Part 6. Altimetry

TOPICS

ISA and the Aircraft Altimeter

Height, Pressure, and the Aircraft Altimeter

Temperature and the Aircraft Altimeter

4

4

4

Altimeter Settings and Terminology4

Pressure, Humidity & Temperature4

PRESSURE, HUMIDITY AND TEMPERATURE

The study of pressure variation within the atmosphere is called

ALTIMETRY.

Pressure decreases with increasing height.

Not only does the pressure decrease at altitude, butthe density of the atmosphere does too.

However, there are a number of other factors thataffect density -

PRESSURE

The greater the pressure, the greater the density.This is because, as you increase the pressure of a gas,

the molecules are squashed together within thegas and it’s weight for a given volume

must also increase.

PRESSURE, HUMIDITY AND TEMPERATURE

HUMIDITY

Water vapour is less dense than dry air becausethe molecules are further apart.

However, it combines readily with dry air sothe higher the water vapour content of the air

the lower the overall density.

PRESSURE, HUMIDITY AND TEMPERATURE

TEMPERATURE

The lower the temperature the greater the densitybecause the atoms take up less space as

temperature is reduced.

PRESSURE, HUMIDITY AND TEMPERATURE

A column of cold air will weigh more than an identicalcolumn of warm air and the pressure at thebottom of the cold column will be higher.

Similarly, if the pressure at the bottom of the warmcolumn was the same as the cold column

they would have to weigh the same and the warm column

must be taller to achieve this.

That being the case, the pressures would alsobe the same at the top of each column.

PRESSURE, HUMIDITY AND TEMPERATURE

ColdAir

WarmAir

London Bath

700mb (hPa)

700mb (hPa)

1000mb (hPa)

9 50

0ft

10 0

00ft

London and Bath have different atmospheric conditionsalthough the surface pressures are the same at

1000 hPa.

This standard atmosphere, which has beeninternationally agreed, is a set of average values

which are utilised for thecalibration of aircraft altimeters,

the cockpit instrument that indicatesheight

by sampling thestatic (undisturbed air) pressure.

Thus in theory, all aircraft altimeters should reactin exactly the same manner to any change

in air conditions.

INTERNATIONAL STANDARD ATMOSPHERE

The ICAO defined values are -

Mean Sea Level Temperature - + 15°C

MSL pressure - 1013.25 hPa/mb (29.92 ins)

MSL density - 1225 gm cu m

Lapse rate - temp decreasing at 1.98°C/1000ftup to 11kms (36 090ft)

- remaining at -56.5°C thereafterup to 20kms (65 617ft)

- increasing at 0.3°C/1000ftthereafter up to 32kms (104 987ft)

INTERNATIONAL STANDARD ATMOSPHERE

The principle of the aircraft altimeter is exactlythe same as that of the aneroid barometer.

An evacuated capsule reacts to changes in airpressure and these changes are transmitted

to a pointer on a dial that is suitablycalibrated in feet or metres.

AIRCRAFT ALTIMETER

Altimeters are fitted with a digital subscale, that is set by a rotating knob to indicate the pressuredatum above which the altimeter is operating.

This is necessary because air pressure doesnot remain constant at any place and

varies from hour to hour.

HEIGHT AND PRESSURE

MET02/16

1

2

3

45

6

7

8

9

1020

0

1020hPa

1

2

3

45

6

7

8

9

1020

0

1000hPa

A B

HEIGHT AND PRESSURE

Sub scale setting level 1000 hPa

1000hPa990 hPa300ft True

600ft

980 hPa level

980 hPa level

600ft

Indi

cate

d

SURFACE

Flight path

HEIGHT AND PRESSURE

Cold air is denser than warm air. Consider threecolumns of air with identical pressures at MSL, if the

temperatures of the columns are different thenthe height at which the pressure has fallen to a

specified level will also be different.

TEMPERATURE VARIATION

COLDER THAN ISA10 000ft ISA

WARMER THAN ISA

697hPA

697hPA

697hPA

MET02/20

The altimeter subscale setting depends upon thephase of flight that the aircraft is undergoing.

When operating at, or near an airfield thesubscale setting may be set on either the pressure

at the official aerodrome elevation, which is known as QFE, or at MSL which is known as

QNH.

ALTIMETER SETTINGS AND TERMINOLOGY

When flying at higher levels all aircraft set the ISApressure of 1013.2 hPa because their verticalseparation from each other is more important

than their separation from the ground.

1013.2

1013.2

QNH

QFE

MSL

altitude

flightlevel

elevation

height

ALTIMETER SETTINGS AND TERMINOLOGY

QFE

“Atmospheric pressure at officialaerodrome level. When set on the

subscale of a pressure altimeter it willread zero when the aircraft is on

the ground at the station.”

QNH

“Atmospheric pressure at mean sea level.When set on the subscale of a pressure

altimeter it will read aerodromeelevation when the aircraft is

on the ground at thestation.”

HEIGHT

“The vertical distance of a level, pointor object considered as a point

measured from a specifieddatum.”

ALTITUDE

“The vertical distance of a level, pointor object considered as a point

measured from mean sealevel.”

ELEVATION“The vertical distance of a point or level,

on or affixed to the surface of theearth, measured from mean

sea level.”NOTE - Aerodrome elevation is the

elevation of the highest point on thelanding area. A separate threshold elevation is published if it is 7ft or

more BELOW aerodrome elevationand for precision approach runways.

FLIGHT LEVEL

“A level of constant atmosphericpressure above a datum of 1013.2hPa

and separated from other levels byspecific pressure intervals.”

CHANGES OF REFERENCE

Flight Levels

Transition Layer

TransitionAltitude

TransitionLevel

1013.2

1013.2

MSL

QFE

QNH

Transition Level

Transition Altitude

TRANSITION ALTITUDE

“The altitude at or below which thevertical position of an aircraft is controlled

by reference to altitudes. Thetransition altitude is located at a

fixed level and published inaeronautical information

publications.”

TRANSITION LEVEL

“The lowest flight level available foruse above the transition altitude.”

TRANSITION LAYER

“The airspace between the transitionaltitude and the transition level.”

NOTE - The actual depth of the transitionlayer varies as the pressure at MSLchanges and vertical separation of

1000ft does not always exist between the transition altitude and the

transition level.

EXAMPLE 1If the Transition Altitude is 2000ft, the QNH is 995 hPa

and the Transition Layer at least 1000ft in depth,what is the Transition Level ?

(Assume 1hPa = 30ft)

540ft

2000ft

1000ftTransition Layer

Transition Level

Transition Altitude

Mean Sea Level 995hPa

1013hPa

1000 + 2000 + 540 = 3540

Therefore the top of the TL is 3540ft above a pressure datum of 1013hPa

The next available flight level above 3540ft is FL40 - The TL is FL40

EXAMPLE 2If the Transition Altitude is 3000ft, the QNH is 1008 hPa

and the Transition Layer at least 1000ft in depth,what is the Transition Level ?

(Assume 1hPa = 30ft)

150ft

3000ft

1000ftTransition Layer

Transition Level

Transition Altitude

Mean Sea Level 1008hPa

1013hPa

1000 + 3000 + 150 = 4150

Therefore the top of the TL is 4150ft above a pressure datum of 1013hPa

The next available flight level above 4150ft is FL45 - The TL is FL45

EXAMPLE 1If aircraft A is flying at FL55 and aircraft B is operating at 4700ft

on the QNH of 1004hPa, what is their vertical separation ?(Assume 1hPa = 30ft)

Calculate distance Z 1013 - 1004 = 9 9 x 30 = 270Therefore Z = 270ft

Aircraft A is at 5500 - 270 = 5230ft above a pressure datum of 1004hPa

Therefore the vertical separation between the aircraft is 5230 - 4700 = 530ft

Z

4700ft

Mean Sea Level1004hPa

1013hPa

A

B

5500ft

MET03/20

EXAMPLE 2If aircraft A is flying at FL55 and aircraft B is operating at 4700ft

on the QNH of 1024hPa, what is their vertical separation ?(Assume 1hPa = 30ft)

Calculate distance Z 1024 - 1013 =11 11 x 30 = 330Therefore Z = 330ft

Aircraft A is at 5500 + 330 = 5830ft above a pressure datum of 1024hPa

Therefore the vertical separation between the aircraft is 5830 - 4700 = 1130ft

Z

4700ft

Mean Sea Level1024hPa

1013hPa

A

B

5500ft

Questions…