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SYSTEMATIC AIR TRAFFIC MANAGEMENT IN A REGULAR LATTICE

Richard Irvine, Horst Hering,

EUROCONTROL Experimental Centre, Bretigny sur Orge, France

Abstract A regular lattice combines two ideas: a

repeating (or regular) airspace structure and layers of parallel tracks. A repeating or regular airspace structure has several advantages over an irregular structure. The skills or methods used to control traffic in one part of the structure are applicable throughout the structure. Regularity gives rise to multiple routes between two points with similar distances flown in each direction and at each flight level. Flow management can select routes which distribute traffic over a region. The same mechanism could be used to choose routes which avoid reserved areas. Since the possible routes between two points have similar properties, the selection of an alternative route has a small impact on flight time, contributing to the predictability of airline operations. Properties which apply to an element of the airspace apply wherever that element is repeated, so that reasoning about a small region of the airspace can immediately be scaled up to apply to a much larger region. Layers of parallel tracks eliminate crossing conflicts between aircraft which are flying straight and level. Together with measures to preserve the stability of traffic flows (sufficient spacing, speed regulation, or ASAS sequencing procedures), traffic may be separated into two easily identifiable populations: a "stable" population of cruising aircraft, which require low controller monitoring per aircraft, since there are no crossing conflicts between cruising aircraft, and a population of aircraft in "transition" to or from the stable state, which require greater monitoring. Some preliminary results from fast-time simulations are reported.

Introduction: goals, problems and opportunities

SESAR Goals The European Commission’s (EC) expectation

for Single European Sky ATM Research (SESAR) is that it will deliver a European Air Traffic Management (ATM) System for 2020 and beyond which can meet the following goals relative to today’s performance [SESAR1]:

• Safety Increase 10 times

• Capacity Increase 3 times

• ATM costs Divide by 2

• Environmental impact Reduce by 10%

Safety On July 1st 2002 a Tupolev TU154M

passenger aircraft and a Boeing 757-200 cargo aircraft collided near the German town of Uberlingen killing all passengers and crew. [AX001-1-2/02].

Figure 1 [taken from AX001-1-2/02 - A1&3] shows the paths of the aircraft, and their positions about 3 minutes before the accident. Both aircraft were at the same flight level.

Figure 1: Radar tracks of flights in the Uberlingen mid-air collision

If, at this time, an instruction had been given to either aircraft to modify its trajectory the situation would have been quite nominal. The accident illustrates a feature of the air traffic control system: aircraft with converging headings may cruise at the same flight level. There are many barriers to the occurrence of conflict and collision, and even should all the barriers fail collision is unlikely. One of the most important barriers is the vigilance of the air traffic controller. However, human vigilance cannot be infallible and the airborne collision avoidance system (ACAS/TCAS) cannot guarantee the prevention of collision. It would be desirable, wherever feasible, to reduce the

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dependence of the safety of the air traffic control system on human vigilance.

An opportunity – area navigation Navigation from departure to destination has

been one of the important challenges in the history of aviation. Simple radio navigation beacons allowed aircraft to fly from point to point along their routes, but have also constrained airspace design. The advent of area navigation, which allows aircraft to fly arbitrary routes, opens the door to new airspace design possibilities. Airspace design as an approach to improving the capacity of the air traffic management system is also attractive in view of the difficulties involved in conceiving, approving and deploying technology-based solutions.

SuperHighway The approach described is being investigated

as the second operational concept scenario of the European Commission SuperHighway [SHP] project.

Background [LPT] proposed the use of layers of parallel

tracks. Layers are vertically separated from one another. Within a layer aircraft are only allowed to fly along parallel tracks which are laterally separated, either in one direction or the opposite direction. The directions of the tracks change from one layer to another. This scheme eliminates crossing conflicts between aircraft flying straight and level in layers.

Layer 1

Layer 2

Layer 3

Layer 4

Figure 2: Layers of parallel tracks

To navigate from one point to another in such a system, it is necessary to approximate the direct route between the points by first flying in one direction in one layer and then follow a second direction in an adjacent layer.

Figure 3: Following two allowed directions to reach a destination

The greater the number of allowed directions the closer the approximation which is possible and hence the smaller the extra distance flown compared with the direct route. At first sight it seems that such a scheme must entail unacceptably large extra distances. However, it was shown that the extra distance flown in a flat world with just four pairs of allowed directions was on average 5.5% compared with the direct route. According to [PRR2005] the TMA to TMA (en-route) horizontal inefficiency of the current system is about 4%.

The greater the number of pairs of directions, the further away from its preferred or optimum flight level an aircraft will, on average, have to cruise.

In [LPT] some problems were recognised: aircraft must be able to climb to the layer which is appropriate to their initial direction of flight and descend from their final cruise direction. To enable this, and also to facilitate changes of flight level and direction, intermediate levels were introduced. However, as a consequence of the intermediate levels, aircraft would, on average, cruise 3000 feet away from their preferred cruise levels.

The principle of layers of parallel tracks as presented in [LPT] does not, by itself, require a regular route network and consequently track spacing did not depend on the orientation of tracks.

[Freeway] proposed a dual airspace concept consisting of district airspaces, similar to conventional airspace, for short haul and TMA traffic and a Freeway airspace for cruising traffic.

Figure 4: Air traffic freeways

Freeways are independent, isolated airspaces at high altitude with special rules. The proposed freeways follow the main European traffic flows

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and end at intercontinental connection areas. Freeways consist of parallel lanes, which have no crossings with other traffic. The 'Red' and 'Blue' freeways (see figure 4) use different flight levels. In defined areas, a Freeway has intersections with the district traffic. Intersection areas are reserved for joining and leaving the freeway. All intersections have the same layout.

Freeways would be operated by a single control authority. Their capacity c be increased by adding lanes. The concept favours catchment areas along the Freeways.

Lattice Design Basics The airspace structure presented here is

destined for dense, upper airspace regions in which aircraft are flying "from everywhere to everywhere". The essential problem here is one of crossing trajectories. The idea of layers of parallel tracks is retained. In contrast to [LPT] intermediate levels are removed so that actual cruise levels can be closer to preferred cruise levels. The design of the lower airspace structure in which aircraft climb from departure airports to the lattice, and descend from the lattice to destination airports is not addressed here.

Number of pairs of directions The number of pairs of directions is a basic

parameter in the design of the lattice. An important consideration in the choice of the number of pairs of directions is fuel efficiency, for reasons of cost and environmental impact. Fuel efficiency can be broken into horizontal and vertical efficiency.

There will be a loss of horizontal efficiency relative to an airline's preferred trajectory if an aircraft must fly a greater distance. Average extra distance flown as a function of the number of pairs of directions was calculated in [LPT] and is repeated in table 2.

There will be a loss of vertical efficiency if an aircraft is constrained to cruise at a level below its preferred cruising level. Typical fuel efficiency as a function of flight level is shown in table 1.

% cruise fuel efficiency relative to FL330 Flight level Airbus A320

(nominal mass – 62,000 kg)

Boeing 777 - 200 (nominal mass –

208,700 kg)

260 -12% -14% 280 -10% -10% 290 -8% -8% 310 -5% -5% 330 0% 0% 350 +5% +5% 360 +7% +7%

Table 1: Fuel efficiency against flight level

Although the two aircraft types considered in table 1 are very different in size, it can be seen that the variation of fuel efficiency with flight level is very similar. There is a variation of 17% between FL280 and FL360 which is equivalent to a 2.1% loss of efficiency for every 1000 feet below the preferred level.

If there are n pairs of directions, each separated by 1000 feet, then, on average, an aircraft will have to cruise ( )[ ] 100021 ×−n feet below its preferred level, which will result in a loss of cruise efficiency of about ( )[ ] %1.221 ×−n .

The total loss of fuel efficiency within layers of parallel tracks, relative to an airline preferred trajectory, is the sum of the extra distance flown and the loss of efficiency due to cruising at a sub-optimal level. See table 2 and figure 5. This loss of efficiency applies to that part of the flight which is within the layers of parallel tracks. Number of allowed directions

Smallest number of parallel track layers needed

Average percentage extra distance flown (%)

Loss of cruise efficiency (%)

Total loss of fuel efficiency (%)

4 2 27 1.05 28.05 6 3 10 2.1 12.1 8 4 5 3.15 8.15 10 5 3 4.2 7.2 12 6 2 5.25 7.25

Table 2: Loss of fuel efficiency in layers

Total loss of fuel efficiency against number of allowed directions

0

5

10

15

20

25

30

0 5 10 15

Number of allowed directions

Los

of fu

el e

ffici

ency

Figure 5: Loss of fuel efficiency in layers of

parallel tracks

The loss of fuel efficiency is a minimum for 10 directions (5 pairs of directions) but the minimum is in fact a very flat minimum. Schemes with 8 or 12 directions have very similar fuel efficiency. The lattices presented in this paper have 8 directions, which has the slight advantage that the directions can be chosen to correspond to traditional compass directions N, NE, E, SE, S, SW, W and NW.

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The fast-time simulation results reported later include figures for total fuel consumption of various airspace schemes.

Vertical structure

FL 300

FL 310

FL 320

FL 330

FL 340

FL 350

FL 360

FL 370

FL 380

N

E

S

W

Figure 6: Repeated stack

The stack of four layers may be repeated, so that for each allowed direction more than one flight level is possible.

As shown in the above scheme, the direction of the tracks increases by 45 degrees in a clockwise direction on climbing from one layer to the layer above. When a cruising aircraft turns 45° to the right [clockwise] it has to climb 1000 feet to reach a level consistent with its new direction. Similarly, if it turns 45° to the left [anti-clockwise] it has to descend 1000 feet.

Horizontal structure If the spacing between diagonal tracks is

chosen appropriately (relative to the spacing between horizontal and vertical tracks) a regular lattice can be constructed:

Figure 7: Possible horizontal structure

(Alignment 1)

In a regular lattice the layout of junctions is identical throughout the network. A repeating or regular airspace structure has several advantages over an irregular structure: operating procedures are the same throughout the structure, so that the skills required to control traffic in one part of the network are applicable throughout the network; regularity gives rise to many routes between two nodes with

similar lengths. Flow management can select routes which distribute traffic evenly over a region. The same mechanism could be used to choose routes which avoid reserved areas. Furthermore, properties (e.g. safety) which apply to a an element of the airspace may apply wherever that element is repeated, so that reasoning about a small region of the airspace can be scaled up to apply to a much larger region. Different lattice designs are possible. The challenge is to find a lattice design and corresponding operating procedures which are safe and feasible. A regular route structure suggests a corresponding, regular sectorisation. It is possible that because of its simple geometrical properties, both horizontally and vertically, a lattice can be divided into a greater number of control volumes than conventional sectors, so dividing the workload associated with increased traffic amongst a greater number of controllers.

Grids The lattice can be considered to be made up of

two grids (of perpendicular lines) - the blue-red grid and the brown-green grid.

Blue-red grid

Brown-green grid

Figure 8: Grids

Orientation As shown the spacing of tracks in the blue-red

grid is greater than in the brown-green grid. The lattice could also be turned through 45 degrees. However, the orientation shown is the preferred one as it gives a greater density of routes in the North-West to South-East direction. It is thought that this direction corresponds better to the major traffic flows in Europe.

Alignment Depending upon how the vertices in the grids

are aligned, distinct variations of the lattice are possible. One alignment, termed "alignment 1", is shown in figure 7. In this alignment the shortest uninterrupted distance along the lattice corresponds to the edge of a brown-green cell. The longest uninterrupted distance along the lattice corresponds to the edge of a blue-red cell. There are places

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where vertices of both grids coincide. At these places four tracks intersect.

lattice spacing Figure 9: Alignment 2

A second alignment is shown in figure 9. Half of the cells of the brown-green grid lie within cells of the blue-red grid, and the other half lie across four cells of the blue-red grid. The cells of the brown-green grid which are not crossed by the blue-red grid effectively constitute “holes” or “shafts”. If these holes or shafts are large enough, they could be used as places where slow climbing aircraft can climb into the lattice without interfering with traffic which is on the lattice.

The shortest uninterrupted distance along the lattice corresponds to half of the edge of a blue-red cell. The longest uninterrupted distance along the lattice corresponds to the edge of a brown-green cell. The longest uninterrupted distance (not along the lattice) corresponds to a diagonal of a brown-green cell, that is, a “hole”.

The vertices of the grids do not coincide. The vertices of the brown-green grid lie on edges of cells in the blue-red grid. As a consequence, at most three tracks intersect at any one point.

Route cross-sections For a given basic lattice structure, different

horizontal route cross-sections or lane layouts can be considered. For example, a route might have one or more lanes; it might be mono-directional or bi-directional; it might have joining and leaving lanes. Some examples are shown in figure 10.

Basic lattice element

Mono-directional route

Bi-directional route

Bi-directional route with joining lanes

Mono-directional route withjoining and leaving lanes

Figure 10: Horizontal route cross-sections

By replacing lines in a basic lattice with a route cross-section more elaborate lattices can be generated. For example, the following diagram shows an alignment 2 lattice of bi-directional routes:

Figure 11: Bi-directional routes

This paper will describe mono-directional routes primarily because procedures for turning (left or right) are symmetrical and more straightforward in this case (see later).

Turning places With alignment 1, four tracks meet at each

junction. With alignment 2, three tracks meet at each junction. Depending upon the directions of the tracks, none, two or four turns are possible at junctions. On average two turns are possible at each junction. There are twice as many junctions per unit area with alignment 2 compared with alignment 1, although the average number of turns possible at each junction is half that of alignment 1.

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Figure 12: Alignment 2, turning pattern A

When considering an alignment 2 lattice, the choice of directions of the tracks can give rise to two basic patterns of turning places, termed pattern A and pattern B. In the example shown below (figure 13, pattern B), the directions of the brown tracks are reversed compared with that shown above (figure 12, pattern A).

M M

M M

M M

M M

A A

A A

C C

C C

Figure 13: Alignment 2, turning pattern B

In pattern B two turns are possible at all junctions. This results in simple junctions and a uniform airspace. At each junction only traffic from one other track joins a given track, and furthermore traffic may leave the track before other traffic joins. The turns are in a clockwise direction around a quarter of the holes (marked ‘C’ in the diagram above), in an anti-clockwise direction around another quarter of the holes (marked ‘A’) and in mixed directions around the remaining half of the holes (marked ‘M’).

Aircraft Performance Some familiarisation with aircraft

performance, specifically the rate of climb of heavy aircraft and the turn radius of fast aircraft, is helpful before further considering lattice dimensions and operation.

Rate of climb Figure 14 shows the rate of climb of a B747 as

a function of flight level, for three different loadings.

0

1000

2000

3000

4000

5000

6000

0 200 400 600

Flight level

Feet

per

min

ute

LightNominalHeavy

Figure 14: Rate of climb of B747

Note that for a heavily loaded B747 the rate of climb is about 1000 feet per minute at FL290 and it decreases from there to the aircraft's ceiling.

Turn radius The radius of turn r of an aircraft is given by

r = φtan

2

gv

where v is the speed of the aircraft

g is the acceleration due to gravity

φ is the bank angle

For example, an aircraft cruising at 480 knots and banking at 15° has a radius of turn of 12.5 nautical miles.

Lattice Design and Operation Consideration of how a lattice might be

operated leads to further constraints on the design of a lattice. Basic operations which must be accommodated are:

• Joining the lattice

• Turning and merging

Joining the lattice Aircraft, including slow climbing aircraft,

should be able to climb to appropriate cruise levels without encountering traffic on crossing tracks. Crossing tracks are not an obstacle for aircraft joining the lowest layer of the lattice. Access to the lowest layer is straightforward for all aircraft types.

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The lattice spacing can be defined to be the distance between adjacent crossings in the larger of the two grids (see figure 9). If the lattice spacing is reduced the number of tracks (per unit distance) increases, and so, therefore, does the physical capacity of the lattice. On the other hand, slow climbing aircraft require a large lattice spacing if they are to be able to join the lattice in a single climb without encountering crossing traffic. Lattice spacing also has some impact on distance flown (see later).

With both alignments the greatest uninterrupted distances (other than in the lowest layer) are equal to the lattice spacing. In an alignment 1 lattice, the greatest uninterrupted distances correspond to the edges of the blue-red cell. In an alignment 2 lattice the longest uninterrupted distances correspond to diagonals of the brown-green cells.

Where should an aircraft join a lattice? It is assumed that when an aircraft reaches the lattice flight levels it will be heading towards its destination. Suppose that lattice layers begin at FL300. An aircraft must climb from its departure airport to reach the lattice. To climb to FL300 takes 10 to 25 minutes, depending upon the rate of climb of the aircraft. The distance flown during this time is of the order of 60 to 150 nautical miles. Placing circles with these radii around the departure airport allows likely joining places to be identified. Similarly the places at which aircraft would leave the lattice can be identified:

Figure 15: Joining and leaving places

The places at which aircraft can join and leave the lattice are evenly distributed in relation to the airport. Furthermore, each airport will, in general, have a set of places at which traffic joins and leaves the lattice which is independent of other airports.

There are various ways in which an aircraft can climb into a lattice, depending upon its rate of climb. An aircraft can climb directly to a segment. Alternatively it can climb in steps first to one segment (e.g. green) and then turn and climb to

another (e.g. blue). Having joined the lattice an aircraft may also climb to a level in the same direction but 4000 feet higher.

Figure 16: Spiral ramps for slow climbers

With an alignment 2 lattice there is the additional possibility that the “holes” be used by slow climbing aircraft to turn and climb to the appropriate level. In this case the lattice resembles a multi-storey car park in which slow climbers reach different levels via spiral ramps. A fast aircraft (480 knots) requires a turning circle with a diameter of at least 25 nautical miles (see earlier). A turning circle with this diameter has a circumference of 79 miles. At 480 knots this corresponds to 10 minutes flying time. To climb the 4000 feet from beneath the lattice to the uppermost level of the lower stack of four layers in 10 minutes a slow climber would require an average rate of climb of 400 feet per minute. The turning circle should be laterally separated from the tracks which enclose it. This suggests that the inner square enclosing each hole have a side of about 50 nautical miles, which leads to a lattice spacing of 7050*2 = nautical miles.

A lattice spacing of this size would also allow aircraft on diagonal tracks, with a rate of climb of 1000 feet or more, to climb the 4000 feet between levels without encountering crossing tracks. Aircraft on North-South or East-West directions could also be climbed in two steps (1000 feet and 3000 feet) without meeting crossing traffic.

With turn pattern B, the clockwise holes are natural places for slow climbers to turn and climb (assuming that the directions move clockwise when moving upwards from one layer to another) since aircraft turning in such holes will not approach traffic head-on, i.e. the closing speed is low. However, note that for an aircraft turning clockwise and climbing in a clockwise hole only three of the four pairs of directions are easily accessible. For example, in the clockwise holes shown above, it is not possible to turn clockwise and join the brown tracks. However, the bottom layer is accessible to

C C

C C

A A

A A

M M

M M

M M

M M

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all aircraft types without turning and we can choose which colour of track is the bottom layer. The above diagram is drawn with the intention that the brown layer be the bottom layer. The brown layer is chosen as the bottom layer because it is thought that the North-West – South-East direction is probably the predominant axis in Europe.

Turning and merging Consider traffic turning from brown and green

tracks at the following junction in an alignment 2 lattice of mono-directional tracks with turn pattern A:

Vertically separated joining lanes

Vectoring

Figure 17: Turning from brown and green. Horizontal and vertical views.

If, at a junction, an aircraft turns from one direction to another, without changing flight level, then it will be vertically separated from traffic cruising at the correct flight level in the new direction, at least until the next junction is reached. For example, an aircraft initially flying to the North-East on a green track, which turns to the East without changing flight level, will find itself beneath traffic cruising Eastwards on a blue track.

Aircraft turning from the brown and green tracks can be considered to turn onto joining lanes which are vertically separated from the blue track. The task of the controller is to merge aircraft from the brown and green joining lanes onto the blue track. This can be done by vectoring before moving the aircraft vertically into gaps in the traffic on the blue track. A basic assumption is that gaps will be available. The likely availability of gaps in the traffic would be a goal of flow management. In the worst case, an extreme form of vectoring – a go-around – could be used to delay an aircraft until a suitable gap becomes available.

With turn pattern B junctions traffic joins a track from at most one other direction.

Vertical movement and merging It is desirable that climbing and descending

traffic be separated from one another and from cruising traffic. One way of doing this is to allow for climbing and descending regions offset from each track by at least the required horizontal separation. Continuing the preceding example, figure 18 shows horizontal and vertical views of the climbing and descending regions.

When a climbing or descending aircraft reaches the level at which it wishes to cruise it levels off into what is effectively a laterally offset joining lane. As before, the task of the controller is then to merge the aircraft into the blue track, by vectoring it into a suitable gap. The same approach could be used to climb aircraft into the lattice. climbing

descending

climbingdescending

Horizontalview

Vertical view

Figure 18: Climbing and descending regions

Impact of lattice spacing on fuel efficiency As discussed earlier, trajectories through a lattice have lower fuel efficiency than fuel optimal trajectories, since approximating an ideal direction by two allowed directions creates extra distance flown and because aircraft may not always cruise at their fuel-optimal flight levels. There is also a small contribution to distance flown when flying to and from tracks in the lattice. On joining the lattice an aircraft must fly to a track in the appropriate direction. Consider an aircraft flying Northwards:

D

l

θ

Figure 19: Average distance to join lattice

Suppose that ideally the aircraft would join the lattice at the top of climb distance from the departure airport. Let this distance be D . Suppose that to join a track in the appropriate direction the aircraft must fly at an angle θ to that direction.

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The extra distance flown to join the lattice ( )θε is given by

( )θε = DD−

θcos ≈

2

2θD for small θ

The average extra distance flown to reach a joining or leaving point is approximately

ε = ∫max

0

2

max 21

θ

θθθ

dD=

6

2maxθD

where Dl=maxtanθ , l is the lattice spacing.

Taking l = 70 nautical miles, D = 100 nautical miles, then maxθ = 35° = 0.61 radians, then

the worst case extra distance to join is ( ) =maxθε 22 nautical miles and the average is ε = 6.2 nautical miles. Since an aircraft must both join and leave the lattice, the worst case extra distance flown to join and leave is ( ) =max2 θε 44 nautical miles, and the average extra distance flown to join and leave is approximately 2ε = 12.4 nautical miles. Taking into account aircraft which arrive and depart in diagonal directions, where the spacing between

tracks is 2l , the overall average extra distance would be about 10 nautical miles.

Mapping a lattice onto a region of a round world

A lattice can be built up around a set of grid points, each defined by a latitude and longitude.

One approach is to define one set of parallel tracks along lines of longitude and another along lines of latitude. The cells so defined should be big enough to allow the possibility of enclosing a climbing circle for slow climbers (see earlier). In particular, the longitude difference between adjacent North-South tracks should be sufficient, at the Northernmost end of the lattice, to enclose a turning circle. As one moves Southwards the distance between adjacent lines of longitude increases. If the distance between adjacent East-West tracks increases in the same way, then cells will remain square, and the allowed directions of flight will be the same across the lattice.

Figure 20 shows such a lattice in a Mercator projection. In this projection, the cells appear to be the same size. On the Earth the cells get bigger as one moves Southwards. On the other hand the directions of the tracks are correctly represented.

Figure 20: Regular lattice in a Mercator projection

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The Mercator projection is a conformal projection, i.e. angles or (small) shapes on the Earth are preserved in the map. This suggests a more general technique for choosing the latitude and longitude of grid points: construct a lattice with the required angular properties on a map drawn with a conformal projection. Then use the inverse of the projection to find the corresponding latitudes and longitudes.

Multiple routes The principle of layers of parallel tracks

requires a flight to approximate a straight line between two points by two flight legs in (adjacent) directions. In general there is a parallelogram of possible routes from one point to another. On a flat plane these routes would have identical lengths. Furthermore, they would have identical distances flown in each direction. Because of the link between flight level and direction all routes would have the same distances flown at each flight level. On a sphere these properties will not be identical, but they can be expected to be similar.

A

B

Figure 21: Alternative routes around a reserved area

The existence of multiple routes with similar properties has various advantages:

• Flow management can select routes which contribute to a desired traffic distribution over a region.

• Similarly, routes can be selected which are not affected by active reserved areas.

• Because of the similar distances flown in each direction and at each flight level the flight times for all of the routes in the parallelogram will be similar. This would contribute to the predictability of airline operations. Also the effect of the wind can be expected to be similar

for all the routes in the parallelogram, since wind speed and direction are a function of flight level, and since their impact depends upon the direction of flight.

In the case that a route through the lattice which corresponds to a single direction is blocked by a reserved area, additional distance can be added to yield alternative routes.

Another approach would be to deform the lattice around the reserved area, but this would require a redefinition of routes as opposed to re-routing through routes with a fixed definition.

Ideal encounters for medium-term conflict detection

Although layers of parallel tracks eliminate conflicts between cruising aircraft there remains the possibility of catch-up conflicts between aircraft on the same track. Two aircraft flying in the same direction along straight tracks at the same flight level, experiencing the same wind variations is exactly the situation in which conflict detection can be expected to perform with high reliability and long look-ahead times. In these circumstances medium-term conflict detection can be expected to be a very effective support to controller vigilance.

Some preliminary fast-time simulation results

A prototype fast-time simulator has been adapted, and some preliminary simulations performed, to allow comparison of the lattice to other airspace schemes.

A volume of interest was defined corresponding to a lattice which approximately covers the European "core area", see figure 20. The floor of the volume of interest was set at 30 000 feet.

To construct a traffic sample, a preliminary simulation was conducted using the SESAR 2005 (regulated, noisy) traffic sample. Flights were direct. It was assumed that all aircraft had nominal mass, and that they wished to climb to the highest flight level consistent with the length of flight and the ceiling of the aircraft. No flight level allocation scheme was applied. Flights which transited the volume of interest during the 24 hour period of the 19th July were added to a traffic subset. There were 12 717 such flights.

A series of simulations was run using this traffic subset, in which the routing and cruise level allocation schemes within the volume of interest were varied. Outside the volume of interest aircraft flew directly and there were no constraints on cruise level, so that changes in total fuel

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consumption were attributable only to changes of routing and cruise level schemes within the volume of interest. Conflict detection was only preformed within the volume of interest. The various schemes applied in the volume of interest are summarised in table 3.

Figure 22 shows a snapshot from a lattice simulation during a busy period. Aircraft in level flight are shown in colours which correspond to flight levels. If an aircraft is flying along a track at one of the allowed levels for that track, then the colour of the aircraft will be the same as that of the

track. Climbing aircraft are shown in purple and descending aircraft in black. The circle around each aircraft has a radius of 5 nautical miles. The rectangles show conflicts which begin when aircraft join tracks (there being no merging in the simulation) and which persist for long periods because of the small speed differences between aircraft flying in the same direction at the same flight level. If the merging problem can be solved, then stable, conflict-free flows will result, particularly if speed regulation can be applied to aircraft on the same track at the same level.

Figure 22: Snapshot from simulation showing long duration (merging) conflicts

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# Horizontal

navigation Cruise level allocation scheme

Total number of flights entering volume of interest

Total number of conflicts

Conflicts in which one or both aircraft are moving vertically

Conflicts in which both aircraft are level

Total fuel consumption / kilotonnes

Total Same direction

Crossing

Total Same direction

Crossing

1 Fly direct Unconstrained 12710 5266 2687 119 2568 2579 18 2561 112.055 2 Fly direct Semi-circular 12641 3469 2627 111 2516 842 17 825 112.063 3 Flight plan

routes Semi-circular 12512 4506 3441 967 2474 1065 93 972 114.4

4 Lattice, spacing >= 70 NM

Lattice 11855 3631 3201 2006 1195 430 430 0 113.5

5 Lattice, spacing >= 35 NM

Lattice 11996 2514 2274 1042 1232 240 240 0 113.1

Table 3: Summary of preliminary fast-time simulation results

Safety No attempt has been made to perform a

safety analysis for a lattice. However, one of the objectives of the lattice structure is to eliminate crossing conflicts between cruising aircraft. As can be seen from table 3, the lattice simulations (simulations 4 and 5) confirm this property.

Fuel consumption Fuel consumption is an important indicator

from both economic and environmental perspectives.

Simulation 1 (direct routes, cruise level unconstrained) gives a figure (112.055 kilotonnes) for total fuel consumption in the absence of horizontal and vertical constraints. Not surprisingly, this is the scheme with the greatest number of conflicts.

Simulation 2 (direct routes, semi-circular flight level allocation) has marginally increased total fuel consumption. This scheme may be considered an approximation to conventional routing and flight level allocation in the absence of reserved areas and will serve as a baseline for comparison with the lattice simulations.

Simulation 3 (flight plan routes, semi-circular flight level allocation) fulfils the requirement to avoid reserved areas. The total fuel consumption is 114.4 kilotonnes, which is about 2% greater than when following direct routes (together with the semi-circular rule, simulation 2).

Simulation 4 (lattice navigation, with a longitudinal lattice spacing of 70 nautical miles, without any consideration of reserved areas) has a

total fuel consumption of 113.5 kilotonnes, which is about 1.3% greater than in simulation 2 (direct routes, semi-circular flight level allocation). Short haul flights suffer greater horizontal inefficiencies than long haul flights, but they are also better candidates for level capping beneath the lattice.

ATM efficiency In the lattice simulation with a lattice spacing

of 70 nautical miles (simulation 4), the total number of conflicts is 3631. Comparing this with simulation 2, the approximation to conventional routing and flight level allocation in the absence of reserved areas, the number of conflicts (in the lattice simulation) is about 5% greater.

0

1000

2000

3000

4000

5000

6000

Dire

ctun

cons

train

ed

Flig

ht p

lans

,se

mi-c

ircul

ar

Dire

ctse

mic

ircul

ar

Latti

ce 7

0

Latti

ce 3

5

Navigation schemes

Con

flict

s

Total conflictsLevel level crossing conflicts

Figure 23 Conflict counts for different schemes

Looking more closely into the breakdown by conflict type, there are 2006 conflicts (about 55%) in which one or both aircraft are moving vertically (when the conflict is first detected), but which are moving in the same direction horizontally. These

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conflicts are due to the mixing of climbing, descending and cruising aircraft flying along the same horizontal paths. A possible solution to this problem, which will be the subject of a future simulation, would be an improved lattice design with the addition of joining and leaving lanes, as discussed earlier, in order to separate climbing, descending and cruising aircraft. In simulation 4 there were 950 short-lived (i.e. not merging) conflicts between aircraft with differing attitudes flying in the same direction. Other things being equal, the addition of joining and leaving lanes could potentially reduce the total number of conflicts by this amount, giving a total of 2681, which would represent a reduction of about 22% compared with direct routes and the semi-circular rule (simulation 2).

Capacity Total workload has not yet been estimated. In

considering the possible capacity benefits of a lattice, what is important is not simply the total workload needed, but also how that total workload will be shared between sectors. Sectorisation has not yet been addressed. The objective will be to find a decomposition into control volumes which would be both operationally feasible and which, at current levels of workload per sector, would allow a higher traffic level.

Conclusions and future work A regular lattice could have several

advantages compared with a conventional, irregular airspace structure:

• A regular structure allows airspace design solutions to be replicated throughout a region.

• Operating procedures or methods would be common throughout a region.

• A regular lattice can eliminate, by design, crossing conflicts between cruising aircraft.

• Together with measures to preserve the stability of traffic flows (sufficient spacing, speed regulation, or ASAS sequencing procedures), traffic may be separated into two easily identifiable populations: a "stable" population of cruising aircraft, which require low controller monitoring per aircraft, since there are no crossing conflicts between cruising aircraft, and a population of aircraft in "transition" to or from the stable state, which require greater monitoring.

• The existence of equivalent routes could facilitate traffic distribution and rerouting around reserved areas, with small impact on the predictability of airline operations.

Preliminary fast-time simulation of a lattice with a minimum longitudinal spacing of 70 nautical miles using SESAR 2005 traffic confirms the elimination of crossing conflicts between cruising aircraft. Compared with a direct routing baseline, total fuel consumption increased by 1.3%. There was a 5% increase in the total number of conflicts. However, it is thought that a decrease of about 22% could be achieved through an improved design in which joining and leaving lanes separate same direction climbing, cruising and descending aircraft. This will be a subject of future simulations.

Sectorisation will be a key issue for future work, not least in order to investigate whether higher traffic levels can be accommodated with current levels of workload per sector (in the busiest sectors).

References [SESAR1] SESAR Definition Phase Deliverable 1 “Air Transport Framework - The Current Situation”, DLM-0602-001-03-00, July 2006.

[AX001-1-2/02] Investigation Report AX001-2/02, German Federal Bureau of Aircraft Accidents Investigation, May 2004

[AX001-1-2/02 - A1&3] Appendices 1 & 3

[LPT] R. Irvine, C. Shaw, "Layers of Parallel Tracks: A Speculative Approach to the Prevention of Crossing Conflicts Between Cruising Aircraft", EUROCONTROL Experimental Centre Technical Note 2004-09, September 2004.

[Freeway] H. Hering, "Air Traffic Freeway System for Europe", EUROCONTROL Experimental Centre Technical Note 2005-20, November 2005.

[PRR2005] EUROCONTROL Performance Review Report 2005, published April 2006.

[SHP] European Commission SuperHighway project web-site http://www.sh.isdefe.es/