Part 6. Altimetry
TOPICS
4
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
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 -
PRESSURE, HUMIDITY AND TEMPERATURE
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 the
gas and it’s weight for a given volume
must also increase.
PRESSURE, HUMIDITY AND TEMPERATURE
HUMIDITY
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.
PRESSURE, HUMIDITY AND TEMPERATURE
TEMPERATURE
The lower the temperature the greater the density
because the atoms take up less space as
temperature is reduced.
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.
London and Bath have different atmospheric conditions
although the surface pressures are the same at
1000 hPa.
700mb (hPa)
9 500ft
Cold
Air
10 000ft
700mb (hPa)
Warm
Air
1000mb (hPa)
London
Bath
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.
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/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 (104 987ft)
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.
MET02/16
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.
HEIGHT AND PRESSURE
9
0
1
8
9
2
0
8
2
1020
7
1020
3
6
5
4
1
7
3
6
5
4
1020hPa
1000hPa
A
B
HEIGHT AND PRESSURE
980 hPa level
990 hPa
600ft Indicated
980 hPa level
600ft
300ft True
SURFACE
1000hPa
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.
697hPA
697hPA
697hPA
COLDER THAN ISA
WARMER THAN ISA
10 000ft ISA
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 1013.2 hPa because their vertical
separation from each other is more important
than their separation from the ground.
ALTIMETER SETTINGS AND TERMINOLOGY
flight
level
height
altitude
QFE
1013.2
elevation
MSL
QNH
1013.2
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.”
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.”
HEIGHT
“The vertical distance of a level, point
or object considered as a point
measured from a specified
datum.”
ALTITUDE
“The vertical distance of a level, point
or object considered as a point
measured from mean sea
level.”
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.
FLIGHT LEVEL
“A level of constant atmospheric
pressure above a datum of 1013.2hPa
and separated from other levels by
specific pressure intervals.”
CHANGES OF REFERENCE
Flight Levels
Transition Level
Transition Layer
Transition Altitude
Transition
Altitude
Transition
Level
QFE
1013.2
MSL
QNH
1013.2
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.”
TRANSITION LEVEL
“The lowest flight level available for
use above the transition altitude.”
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.
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)
Transition Level
Transition Altitude
Transition Layer
1000ft
2000ft
Mean Sea Level
540ft
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 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)
Transition Level
Transition Altitude
Transition Layer
1000ft
3000ft
Mean Sea Level
150ft
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 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
A
B
5500ft
4700ft
1004hPa
Mean Sea Level
Z
Calculate distance Z
 1013 - 1004 = 9
Therefore Z = 270ft
1013hPa
9 x 30 = 270
Aircraft A is at 5500 - 270 = 5230ft above a pressure datum of 1004hPa
Therefore the vertical separation between the aircraft is 5230 - 4700 = 530ft
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)
A
B
5500ft
4700ft
1013hPa
1024hPa
Mean Sea Level
Z
 1024 - 1013 =11
11 x 30 = 330
Therefore Z = 330ft
Aircraft A is at 5500 + 330 = 5830ft above a pressure datum of 1024hPa
Calculate distance Z
Therefore the vertical separation between the aircraft is
5830 - 4700 = 1130ft
Questions…