2. Part 6. Altimetry

3. TOPICS Pressure, Humidity & Temperature ISA and the Aircraft Altimeter4
Pressure, Humidity & Temperature 4
ISA and the Aircraft Altimeter 4
Height, Pressure, and the Aircraft Altimeter 4
Temperature and the Aircraft Altimeter 4
Altimeter Settings and Terminology

4. Pressure decreases with increasing height.
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, but
the density of the atmosphere does too.
However, there are a number of other factors that
affect density -

5. PRESSURE, HUMIDITY AND TEMPERATURE
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 the
gas and it’s weight for a given volume
must also increase.

6. PRESSURE, HUMIDITY AND TEMPERATURE
Water vapour is less dense than dry air because
the molecules are further apart.
However, it combines readily with dry air so
the higher the water vapour content of the air
the lower the overall density.

7. PRESSURE, HUMIDITY AND TEMPERATURE
The lower the temperature the greater the density
because the atoms take up less space as
temperature is reduced.

8. PRESSURE, HUMIDITY AND TEMPERATURE
A column of cold air will weigh more than an identical
column of warm air and the pressure at the
bottom of the cold column will be higher.
Similarly, if the pressure at the bottom of the warm
column 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 also
be the same at the top of each column.

9. London and Bath have different atmospheric conditions
although the surface pressures are the same at
1000 hPa.
Cold
Air
Warm
London
Bath
700mb (hPa)
1000mb (hPa)
9 500ft
10 000ft

10. INTERNATIONAL STANDARD ATMOSPHERE
This standard atmosphere, which has been
internationally agreed, is a set of average values
which are utilised for the
calibration of aircraft altimeters,
the cockpit instrument that indicates
height
by sampling the
static (undisturbed air) pressure.
Thus in theory, all aircraft altimeters should react
in exactly the same manner to any change
in air conditions.

11. INTERNATIONAL STANDARD ATMOSPHERE
The ICAO defined values are -
Mean Sea Level Temperature °C
MSL pressure hPa/mb (29.92 ins)
MSL density gm cu m
Lapse rate temp decreasing at 1.98°C/1000ft
up to 11kms (36 090ft)
- remaining at -56.5°C thereafter
up to 20kms (65 617ft)
- increasing at 0.3°C/1000ft
thereafter up to 32kms ( ft)

12. AIRCRAFT ALTIMETER The principle of the aircraft altimeter is exactly
the same as that of the aneroid barometer.
An evacuated capsule reacts to changes in air
pressure and these changes are transmitted
to a pointer on a dial that is suitably
calibrated in feet or metres.

13. Altimeters are fitted with a digital subscale, that is
HEIGHT AND PRESSURE
Altimeters are fitted with a digital subscale, that is
set by a rotating knob to indicate the pressure
datum above which the altimeter is operating.
This is necessary because air pressure does
not remain constant at any place and
varies from hour to hour.

14. A B HEIGHT AND PRESSURE 1020hPa 1000hPa 1 2 3 4 5 6 7 8 9 1 2 3 4 5 6
1020hPa 1 2 3 4 5 6 7 8 9
1020
1000hPa A B

15. HEIGHT AND PRESSURE 980 hPa level Flight path 600ft 300ft True 990 hPa
Sub scale setting level 1000 hPa
1000hPa
990 hPa
300ft True
600ft
980 hPa level
600ft Indicated
SURFACE
Flight path

16. TEMPERATURE VARIATION
MET02/20
TEMPERATURE VARIATION
Cold air is denser than warm air. Consider three
columns of air with identical pressures at MSL, if the
temperatures of the columns are different then
the height at which the pressure has fallen to a
specified level will also be different.
COLDER THAN ISA
10 000ft ISA
WARMER THAN ISA
697hPA

17. ALTIMETER SETTINGS AND TERMINOLOGY
The altimeter subscale setting depends upon the
phase of flight that the aircraft is undergoing.
When operating at, or near an airfield the
subscale 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.
When flying at higher levels all aircraft set the ISA
pressure of hPa because their vertical
separation from each other is more important
than their separation from the ground.

18. ALTIMETER SETTINGS AND TERMINOLOGY
flight
level
height
altitude
QFE
1013.2
elevation
QNH
MSL
1013.2

19. QFE “Atmospheric pressure at official aerodrome level. When set on the
subscale of a pressure altimeter it will
read zero when the aircraft is on
the ground at the station.”

20. QNH “Atmospheric pressure at mean sea level.
When set on the subscale of a pressure
altimeter it will read aerodrome
elevation when the aircraft is
on the ground at the
station.”

21. HEIGHT “The vertical distance of a level, point
or object considered as a point
measured from a specified
datum.”

22. ALTITUDE “The vertical distance of a level, point
or object considered as a point
measured from mean sea
level.”

23. ELEVATION “The vertical distance of a point or level,
on or affixed to the surface of the
earth, measured from mean
sea level.”
NOTE - Aerodrome elevation is the
elevation of the highest point on the
landing area. A separate threshold
elevation is published if it is 7ft or
more BELOW aerodrome elevation
and for precision approach runways.

24. FLIGHT LEVEL “A level of constant atmospheric
pressure above a datum of hPa
and separated from other levels by
specific pressure intervals.”

25. CHANGES OF REFERENCE Flight Levels Transition Level Transition Layer
Altitude
Level
1013.2
MSL
QFE
QNH
Transition Level
Transition Altitude

26. TRANSITION ALTITUDE “The altitude at or below which the
vertical position of an aircraft is controlled
by reference to altitudes. The
transition altitude is located at a
fixed level and published in
aeronautical information
publications.”

27. TRANSITION LEVEL “The lowest flight level available for
use above the transition altitude.”

28. TRANSITION LAYER “The airspace between the transition
altitude and the transition level.”
NOTE - The actual depth of the transition
layer varies as the pressure at MSL
changes and vertical separation of
1000ft does not always exist
between the transition altitude and the
transition level.

29. The next available flight level above 3540ft is FL40 - The TL is FL40
EXAMPLE 1
If 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
1000ft
Transition Layer
Transition Level
Transition Altitude
Mean Sea Level
995hPa
1013hPa
= 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

30. The next available flight level above 4150ft is FL45 - The TL is FL45
EXAMPLE 2
If 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
1000ft
Transition Layer
Transition Level
Transition Altitude
Mean Sea Level
1008hPa
1013hPa
= 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

31. If aircraft A is flying at FL55 and aircraft B is operating at 4700ft
EXAMPLE 1
If 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)
MET03/20 Z
4700ft
Mean Sea Level
1004hPa
1013hPa A B
5500ft
Calculate distance Z  = x 30 = 270
Therefore Z = 270ft
Aircraft A is at = 5230ft above a pressure datum of 1004hPa
Therefore the vertical separation between the aircraft is = 530ft

32. If aircraft A is flying at FL55 and aircraft B is operating at 4700ft
EXAMPLE 2
If 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) Z
4700ft
Mean Sea Level
1024hPa
1013hPa A B
5500ft
Calculate distance Z  = x 30 = 330
Therefore Z = 330ft
Aircraft A is at = 5830ft above a pressure datum of 1024hPa
Therefore the vertical separation between the aircraft is
= 1130ft

33. Questions…