amount of fuel saved per year and per flightwhich can be attributed to nothing more than good coordination.

Engine start-up too should, if possible, be carefully planned in conjunction with A.T.C. Ifpush-back is delayed, it is preferable to wait and use A.P.U. for air conditioning andelectrical requirements.

The following table shows extra fuel consumption per minute for using a single enginerather than the A.P.U.:

Table 3: Extra fuel for single engine use rather than the A.P.U.

A.P.U. typesOne engineconsumption

(kg/h)∆consumption (kg/h)

GTCP 36-300A319/A320/A321

340 +210

APS 3200A319/A320/A321

340 +225

GTCP 331-350A330GE

550 +335

A330 PW 580 +365

A.P.U. typesOne engineconsumption ∆consumption (kg/h)


(kg/h)

A330 RR 820 +605

GTCP 331-350

A340300 +80

3.3.4 Summary

It is quite difficult to give advice for using A.P.U. rather than ground equipment because itdepends on several parameters. Operators have to define the most economical solutions,depending on their own aircraft operations.

3.4 Taxiing

3.4.1 Preliminary

Jet engine performance is optimized for flight conditions. Nevertheless, all aircraft spendsignificant amounts of time on ground for various operations.

As regards taxiing conditions such as:

• congestion of ground traffic,

• ramp to runway distance,

• holding point with dozens possibly waiting for take-off,

all lead to a waste of precious time and fuel.

One (or two) engine(s) taxi can help. But such procedures need to be discussed, andoperators have to define their field of application.

Airbus provides standard procedures to operators (FCOM 3.04.90). Regardless of these,operators have to keep in mind some specifics.


Taxi is split into two distinct phases:

• taxi out: from ramp to runway

• taxi in: from runway to ramp

3.4.2 Engine-out taxi operation

Taxiing out with one (or two) engine(s) shut down

a) No fire protection from ground staff is available when starting engine (s) awayfrom the ramp.

b) Potential loss of:

− braking capability (brake accumulators are nevertheless operational), nose wheelsteering (in case APU is not used),

could lead to a taxiway excursion.

c) FCOM (3.04.90) requires not less than a defined time (from 2 to 5 minutesdepending on the aircraft) to start the other engine(s) before take off. On engineswith a high bypass ratio, warm-up and cool-down time prior to applying maximumtake off thrust, is vital for engine safety and lifetime.

d) Mechanical problems can occur during start up of the other engine(s), requiring agate return for maintenance and delaying departure time.

Taxiing in with one (or two) engine(s) shut down

a) FCOM requires APU start before shutting down the engine, to avoid an electricaltransient (A319/A320/A321).

b) FCOM (3.04.90) requires not less than a defined time before shutting down theother engine(s). On engines with a high bypass ratio, the cool-down time afterreverse operation, prior to shut down is vital for the engine safety and lifetime.

During taxi in and out, one (or two) engine(s) shut down

a) Caution must be exercised when taxiing one (for twin engine) or two engine(s) shutdown (A340) to avoid excessive jet blast and FOD.

b) Slow and/or tight taxi turns in the direction of the operating engine may not bepossible at high gross weight.

c) More thrust is necessary for breakaways and 180 degrees turn. Be aware of higherblast effect.


d) For the A340, it is recommended to taxi with the outer engines to pressurize thegreen hydraulic system, so as to allow normal operation of braking and nose wheelsteering.

e) For the A319/A320/A321, it is preferable to use engine 1 for taxi to pressurize thegreen hydraulic system, and without using the PTU.

3.4.3 Summary

Operators have to define their own taxiing policy depending on airport configurations(taxiways, runways, terminals and ramps,...) and crew training with an eye on FCOMprescriptions.

3.5 Conclusion on pre-flight procedures

• On all aircraft except the A320 family, it is advised to load the airplane so that itscenter of gravity is further aft, provided it is still within the allowable range.

• It is advised to avoid excess weight by diminishing zero fuel weight and embarkedfuel due to accurate flight planning.

• Operators have to decide whether the use of A.P.U. is appropriate depending onturn around time, quality of ground equipment and airport specific procedures.However, whenever possible, the use of ground equipment is recommended tosave both fuel and A.P.U. life.

• Taxiing with one engine out saves fuel but has some drawbacks. Operators haveto define their own taxiing policy depending on airport configurations (taxiways,runways, ramps…).


4. WITHIN THE FLIGHT ENVELOPE

As aircraft spend more time airborne than on ground, much fuel can be saved bydisciplined flight crews.

This part intends to give flight crew recommendations on how to save fuel during flight. Itreviews the different flight phases, that is to say:

• Climb

• Step climb

• Cruise

• Descent

• Holding

• Approach

4.1 Climb

4.1.1 Preliminary

Depending on speed laws, climb profiles change. The higher the speed, the lower theclimb path, the longer the climb distance.

Figure 8: Climb profiles

TOC: top of climb.

It is suggested to climb at "best rate of climb" on a constant IAS/Mach climb speedschedule. Climb is performed in three phases at max climb thrust:

• indicated air speed is maintained at 250 KT until flight level 100, then the aircraftaccelerates to the chosen indicated air speed,

• constant indicated air speed is maintained,

• constant Mach number is maintained.


The crossover altitude is the altitude where we switch from constant IAS climb to theconstant Mach number climb. It only depends on the chosen IAS and Mach number, anddoes not depend on ISA variation.

Climb can be schematized as below:

Figure 9: Climb laws

TAS: True Air Speed

IAS: Indicated Air Speed

During climb, at constant IAS, both TAS and Mach number increase. Then, during climb atconstant Mach number, both TAS and IAS decrease up to the tropopause.

4.1.2 Managed mode

The Flight Management System computes the climb speed law taking into account costindex, wind, temperature, and take-off weight. It allows a climb schedule with a continuousevolution of speed during climb to be determined.

To get conclusive information, climb and cruise flight must be viewed in relation to eachother. A short climb distance for example extends the cruise distance, a low climb speedrequires more acceleration to cruise speed at an unfavorable high altitude. One hastherefore to consider sectors that cover acceleration to climb speed, climb, acceleration tocruise speed and a small portion of the cruise as depicted in figure 10.

a) A300-600, A310, A320 family, A330

The following table gives the different relevant accurate climb parameters (time, fuel, anddistance). The first column gives climb parameters relative to the climb part only, and thesecond column take into account the remark in § 4.1.1. These results come fromtheoretical aircraft models.


Table 4: Climb parameters for different cost indices (climb to FL330)

Only climb segment Climb with cruisesegment

Aircraft type CIFuel(kg)

Time(min)

Dist(NM)

Fuel(kg)

Time(min)

CAS/MachRATE at

TOC(ft/min)

A300-600 0 2891 17 115 2977 18 320/.777 869

(PW 4158) 30 2959 17.5 119 2993 17.8 325/.791 842

160 000 Kg 60 3004 17.8 122 3004 17.8 325/.800 810

0 2787 17.4 114 2922 19 302/.791 1037

30 2833 17.6 118 2929 18.7 311/.8 1024

60 2870 17.7 121 2938 18.5 320/.803 1009

100 2920 17.9 124 2952 18.3 330/.807 991

150 2942 18.1 125 2958 18.3 330/.811 968

A310

(CF6-80)

140 000 Kg

200 2965 18.2 127 2965 18.2 330/.814 936

0 1757 22.4 150 1984 27.5 308/.765 584

20 1838 23.1 159 2009 26.9 321/.779 566

40 1897 23.7 165 2030 26.6 333/.783 550

60 1980 24.7 175 2056 26.3 340/.791 506

80 2044 25.6 183 2072 26.2 340/.797 461

A320

(CFM 56)

75 000 Kg

100 2080 26.1 187 2080 26.1 340/.8 439

0 3568 19.1 122 3927 23 293/.761 963

50 3773 20 135 3984 22.2 309/.8 943

80 3886 20.5 141 4018 21.8 320/.812 917

100 3927 20.7 143 4031 21.8 320/.818 896

150 4005 21.3 148 4053 21.7 320/.827 837

A330

(PW 4168)

200 000 Kg

200 4068 21.7 152 4068 21.7 320/.833 786


Since these values depend a lot on flight conditions (first assigned flight level, take-offweight, temperature, wind), the most representative values are delta in time, delta in fuelbetween high and low cost indices which are almost constant even with externalconditions. The results are summed up in the following table:

Table 5: Delta in fuel and in time between a high and a low cost index

Aircraft types Time gain Fuel increment (kg)

A320 1min30s 100

A330 1min20s 140

A300-600 10s 30

A310 50s 40

Time to climb is only slightly affected by the cost index (less than one minute) for theA300-600 and A310 between low and high cost indices.

Moreover, climbing at high cost indices is only valuable if time to climb is really essentialsince time differences between low and high cost index climb are very small.

b) A340 family

A340 family aircraft have a different climb behavior than twin engine aircraft. Indeed, twinengine aircraft have a higher thrust than four engine aircraft, as they must satisfy morestringent climb requirements with only one engine operative. Hence, twins climb faster thanfour engine aircraft and reach their allotted cruise flight level with higher vertical speedbecause of extra thrust.

As the climb performance of the A340 was not fully satisfying, and as ECON climb was notfully optimized, another ECON climb law was implemented so as to reduce time to climb(FMGC L7).


The following table gives the relevant parameter with regard to the previous climb laws fora climb towards flight level 330.

Only climb segment Climb with cruisesegment

Aircraft type CIFuel(kg)

Time(min)

Dist(NM)

Fuel(kg)

Time(min)

CAS/MachRATE at

TOC(ft/min)

0 5363 25.4 168 5532 26.8 298/.793 503

50 5450 26 172 5551 26.7 298/.805 485

80 5492 26.2 174 5560 26.7 298/.810 475

100 5510 26.3 175 5563 26.7 298/.812 469

150 5547 26.5 177 5570 26.7 298/.816 457

A340

(CFM 56)

250 000 Kg

200 5574 26.7 178 5574 26.7 298/.819 447

The following table gives the new ECON climb laws for various take-off weights and flightlevels:

TOW (1 000 kg)CRUISE FL

280 260 240 220 200

270 315/.78 315/.78 315/.78 315/.78 313/.775

280 309/.78 309/.78 309/.78 309/.78 309/.78

290 302/.78 302/.78 302/.78 302/.78 302/.78

310 291/.784 289/.78 289/.78 289/.78 289/.78

330 290/.79 286/.78 286/.78 286/.78

350 293/.797 286/.78 286/.78

370 295/.803 286/.78

390 294/.8


The following table compares the new ECON climb laws to the previous ones for a climbtowards flight level 330 and a take-off weight of 250 000 kg:

Table 6: Comparison between the previous climb speed laws and the new ones

Cost index FMGC L6 FMGC L7

0 298/.793 288/.78

50 298/.805 288/.78

80 298/.810 288/.78

100 298/.812 288/.78

150 298/.816 288/.78

200 298/.819 288/.78

ECON climb law is no longer function of the cost index. Mach number at the end of theclimb is in most cases M0.78 and the acceleration to cruise Mach number is made in levelflight.

This new climb law enables to reach cruise flight level some 6 to 20 NM earlier than withprevious climb laws, and rate of climb is also improved above 29 000 ft.

One must remember than the A340 is a four engine aircraft and thus cannot have climbperformance similar to that of a twin.

4.1.3 Selected mode

Under many circumstances, the ideal scenario computed by the FMS cannot be sustained.A.T.C. may impose speed or altitude constraints, and crews will consequently have toperform climb in selected mode.

The following charts give fuel consumption as well as time variations depending ondifferent climb laws, taking into account the acceleration and the cruise to reach thefurthest top of climb. As already explained in § 4.1.2 the fuel consumption for the climbsegment was added to the fuel consumption to accelerate to the cruise Mach number andto the fuel consumption to cruise till the furthest top of climb.


Figure 10: Profile used for the computation

The following graphs show delta in fuel and delta in time for different climb laws incomparison with the optimal one. As they may be not very clear, the following figure aimsat explaining them.

If you want to know the increase in fuel consumption when performing a climb with acertain speed law rather than of the optimum speed law, you will have to find the pointwhich represents the speed law on the graph. You will then be able to read thecorresponding fuel consumption increment (in percent) on the vertical axis.

EXAMPLE: You decide to perform a climb at 250Kts/300Kts/0.78.Here the optimal climb law is: 250Kts/280Kts/0.76 (figure 12).

To find the point which represents your climb law, you just have to find the intersection ofthe "isolAS 300Kts" and of the "isoMach 0.78", as shown on the graph:

Figure 11: ∆consumption between the optimum climb law anddifferent climb laws


As can be noticed, the chosen climb law consumes 1 % more than the optimal one. Thecolored legend helps to directly appreciate the range of fuel increment. For example theblue color highlights the speed laws which lead to an increase in fuel consumption for theoptimum climb law.

The same graphs were plotted for time variations:

Figure 12: ∆time between the optimal climb law and different climb laws

This particular document contains graphs characterizing the A319 CFM, the A320 CFM andthe A330 GE.

Figure 13: ∆time between the optimum climb law anddifferent climb laws


Figure 14: ∆ consumption between the optimal climb law and different climb laws

Figure 15: ∆ time between the optimum climb law and differentclimb laws


Figure 16: ∆ consumption between the optimum climb law and different climb laws

Consumption curves are stretched for high climb laws, while time curves are quite regular.This means that climbing at high speed costs more in fuel for an identical time gain. Wealso notice that for slow climb laws, fuel curves are shrinking whereas time curves areregular. So slow climb saves less fuel for equivalent time loss. In other words, timevariations are linear with respect to speed increase whereas fuel consumption variationsare not. Fuel consumption increases a lot for high climb laws and after being at its best foran optimum climb law, increases for low speed laws.

To conclude, it is neither profitable to climb at high climb laws except for time imperatives,nor to climb at very slow climb laws.

The following table gives delta in fuel and in time between the optimum climb law and themost unfavorable one:

Table 7: Delta in fuel and in time between the optimum climb law andthe most unfavorable one

Aircraft types ∆fuel (kg) ∆time (min)

A319 (60,000 kg) 110 2

A320 (70,000 kg) 110 3

A321 (78,000 kg) 120 3

A330 (190,000 kg) 170 2


Aircraft types ∆fuel (kg) ∆time (min)

A340 (250,000 kg) 250 2

A300-600 (160,000 kg) 100 1.5

A310 (130,000 kg) 130 1.5

However, even if the adjustment of climb speed saves only 10 kg fuel per flight, the annualsavings are bound to be somehow significant:

Table 8: Annual fuel savings corresponding to 10 kg fuel savings

Aircraft types Annual fuelsavings (kg)

A319 18 000

A320 17 000

A321 20 000

A330 12 000

A340 7 000

A300-600 13 000

A310 11 000


The following figure gives the potential money per year per aircraft for a saving of 10 kgfuel.

Figure 17: Annual potential savings per aircraft if 10 kg fuelis saved for each climb

Hence the optimal climb law enables significant savings.

The optimal climb law mainly depends on the aircraft take-off weight. The following tablegives the recommended climb laws with respect to the aircraft take-off weight:

Table 9: Recommended climb laws

Aircraft types BELOW ABOVE

TOW<60t TOW>60tA319

250/260/0.78 250/280/0.78

TOW<65t TOW>65tA320

250/260/0.78 250/280/0.78

TOW<70t TOW>70tA321

250/260/0.78 250/280/0.78


Aircraft types BELOW ABOVE

TOW<190t TOW>190tA330

250/280/0.8 250/300/0.8


250/300/0.78 250/320/0.78


250/300/0.8 250/320/0.8

TOW<150t TOW>150tA300-600

250/300/0.78 250/320/0.78

4.1.4 Crossover altitude versus optimum altitude

In managed mode, the crossover altitude varies with the cost index; in selected mode, itdepends on the speed law set on the FCU.

Figure 18: Climb law

This graph clearly shows that the TAS is maximum at the crossover altitude. One canwonder whether it is profitable to stay at this altitude, instead of climbing to the firstoptimum altitude.

Considering standard climb laws, it is possible to know the crossover altitude and the firstoptimum flight level.


The standard speed laws are summed up in the following table:

Table 10: Standard climb laws

Aircraft types Speed law

A319/A320/A321 250kts/300kts/M0.78

A330 250kts/300kts/M0.8

A340 250kts/290kts/M0.78

A310 250kts/300kts/M0.8

A300-600 250kts/300kts/M0.78

The next table exhibits the crossover altitude and the first optimum flight level for an ISAvariation below 10 degrees Celsius and for an aircraft weight near the maximum takeoffweight.

Table 11: Crossover altitude and first optimum altitude

Aircraft types Crossover altitude 1st optimum FL(ISA<10)

A319 370

A320 360

A321

29000 ft

340

A330 GE

A330 PW

A330 RR

31000 ft 370

A340 32000 ft 330

A310 30000 ft 330

A300-600 29000 ft 300

To be aware that it is of no use staying at the crossover altitude to gain time as the TAS ishigher, the table shows the increase in consumption and the decrease in time when flyingat crossover altitude rather than at the optimum altitude (ISA<10) on a 1000Nm stagelength.


Table 12: Delta in fuel and time when flying at crossover altitudeinstead of flying at optimum altitude.

Stage length: 1000 Nm. Cruise Mach number.

Aircraft type Gainedtime (min)

Increase in fuelconsumption (kg)

A319 (60,000 kg) 5 950

A320 (65,000 kg) 5 800

A321 (75,000 kg) 4 650

A330 GE (190,000 kg) 3 1300

A330 PW (190,000 kg) 3 1200

A330 RR (190,000 kg) 3 1200

A340 (250,000 kg) 0.5 80

A310 (140,000 kg) 2 450

A300-600 (160,000 kg) 0.5 100

The table shows that flying at crossover altitude is not valuable particularly for A319/A320and A330 because their first optimum flight levels are far above the crossover altitude. Thisis not true for the A340: due to its particular climb speeds, the A340 crossover altitude (wellabove the other) corresponds to the first optimum flight level for high take-off weights. Forthis aircraft indeed, it may be beneficial to stay at the crossover altitude at the beginning ofany flight.

The following table expresses the results above (table 13) in percentages:

Table 13: Percentage of fuel increment and of time gain when flying at crossoveraltitude instead of flying at optimum altitude.Stage length: 1000 Nm. Cruise Mach number.

Aircraft type Gained time(%)

Increase in fuelconsumption (%)

A319 (60 000 kg) 4 20

A320 (65 000 kg) 4 15

A321 (75 000 kg) 3 10

A330 GE (190 000 kg) 2.5 10

A330 PW (190 000 kg) 2.5 10

A330 RR (190 000 kg) 2.5 10

A340 (250 000 kg) 0.5 0.5

A310 (140 000 kg) 1.5 4

A300-600 (160 000 kg) 0.5 1


4.2 Step climb

4.2.1 Preliminary

When plotting specific range for different altitudes and weights, we readily see that, foreach weight there is an altitude that maximizes specific range.

Figure 19: Specific range variations for different weights and altitudes

Thus, an optimum altitude can be found for each weight:

Figure 20: Optimum altitude

As for specific range, the optimum altitude is mostly independent of temperature.The ideal scenario would be to follow the optimum altitude as in climbing cruise. ButA.T.C. constraints do not make this possible for the moment. So aircraft should remainas close as possible to their optimum altitude. The closest way to do so is to performstep climbs. In general, large deviations from the optimum altitude should be avoided,as the maximum altitude is high enough to enable a good altitude selection.

4.2.2 Trade-off between manoeuvrability and economy

Several parameters such as weather conditions, ATC requirements, may influence anydecision made by the crew with regard to three fundamental priorities: manoeuvrability,passenger comfort, and economics.


This pertains to the choice of cruise flight level, which can be made according to the threefollowing climb profiles:

Figure 21: Step climb profiles

The best order to improve manoeuvrability margins is: 3, 2, 1.

The best order for money savings is: 2, 3, 1.

Thus pilots are advised to perform step climbs around the optimum altitudes.FCOM (3.05.15) facilitates this by providing the optimum weight for climb to the next flightlevel.

On all Airbus FMS-equipped aircraft, both optimum altitude (OPT FL) and maximum flightlevel (MAX FL) are displayed on the MCDU progress page. The recommended maximumaltitude in the FMGC ensures a 0.3g buffet margin, a minimum rate of climb of 300ft/min atMAX CLIMB thrust and a level flight at MAX CRUISE thrust.


Pilots perform step climbs, traffic and A.T.C. requirements permitting. Values of OPT FLand MAX FL are given in the following table:

Table 14: Delta between optimum FL and MAX FL

Aircraft typesAverage differencebetween OP FL and

MAX FL

A319 4000 ft

A320 3000 ft

A321 3500 ft

A330 GE 3500 ft

A330 PW 3500 ft

A330 RR 3000 ft

A340 2500 ft

A310 3500 ft

A300-600 3500 ft

4.2.3 Delays in altitude follow-up

Let us consider an aircraft which cannot perform its step climb. Being at flight level 330, itshould climb at flight level 370. However, A.T.C. requires a level off at an intermediateflight level or does not allow the climb to flight level 370. Consequently the aircraft cannotfollow its natural step climb profile.

If the aircraft is compelled by A.T.C. to stay at flight level 330, it will consume excess fuelsince its optimum flight level is 370 for its actual weight. This increment in fuel consumptionhas been computed for a trip of one and two hour(s).

The results are brought together in the following table:

Table 15: Fuel increment for delayed climb to FL 370 at optimum weight

Aircraft types FL330 + 1hFuel (kg)

FL330 + 2hFuel (kg)

A319 135 330

A320 120 315

A321 110 320

A330 185 550



FL330 + 2hFuel (kg)

A340 265 720

A300-600 140 450

A310 170 500

The fuel increment is significant and to be aware of it, the following table gives thepercentage increment for a one or two hour flight:

Table 16: Fuel increment in percent for a delayed climb


FL330 + 2hFuel (kg)

A319 5.5% 7.5%

A320 5% 7%

A321 4.2% 6.3%

A330 3.3% 5%

A340 4.5% 6.5%

A300-600 3.2% 5.3%

A310 4% 6.2%

The fuel increment is about 5% of total fuel burn and it increases when time spent at alower level increases. Fuel can hence be saved by anticipating requests to higher flightlevels or by climbing directly to the maximum altitude.

As can be noticed, fuel increments are less important for A340, A330, A310 and A300-600.However, flying at FL370 instead of FL330 can lead to important fuel savings whatever theaircraft type.

To conclude, spending too much time below optimum altitude results in a used fuel/savedtime ratio not at all profitable in terms of costs but performing step climbs as explainedabove always results in a profitable ratio, both in terms of time and costs.

4.3 Cruise

4.3.1 Preliminary

The management of the flight profile (climb schedule, Mach number and flight level incruise, descent schedule, holding, approach) has an important impact both on fuelconsumption and flight time. In the following part, some well-known parameters areidentified which, when correctly managed, can enable airplane utilization to be optimized.


The cruise phase is the most important phase regarding fuel savings. As it is the longestfor long haul aircraft, it is possible to save a lot of fuel. So discipline is particularly importantin this phase.

4.3.2 Managed mode

The flight management system (FMS) optimizes the flight plan for winds, operating costsand suggests the most economical cruise altitude, airspeed, depending on the cost indexchosen by the airline. An airline which wants to save fuel has to choose a low cost index.As said before, this brochure does not aim at helping airlines in their cost index choices.Nevertheless, the next section still intends to highlight the impact of the cost index on fuelconsumption and on trip time.

a) Economy Mach number

Depending on the cost index, predicted aircraft and atmospheric conditions, the OptimumAltitude and the Economy Mach number are computed. From then on, fuel consumptiondepends only of the chosen cost index.

The following charts show the Economy Mach number variations for different cost indicesand for different flight levels. For more clarity, only five graphs have been inserted. But youcan find the graphs which characterize other aircraft in the first appendix.

Figure 22: Economic cruise Mach number for variousflight levels and cost indices



The higher the flight level, the higher the economic Mach number. The charts clearly showthat the economic Mach number changes a lot during flight for low cost indices, whereas itis rather constant for high cost indices. So, if airlines favor time at the expense of fuelconsumption, flights are performed at constant Mach number. The Economy Mach is verysensitive to the cost index when flying below optimum altitude.

However, as aircraft weight decreases, the Economy Mach also decreases particularly forlow cost indices:

Figure 26: Economic cruise Mach number for various weights and cost indices


The charts show that for high cost indices, the Economy Mach number stays fairly constantthroughout the flight. Nevertheless, for low cost indices, the economic Mach number variesa lot. This is quite normal as low cost indices favor fuel consumption at the expense oftime. Moreover, we notice that for low cost indices, a small cost index increment has afar-reaching influence on the economic Mach number, and hence on flight time especiallyfor the A340.

b) Time/fuel relation

To know whether it is profitable to fly at low cost indices, the impact of cost indices on timehas to be considered. The following graphs show trip fuel and time for different flight levelsand cost indices. The computation was done for all aircraft types and restituted the samebasic shape. The aircraft considered here are the A310, the A319 and the A330; thecomputation was performed for a 2000 Nm stage length:

Figure 31: Time/fuel relation for a typical stage length.Stage length: 3000 Nm, M 0.8


As can be seen, it is not really advantageous to fly at very low cost indices as fuel savingsare not significant compared to time loss. The time gain is invariably almost the same fordifferent but high cost indices. For instance, for the A330, flying at flight level 310 with acost index equal to 0 lasts 15 minutes more than flying at flight level 310 with a cost indexequal to 20, for an increase in fuel consumption of a mere 100 kg. In percentage, 15minutes represents 3.5% of the total flight time, and 100 kg represents 0.25% of the totalfuel consumption.

In the same way, for the A330 at FL 350, only 50 kg fuel is saved for 10 minutes ofadditional flight time between CI=0 and CI=20.

4.3.3 From Managed to Selected Mode

Flying at a given cost index rather than at a given Mach number provides the addedadvantage of always benefiting from the optimum Mach number as a function of aircraftgross weight, flight level and head/tailwind components.

This means the ECON mode ("managed" mode) can save fuel relative to fixed Machschedules ("selected" mode) and for an identical block time.

One can wonder whether selecting a higher Mach number than the one chosen by theFMS has a significant impact on fuel consumption. Imagine an aircraft flying at flight level370, in managed mode and at the optimum weight of FL370. The FMS computes theoptimum speed based on cost index, temperature, wind... If the pilot selects another Machnumber (a bigger one), fuel consumption changes as shown with the following charts (for a2000 Nm stage length):

Figure 34: Fuel consumption increment on a 2000 Nm stage length,when the pilot switches in selected mode

and increases Mach number


Figure 35: Fuel consumption increment on a 2000 Nm stage length, when the pilot switches in selected mode and increases

Mach number

Figure 36: Fuel consumption increment on a 2000 Nm stage length, when the pilot switches in selected mode and

increases Mach number


The following table gives fuel increment and time gain for a 2000Nm stage for the case of a0.005 Mach number increase at the beginning of the 2000Nm stage.

Table 17: Delta in fuel and time in the case of a 0.005 Mach increase

Aircraft types Fuel increment Saved time

A319 +150 kg -1 min

A320 +200 kg -2 min

A321 +200 kg -2 min

A330 +250 kg -3 min

A340 +280 kg -3 min

A300-600 +350 kg -2 min

A310 +300 kg -2 min

We notice that fuel consumption increases a lot when increasing cruise Mach number by0.005. This can reach up to 2%. Further away from the economic Mach number, fuelconsumption can even increase beyond 5% for a Mach increment of 10 points.

This can reach 300 kg extra fuel for the A310/A300-600, 100 to 200 kg for theA319/A320/A321 on a 2000 Nm stage, the A330/A340 being in between.

Pilots hence have to be patient and should not change the Mach number even when underthe impression that the aircraft does not fly fast enough.

Moreover, the managed mode must be kept whenever possible.

4.3.4 Selected mode

a) Preliminary

A.T.C may at times constrain aircraft, for it is to maintain safety in the first place. In thesecases, flight crews may have to activate the selected mode and choose flight parametersaccordingly. This section shows the impact of altitude, Mach number and wind on fuelconsumption, to give recommendations to remain as close as possible to the optimum.

The following graphs show fuel consumption as a function of time for different Machnumbers at different flight levels and for typical stage lengths (2000 Nm for the A319 type,3000 Nm for the A330 and A310).


Figure 37: Fuel consumption and time for different flight levels and Machnumbers on a 3000 Nm trip. TOW=130t.




As is well-known, the faster aircraft are flown, the more fuel they consume and the lesstime is taken for the flight. The higher the aircraft are flown, the less they consume. Wenotice that the slopes of these curves are similar. So, whatever the Mach number, fuel totime ratios are identical for each aircraft when climbing from one flight level to the next one.

As aircraft are limited by maximum altitudes, the next sections are intended to recommendmore accurate altitudes and climb Mach numbers as a function of flight situations.

b) Flight at a given Mach number

b.1. Optimum altitude

If A.T.C. imposes a Mach number, flight crews can only optimize the altitude. When notflying at optimum altitude, one must first be aware of fuel consumption increases. Thefollowing charts provide fuel and time increments when flying below the optimum altitudeon a 1000 Nm stage length.

The following charts provide fuel and time increments when flying below optimum altitude.For the whole of the A320 family, an entire trip was simulated at a given flight level.Compared to that, for the other aircraft, graphs were plotted from values of specific rangeat a given weight and flight level in cruise. Otherwise, one would have had to consider astep climb with hence some complicated graphs due to an abundance of possibilities. Thisexplains why no specific stage length is mentioned in the titles of the graphs.


Figure 40: Fuel consumption increment when flying above optimum altitude.Stage length: 1000 Nm. M 0.78

Figure 41: Fuel consumption increment when flying above optimum altitude.Stage length: 1000 Nm. M 0.78


Figure 42: Fuel consumption increment when flying belowoptimum altitude. M 0.8

GE

Figure 43: Fuel consumption increment when flying below optimum altitude. M 0.8


Figure 44: Fuel consumption increment when flying belowoptimum altitude. M 0.8

CFM

We appreciate that identical altitude differences do not always correspond to the samerange loss. For instance, for the A320 IAE, the 2% range loss due to flying at flight level330 rather than at 350 when the weight is 70t may seem acceptable and could lead to thechoice of a lower altitude. However, after a 10 ton fuel burn the range loss reaches 6%.

Evidently, for low weights, the increase in fuel consumption is particularly significant: it canreach up to 20% over a 1000 Nm stage. Nevertheless, flying at 2000 ft above or below theoptimum altitude only increases fuel consumption by 2%.

Flight crews should follow a profile as shown below:

Figure 45: Optimum flight profile


In order to help flight crews, the following charts provide the optimum altitude with respectto aircraft weights and Mach numbers. Pilots can therefore know if they are at the rightaltitudes with regard to fuel savings. Hereby following are graphs characterizing theA300-600 GE, the A310 PW, the A319, the A330 GE and the A340 CFM. The graphs forthe A300-600 PW, for the A310 GE, for the A330 PW, for the A330 RR, for the A320 andthe A321 are presented in Appendix 2.

Figure 46: Optimum altitude with respect to aircraft weight





CFM


We can see that the influence of airspeed on optimum altitude is negligibly small in therange of normal cruise speeds. Since the range optimum altitude closely matches theminimum cost altitude, every effort should be made to reach this altitude both in flightplanning and during flight.

b.2. Optimum altitude on short stage

Optimization should be made for each stage length as it sometimes not worth climbingwhen the cruise is too short to compensate for increased fuel consumption linked to climbto optimum altitude.

For instance, for the A321 with a take-off weight of 83 000 kg, the optimum flight levelwould be 330. However, climbing at this altitude needs 2006 kg of fuel. Climbing at flightlevel 310 needs 1829 kg of fuel.

If the stage length is 275 Nm, it is not valuable to cruise at flight level 330. The latter beingthe optimum flight level:

• total fuel consumption = 2691 kg at FL330.

• total fuel consumption = 2687 kg at FL310.

It is sometimes not at all valuable to climb to the optimum altitude. It is all the moreapplicable as we are near the aircraft's maximum take-off weight.

To help crews, optimum altitudes on short stage are given for each aircraft: Herebyfollowing are the graphs characterizing the A300-600 GE, the A310 PW, the A319, theA330 GE and the A340 CFM. The graphs for the A300-600 PW, for the A310 CFM, for theA330 PW, for the A330 RR, for the A320 and the A321 are presented in Appendix 3.


Figure 52: Optimum altitude on short stageClimb: 250KT/300KT/M0.8Long range cruiseDescent: M0.8/300KT/250KT

Figure 53: Optimum altitude on short stageClimb: 250KT/300KT/M.78Long range cruiseDescent: M.78/300KT/250KT


Figure 54: Optimum altitude on short stageClimb: 250KT/300KT/M0.8Long range cruiseDescent: M0.8/300KT/250KT

Figure 55: Optimum altitude on short stageClimb: 250KT/300KT/M.78Long range cruiseDescent: M.8/300KT/250KT


c) Flight at a given Flight Level

A.T.C. sometimes orders flight crews to stay at a given flight level. The only parameterwhich can be changed is Mach number. If plotting specific range variations with respect toMach number at a given altitude, we can determine the Mach number which gives the bestspecific range. It is called the maximum range cruise Mach (MRC Mach). For practicalpurposes (speed stability since MRC hinges around an optimum), a long range cruiseprocedure was defined that gains a significant speed increase compared to MRC at thecost of a mere 1 % loss in specific range. Like MRC speed, LRC speed also decreaseswith decreasing weight, at constant altitude.

Figure 56: : LRC versus MRC at given altitude

The following graphs show LRC speed as a function of weight, and flight level. Herebyfollowing are graphs characterizing the A300-600 GE, the A310 PW, the A319, theA330 GE and the A340 CFM. The graphs for the A300-600 PW, for the A310 CFM, for theA330 PW, for the A330 RR, for the A320 and the A321 can be found in Appendix 4.


Figure 57: Long Range Cruise Mach number




It is clear from all these charts that LRC speeds remain fairly constant if weight and altitudecorrespond to optimum altitude conditions. It would therefore be possible to fly a constantMach number procedure instead of the variable LRC speed procedure. In order to savefuel however, the exact LRC speed schedule should be maintained.

d) Wind influence

Wind can have a significant influence on flight parameters. Nowadays, meteorologicalforecasts are very reliable and its integration into the FMS provides accurate information toflight crews. Hence, the latter system can best perform flight planning with a view towardsfuel savings.

To be aware of wind influence on both trip time and trip fuel, the following graphs providefuel consumption and time with respect to flight levels, Mach number and wind. Headwindvelocity is preceded by a minus sign whereas tailwind velocity by a plus sign.


Figure 62: Fuel consumption and trip time for various Mach numbers andflight levels in windy conditions. TOW=130t.




As one can imagine and verify, tailwind is very profitable whereas headwind is not. Forinstance, for the A310 GE, for a 2000 Nm stage at flight level 330 and at Mach 0.8, fuelconsumption with no wind is 28 800 kg, whereas it is 32 700 kg for a headwind equal to60 KT and 25 700 kg for a tailwind of the same magnitude. This corresponds to anadditional consumption of some 3900 kg in the case of a headwind (that is to say 13% ofthe total fuel consumption) and to a reduced consumption of some 3100 kg (that is to say10% of the total fuel consumption) in the case of a tailwind.

The following table gives the increment or decrement in fuel consumption for an entireflight at a given Mach number, at flight level 310 and at a take-off weight close to themaximum take-off weight.

Table 18: Fuel consumption variation with head/tailwind

Aircraft types Mach Stagelength (Nm)

Fuel increment ifheadwind (-60 kts)

Fuel decrement iftailwind (+60 kts)

A319/A320 0.76 < M < 0.8 2000 + 1500 - 1100

A321 0.76 < M < 0.8 2000 + 1800 - 1400

A330 0.8 < M < 0.84 3000 + 5300 - 4000

A340 0.8 < M < 0.84 3000 + 6000 - 5000


Aircraft types Mach Stagelength (Nm)

Fuel increment ifheadwind (-60 kts)

Fuel decrement iftailwind (+60 kts)

A300-600 0.78 < M < 0.82 2000 + 3500 - 2700

A310 0.78 < M < 0.82 2000 + 3200 - 2500

In headwind conditions, specific range decreases and the maximum range speedincreases. In tailwind conditions, the maximum range speed decreases and specific rangeincreases.

In the area to the right of the maximum specific range line (figure 62), range losses due toheadwinds can be completely or partially offset for by an airspeed reduction. The higherthe original airspeed, the more applicable this is. Beyond a certain headwind component, areduction in air speed will not improve the specific range over ground. At slow initial speed(e.g. LRC) and very strong headwinds, a speed increase might even prove to beadvantageous.

Figure 65 herebelow, shows that for given initial conditions, a headwind component of VWKT can be compensated for by reducing speed from Mach 0.84 to LRC.

Figure 65: Wind Influence on specific range

The influence of wind is in this case applied to range, which in numerous cases is the soleindicator of how economic a flight really is.

Without wind, specific range is maximum at optimum altitude. In windy conditions, it mayhowever be beneficial to fly at a different cruise altitude. There may be a tailwind at a loweraltitude, which could make up for the increase in fuel consumption when flying lower.


Figure 66: Wind altitude trade for constant specific range


EXAMPLE:

If we consider an aircraft to be at a weight of 190 000 kg, and if there is a wind of a 10 KTintensity at flight level 370, we can assess whether it is valuable to descend to FL 330.Indeed, the minimum wind difference must be equal to:

31-2=29 KT

This result is found by intersecting the vertical line from the abscissa AT 190000 kg of theweight axis, with ordinate flight level lines. The corresponding winds are derived to makethe difference.

Descending to flight level 330 can be contemplated provided the tailwind at this altitude ismore than 29-(wind at FL370)=29-10=19 KT.

Flight crews will find these charts in the flight operating crew manuals (FCOM), morespecifically in the "in flight performance part" (3.05.15).

4.4 Descent

4.4.1 Preliminary

Depending on the descent law, flight paths do vary in steepness.

Indeed, the higher the speed law, the steeper the flight path.

Figure 67: Descent profiles


4.4.2 Managed mode

The FMS computes the Top Of Descent (TOD) as a function of the cost index. We noticethat the higher the cost index:

• the steeper the descent path (the higher the speed)

• the shorter the descent distance

• the later the top of descent point.

Descent performance is a function of the cost index; the higher the cost index, the higherthe descent speed. But contrary to climb, aircraft gross weight and top of descent flightlevel appear to have a negligible effect on the descent speed computation.

The following table shows different relevant accurate descent parameters computed byin-flight performance software for the entire Airbus Family.

Descent from FL 370-ISA conditions:

Table 19 : Relevant parameters for a descent from FL 370 at different cost indices(standard conditions)

Only descent segment Descent with cruisesegment

Aircraft types ClFuel(kg)

Time(min)

Dist(NM)

Fuel(kg)

Time(min)

Mach/CAS

A300-600(PW 4158)120000 Kg

03060

100

317298282276

19.317.315.815

1081029693

317357403427

19.318.117.417

.790/258

.793/285

.800/310

.800/325

A310(CF6-80)

110000 Kg

03060

100

284263239218

21.419.31715.3

11611110395

284301353406

21.419.918.718

.756/246

.801/269

.806/300

.810/332

A320(CFM 56)60000 Kg

0204060

800

138125112137142

191714.914.614.6

10599909292

138157187207210

1917.816.816.416.3

.764/252

.779/278

.786/311

.796/339

.800/342

A330(PW 4168)170000 Kg

05080

>100

449444427420

23.522.720.519.6

135134125121

449463540580

23.522.921.921.4

.774/270

.809/281

.819/307

.823/320

A340(CFM 56)

180000 Kg

<5080

100>150

550524509501

23.22119.719.7

133125120117

550620620663

23.22221.421.2

.767/273

.799/301

.811/323

.817/323


Whereas values for time, distance, Mach number, fuel consumption do vary with flightconditions such as TOD, flight level, temperature and wind, the former are less variablewith respect to gross weight. Delta values with regard to time and distance are largely thesame whatever the initial flight conditions. The following table provides parameters anddifferences in terms of time and fuel from a similar geographical point (TOD correspondingto cost index 0) to summarize descent laws between CI=0 and high cost indices.

Table 20: Delta time and fuel between a descent at a high and a low cost index(decent from FL 370, standard conditions)

Difference between low and high cost indexAircraft types

Time gain Fuel increment (kg)

A320 2 min 40 s 70

A330 2 min 10 s 130

A340 2 min 110

A310 3 min 20 s 120

A300-600 2 min 20 s 110

It can be noticed that time to descent between low and high cost indices is more sensitivethan climb phase.

4.4.3 Selected mode

Fuel consumption with respect to speed

The following curves present the fuel burn during descent, from the same geographicalpoint in cruise. For high speeds, fuel consumption needed to fly from TOD of a shallowdescent to TOD of the steepest descent at cruise level have been added. Moreover, aspeed limitation of 250 KT below 10,000 ft was used in the calculations to comply withA.T.C constraints.

In the following graphs, fuel consumption is shown for the A300-600 GE, the A310 PW, theA319, the A330 GE and the A340 CFM. The graphs for the A300-600 PW, A310 GE,A330 PW, A330 RR, A320 and the A321 are presented in Appendix 6.


Figure 68: Descent consumption from the same point in cruise




Whatever flight conditions, the optimum descent speed is about 280 KT. Nonetheless,charts clearly depict the influence of speed on fuel consumption. For example, 20 kg of fuelcan be saved by performing the descent phase at 280 KT instead of 300 KT. The followingtable gives the annual savings expressed in US dollars per aircraft when performing adescent at 280 KT instead of 300 KT:

Table 21: Annual potential fuel savings for a 20 KT decrease in descent speed

AircraftAnnual fuelsavings peraircraft (kg)

A319 36 000

A320 34 000

A321 40 000

A330 24 000

A340 14 000

A310 22 000

A300-600 26 000


These annual fuel savings enable important money savings as shown on the next figure.

Figure 73: Annual potential money savings for a 20 KT decrease in descentspeed.

In terms of money savings, this represents from $3000 to $8000 per aircraft and per year,depending on aircraft types. We notice that this speed reduction has a bigger impact onaircraft from the A320 family than on any other aircraft.

The following tables present time and fuel variations with respect to a reference(IAS = 300 KT).

Table 22: Comparison of descent A319/A320/A321

∆ fuel (kg)∆ time (min)

FL 390 FL 370 FL 350 FL 330 FL 310

260 KT - 40+ 3

- 40+ 3

+ 40+ 3

- 35+ 3

- 25+ 3

280 KT - 20+ 1.5

- 20+ 1.5

- 20+ 1.5

- 20+ 1.5

- 15+ 1.5

300 KT REF REF REF REF REF

320 KT + 15– 1

+ 15- 1

+ 15- 1

+ 15- 1

+ 20- 1

340 KT + 25– 1

+ 25- 2

+ 30- 2

+ 30- 2

+ 35- 2


Table 23: Comparison of descent A330 GE/PW/RR

FL 390 FL 370 FL 350 FL 330 FL 310

260 KT - 80+ 2.5

- 80+ 2.5

- 80+ 2.5

- 60+ 2

- 35+ 2

280 KT - 55+ 1.5

- 55+ 1.5

- 55+ 1.5

- 50+ 1.5

- 25+ 1


320 KT + 25– 1

+ 25– 1

+ 25– 1

+ 30– 1

+ 30– 1

340 KT + 50– 1.5

+ 50– 1.5

+ 50– 1.5

+ 55– 1.5

+ 55– 1.5

Table 24: Comparison of descent A340 CFM

FL 390 FL 370 FL 350 FL 330 FL 310

260 KT - 60+ 1.5

- 60+ 1.5

- 60+ 1.5

- 50+ 1.5

- 25+ 1

280 KT - 30+ 1

- 30+ 1

- 30+ 1

- 30+ 1

- 20+ 0.5


320 KT + 30– 1

+ 25– 1

+ 25– 1

+ 30– 1

+ 30– 1

340 KT + 55– 1.5

+ 55– 1.5

+ 55– 1.5

+ 60– 1.5

+ 60– 1.5

Table 25: Comparison of descent A300-600

FL 390 FL 370 FL 350 FL 330 FL 310

260 KT - 70+ 3

- 65+ 3

- 65+ 3

- 60+ 2.5

- 50+ 2

280 KT - 30+ 1

- 35+ 1

- 35+ 1

- 40+ 1

- 30+ 1


320 KT + 250

+ 30– 1

+ 30– 1

+ 30– 1

+ 35– 1

340 KT + 40– 0.5

+ 40– 1

+ 40– 1.5

+ 45– 1.5

+ 45– 1.5


Table 26: Comparison of descent A310

FL 390 FL 370 FL 350 FL 330 FL 310

260 KT - 60+ 3

- 60+ 3

- 60+ 3

- 45+ 2.5

- 30+ 2

280 KT - 30+ 1.5

- 30+ 1.5

- 30+ 1.5

- 35+ 1.5

- 25+ 1


320 KT + 25- 1

+ 30– 1

+ 30– 1

+ 30– 1

+ 30– 1

340 KT + 50– 2

+ 50– 2

+ 55– 2

+ 60– 2

+ 55– 2

Whatever the aircraft, the decrease in fuel is higher when diminishing the speed by 20 or40 KT than the increase in fuel when increasing the speed by 20 or 40 KT. In accordancewith the preceding, it is advised to perform the descent phase around 280 KT.

Premature descent

If the aircraft begins its descent too early, the aircraft would leave its optimal flight level,where fuel consumption is at its best.

For examples' purposes, let us consider that an aircraft begins its descent 10 minutes tooearly:

Figure 74: Profile of a too early descent

Two options were simulated:

• a top of descent 10 minutes early followed by a level-off at FL100,

• a cruise segment followed by a descent which starts at the optimum top ofdescent.


The two options were subsequently compared. It appears that the increment of fuel perminute does not depend of the total anticipation time. The following table gives thereforethe average fuel increment for an aircraft beginning its descent one minute too early incomparison with the second option.

Table 27: Fuel increment for a one minute early descent fromflight level 350. Cruise Mach number

Aircraft Fuel consumptionvariation (kg)

A319 CFM (55 000 kg) 20

A319 IAE (55 000 kg) 25

A320 CFM (60 000 kg) 20

A320 IAE (60 000 kg) 25

A321 (65 000 kg) 25

A330 GE (170 000 kg) 30

A330 PW (170 000 kg) 30

A330 RR (170 000 kg) 30

A340 CFM (180 000 kg) 30

A310 (110 000 kg) 30

A300-600 GE (140 000 kg) 20

A300-600 PW (140 000 kg) 20


4.5 Holding

4.5.1 Preliminary

AIRBUS provides airlines with four different holding configurations (FCOM 3.05.25),adapted to each type of aircraft.

Table 28: Recommended holding configurations

Aircraft types Configuration 1 Clean configuration

A319/A320/A321/A330/A310 170 kts S speed 210 kts Green dot

speed

A340/A300-600 210 kts S speed 240 kts Green dotspeed

Green dot speed is the one or two engine out operating speed in clean configuration. Itcorresponds to an approximation of the best lift to drag ratio speed and thus leads to a lowhourly consumption:

Figure 75: Green dot speed definition

The speed which gives the lowest fuel consumption has been replaced by the green dotspeed which helps to obtain a significant increase in speed at the expense of a very limitedfuel consumption increase.


4.5.2 Various configuration / speed combinations

Following graphs compare the hourly consumption for several holding configurations andspeeds. It appears that the best combination is the clean configuration at green dot speed.

Figure 76: Fuel flow with respect to holding altitude for several configurations



Curves reveal that green dot speed leads to the lowest hourly consumption, except for theA330: when arriving in the holding at high gross weight (around 190.000 kg), 210 KT isslightly more economical than green dot speed. In fact, for this weight, 210 KT is the speedwhich gives the lowest fuel consumption (figure 74).

If we compare the fuel flow at flight level 100 in configuration 1 at S speed and the fuel flowat the same altitude but in clean configuration at green dot speed, some differences arenoticed and are summed up in the following table:

Table 29: Percentage of fuel flow increment when holding at S speed in conf 1instead of holding at green dot speed in clean conf. Flight level 100

Aircraft types

A319 CFM 3%

A319 IAE 4%

A320 CFM 6%

A320 IAE 8%

A321 CFM 7%

A321 IAE 8%

A330 GE 13%

A330 PW 11%

A330 RR 8%

A340 CFM 11%

A310 GE 12%

A310 PW 11%

A300-600 GE 11%

A300-600 PW 18%


For a 15-minute holding, the corresponding fuel saved is:

Table 30: Fuel increment when holding at S speed in conf 1instead of holding at green dot speed in clean conf

for a period of 15 minutes. Flight level 100

Aircraft types Fuel saved perholding

Annualsavings

(kg)

Annualsavings

($)

A319 CFM 15 kg 27 000 5 600

A319 IAE 20 kg 36 000 7 500

A320 CFM 25 kg 43 000 9 000

A320 IAE 35 kg 60 000 12 500

A321 CFM 40 kg 80 000 16 500

A321 IAE 45 kg 90 000 18 500

A330 GE 150 kg 180 000 37 000

A330 PW 120 kg 144 000 30 000

A330 RR 90 kg 108 000 22 500

A340 CFM 140 kg 98 000 20 500

A310 GE 110 kg 121 000 25 000

A310 PW 90 kg 99 000 20 500

A300-600 GE 90 kg 117 000 24 000

A300-600 PW 150 kg 195 000 40 500

The table shows that the "green dot speed/clean configuration" combination enablessignificant savings.


However, green dot speed increases with weight and can become higher than themaximum recommended speeds which are reminded below:

Table 31: Maximum recommended speeds

Levels ICAO PAN-OPS FAA France

Up to 6,000 ft inclusive 210 KT 200 KT

Above 6,000 ft to 14,000 ft inclusive230 KT

230 KT 230 KT220 KT

Above 14,000 ft to 20,000 ft inclusive 240 KT



265 KT

Above 34,000 ft M 0.83

240 KT 265 KT

M 0.83

If green dot is higher than these maximum recommended speeds, it is advised to hold inconfiguration 1 at S-speed: keeping clean configuration coupled with a speed reductionwould save fuel but would decrease the safety margin and could become hazardous inturbulent conditions.

As during cruise phase, there is an optimal holding altitude. However holding altitudes areoften imposed by ATC.

4.5.3 Linear holding

If holding is going to be necessary, linear holding at cruise flight level and at green dotspeed should be performed whenever possible since total flight time will remain constant(cruise time is increased but holding time is reduced) and fuel flow is lower at high flightlevels.

If, 15 minutes before reaching a fix, ATC informs that 10 minutes holding is expected, twooptions are possible:

• Either the aircraft is flown 15 minutes at cruise speed and 10 minutes at green dotspeed.

• Or the aircraft performs the cruise to reach the fix and speed remaining time atgreen dot speed.


This can be schematized as below:

Figure 80: Description of the holding options

Table 32: Fuel saved by reduction from M 0.78 to green dot speed at FL 350

Aircraft types CFM/IAE

A319 (55,000 kg) 100 kg

A320 (60,000 kg) 70 kg

A321 (60,000 kg) 100 kg


Aircraft types GE PW RR

A330 (180,000 kg) 90 kg 80 kg 100 kg



Aircraft types CFM

A340 (190,000 kg) 50 kg


Aircraft types GE PW

A310 120 kg 120 kg

A300-600 65 kg 80 kg

This speed reduction would save a great amount of fuel per year, if only we consider that a10' holding is necessary for 10% of the annual flights:

Table 36: Money and fuel saved thanks to the second option

Aircraft type Fuel saved peryear (kg)

Fuel saved peryear

(gallon))

Annualsavings ($)

A319 18 000 4 750 4 750

A320 12 000 3 200 3 200

A321 20 000 5 300 5 300

A330 GE 11 000 2 900 2 900

A330 PW 9 600 2 500 2 500

A330 RR 12 000 3 200 3 200

A340 3 500 900 900

A310 13 200 3 500 3 500

A300-600 GE 8 500 2 200 2 200

A300-600 PW 10 400 2 800 2 800

Note: This speed reduction cannot always be performed, because of ATC restrictions.

These computations do evidently show that the second option is the best one.


4.6 Approach

4.6.1 Decelerated and stabilised approach

The standard recommended approach is the stabilized approach as decribed below:

Figure 81: The stabilized approach

Conditions permitting, decelerated approaches are nonetheless possible. They allow asmooth approach and potential fuel savings compared to the stabilized approach. Theaircraft should intercept the final descent path at S speed in configuration 1. At 1000 ftabove runway elevation, aircraft should be stabilized on the final descent path in thelanding configuration with thrust above idle.


The following figure describes a decelerated approach:

Figure 82: The decelerated approach

The following table gives the increase in fuel consumption when a stabilized approach isperformed, in comparison to a decelerated approach.

Table 37: Fuel increment between a stabilized approach anda decelerated approach

Aircraft Fuel increment(kg)

A319/A320 + 20

A321 + 25

A330 + 40

A340 + 45

A300-600 + 45


These marginal fuel savings can nevertheless produce significant annual fuel savings. Thefollowing table provides the annual fuel savings per aircraft and per type.

Table 38: Potential annual savings due to the decelerated approach

AircraftAnnuals fuelsavings peraircraft (kg)

Annuals fuelsavings per

aircraft (gallon)

Annual savingsper aircraft ($)

A319 36,000 7,400 7,400

A320 34,000 7,000 7,000

A321 50,000 10,300 10,300

A330 48,000 9,900 9,900

A340 31,500 6,500 6,500

A300-600 58,500 12,000 12,000

Figure 83: Money saved by the decelerated approach in comparisonwith the stabilized one

It is obvious that the stabilized approach needs more fuel as flaps and landing gears areextended earlier and thus increase drag. Nevertheless, stabilized approaches are safer asindicated by CFIT and ALAR statistics. Moreover, decelerated approaches can only beperformed in category I weather conditions.


4.6.2 Premature landing gear extension

To be aware of the increase in fuel consumption when the landing gear is extended tooearly, the following procedure was simulated:

Figure 84: Premature landing gear extension

This was then compared with a decelerated approach.


The results are summed in the following table:

Table 39: Fuel increment in case of premature extension

Aircraft Fuel increment(kg)

A319 + 15

A320 + 20

A321 + 25

A330 + 35

A340 + 35

A300-600 + 50

We notice that extending the landing gear too early increases fuel consumption by 15 to50 kg depending on aircraft type.

Annual fuel savings are summed up in the following table:

Table 40: Potential annual savings

Aircraft Annual fuelsavings peraircraft (kg)

Annual fuelsavings per

aircraft (gallon)

Annual savingsper aircraft ($)

A319 27 000 5 600 5 600

A320 34 000 7 000 7 000

A321 50 000 10 300 10 300

A330 42 000 8 700 8 700

A340 24 500 5 100 5 100

A300-600 65 000 13 400 13 400

Pilots are therefore asked to respect this in order to avoid an increase in drag.

Generally speaking, we advise flight crews to avoid premature extensions and to respectrecommended procedures in order to establish a safe situation.


4.7 Conclusion within the flight envelope

4.7.1 With regard to the climb phase:

• Flight crews should try to select a climb law as close as possible to the optimumclimb law. One must not be forget that this depends on the aircraft take-off weight.

• It must be remembered that it is of no use flying at crossover altitude: it increasesfuel consumption significantly for a very small time gain in time.

4.7.2 With regard to the step climb phase:

• Pilots should perform a step climb at around optimum altitude.

• Pilots should avoid delaying climb to the next step.

4.7.3 With regard to the cruise phase:

• In managed mode, it is not advantageous to fly at very low cost indices.

• Whenever possible, it is preferable to fly in managed mode in comparison to theselected mode.

• Pilots should not increase the Mach number chosen by the Flight ManagementSystem.

• At a given Mach number, the aircraft should be flown around the optimum altitude.Care must be taken if stage length is short.

• At a given flight level, the recommended Mach number is the Long Range CruiseMach number.

• One should remember that it is sometimes valuable to fly at a lower altitude thanat the optimum one if the wind trade at lower altitude is found to be beneficial(consult FCOM 3.05.15).

4.7.4 With regard to the descent phase:

• Diminishing descent speed can allow significant fuel savings.

• Premature descent should be avoided.


4.7.5 With regard to holding:

• The best combination is clean configuration at green dot speed.

• However, if green dot speed is higher than the maximum recommended speeds orif there are A.T.C. constraints, it is advised to hold in configuration 1 at S speed.

• If holding is to be anticipated, linear holding should be performed wheneverpossible.

4.7.6 With regard to the approach phase:

• Premature extensions (landing gear, slats, flaps) should be avoided.

• A decelerated approach saves fuel in comparison to a stabilized one, conditionsand safety permitting.


5. GENERAL CONCLUSION

Varying fuel price situations have prompted Airbus to innovate in the field of fuelconservation: whether in the field of design engineering or in flight operations support, wehave always maintained a competitive edge on this specific issue. Whether fuel is in shortor ample supply we have always considered that fuel conservation is a subject worthrevisiting. Fuel conservation permeates in the various areas of flight planning, flightoperations, performance retention and performance recovery. Airbus is both willing andable to support airlines with operational support in all those disciplines no matter howefficient modern jet aircraft have become.


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APPENDIX 1

ECONOMY MACH NUMBER ACCORDING TO COST INDEX AND FLIGHT LEVELS

In this appendix, one finds the same type of graphs as exemplified for other aircraftin 4.3.2.

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APPENDIX 1



APPENDIX 1


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APPENDIX 1



APPENDIX 2

OPTIMUM ALTITUDES

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APPENDIX 2

OPTIMUM ALTITUDES


APPENDIX 2

OPTIMUM ALTITUDES

0.86

0.85


APPENDIX 3

OPTIMUM ALTITUDES ON SHORT STAGES

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APPENDIX 3



APPENDIX 4

LONG RANGE CRUISE MACH NUMBER

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APPENDIX 4



APPENDIX 5

WIND ALTITUDE TRADE FOR CONSTANT SPECIFIC RANGE

Not available for A300-600/A310 models.For all the aircraft refer to FCOM 3.05.15 as a lot of variance exists between various

models of all types involved (A320 family, A330,A340)


APPENDIX 6

DESCENT


APPENDIX 7

HOLDING

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APPENDIX 7

HOLDING

Airbus 1998

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The statements made herein do not constitute an offer. They are based on theassumptions shown and are expressed in good faith. Where the supporting grounds forthese statements are not shown the Company will be pleased to explain the basisthereof.

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