1. Section 03 - Altimetry Lesson 08
The Altimeter
Altimeter Errors
Altimeter Definitions
Altimeter Settings

2. The Altimeter Close connection between pressure and height
It is primarily an instrument merely for measuring pressure.
Similar to an aneroid barometer

3. The Altimeter
The relationship between pressure and height is constantly changing and depends on the surface pressure and the mean temperature of the air column up to the height concerned.
The relationship between pressure and height is governed by the pressure-height equation.
The height indicated on an altimeter is directly dependent on the surface pressure and the mean temperature of the air column from the surface up to the height of the altimeter.

4. Pressure - Height Equation
Note: It is not necessary to memorize this equation for the exams
Calculation of Height/Thickness between two pressure levels
H2 - H1 = T (logP1 - logP2)
‘T’ in the formula is the MEAN temperature of the layer.
Thus the thickness of a layer is directly dependent on this mean temperature ‘T’.
The true altitude of an aircraft will depend on the mean temperature of the air layer below the aircraft.

5. Altimeter Errors Altimeters are calibrated according to the ISA.
Altimeters will only indicate correctly when:
the air column corresponds to ISA
ELR 1.98°C/1000 ft.
The surface pressure is standard;
mb.

6. Altimeter Errors, cont’d
This results in two main errors:
Barometric Error
When the pressure at sea level varies from ISA causing variations aloft.
Temperature Error
When the air column is colder or warmer than ISA.

7. Density Altitude Air behaves according to the gas laws.
If the air is warmed the density will decrease.
If it is cooled the density will increase.
This will affect what is called the “Density Altitude.”

8. Density Altitude
Density Altitude is that height in the ISA which will correspond to the prevailing (ambient) density.
The rule of thumb to calculate this is:
A change of 1°C will result in 120 ft change in density altitude.
If air column is ISA the Density Altitude =ISA Altitude.

9. Variation of Density Altitude with Temperature

10. Influence of Air Temperature on Altimeter Indications
Height indicated on an altimeter is:
directly dependent on the height of the air column above the altimeter.

11. Influence of Air Temperature on Altimeter Indications, cont’d
If the temperature of the air column increases there will be a greater height of air above the altimeter and it will sense a greater pressure and register a lower altitude. (It will under read.)
If the temperature of the air column is decreased there will be less air above the altimeter and this will be sensed as a decrease in pressure and the altimeter will indicate a greater altitude. ( It will over read.)

12. Effect of temperature and pressure variation
If the surface pressure or mean temperature of the air layer below the aircraft is decreasing then true altitude will decrease and the altimeter will over-read on arrival at destination.
If the surface pressure or mean temperature of the air layer below the aircraft is increasing then true altitude will increase and the altimeter will under-read on arrival at destination.

13. Temperature Error In most cases density is unknown
Temperature is easily measured so it is better to speak of “Temperature Error,” rather than density error.
Rule of thumb: 1% of height for 2.5°C (4% per 10°C), or
for every 1°C of ISA deviation the altimeter is in error by 4ft per 1000ft of altitude

14. Temperature error calculation
10°C deviation = 4%
4% of 3000ft = 4x30=120ft.

15. Typical Altimeters

16. Altimeter Definitions (1)
with mb set on its subscale then the Pressure altitude: Is the height of a given level in the ISA above the mb pressure datum.
If an aircraft is flying vertical position is expressed as a Flight Level.
e.g. FL050; FL180.

17. Altimeter Definitions (1)
The setting of mb is referred to as ‘standard altimeter(or pressure)setting.’ (SPS)
Flight levels (FL) are pressure altitudes expressed in units of 100 feet; e.g. FL340 is a pressure altitude of 34,000 ft.

18. Altimeter Definitions (2)
Density Altitude: Is that altitude in the ISA at which the prevailing air density is to be found.
If the air is warmer than ISA it will be less dense and the density altitude will be higher in the ISA than the pressure altitude.
If the air is cooler than ISA then the density altitude will be less than the pressure altitude.

19. Altimeter Definitions (3)
True Altitude: is the exact vertical distance above Mean Sea Level (MSL).
This will differ from indicated altitude if the temperature varies from ISA conditions and the sub-scale setting is different from the value of mean sea level pressure directly below the aircraft.

20. Altimeter Pressure Settings
QFE
QFF
QNH
Regional QNH
Standard Pressure Setting
QNE

21. QFE
Is a pressure setting which when set on the sub-scale of an altimeter will cause it to read zero at the airfield elevation.
An airfield may have a touchdown QFE if there is a difference in height of 2m (7ft) or more between aerodrome reference and runway threshold.

22. QFF
Is the local station barometric pressure adjusted to mean sea level assuming an isothermal column of air at station temperature
It is expressed to one decimal point.

23. QNH
Is a pressure setting which when set on an altimeter sub-scale will cause it to read the airfield elevation above mean sea level.
ISA conditions are assumed for the air column
It is always expressed in integer values.

24. QFF v QNH

25. Regional QNH
Is the lowest forecast QNH for a period of one hour for a designated region

26. Standard Pressure Setting
is defined as mb.
When flying with standard pressure set on the subscale the vertical position of the aircraft is referred to as it’s ‘flight level.’

27. QNE
Is the height indicted on an altimeter with 1013 mb set on the subscale.
It is the same as pressure altitude.

28. Altimeter Calibration
All altimeters are calibrated according to the International Standard Atmosphere.
Any variation from this introduces an error mainly due to temperature deviation from ISA.
Provided all aircraft use the same sub-scale setting, separation can be maintained.

29. Table of ISA Values Height Flight Height Temp Pressure Density
in ft Level in m °C hPa kg/m3
(850) 1.06
(700) 0.90
(500) 0.70
(400) 0.57
(300) 0.458
(200) 0.317

30. Standard Pressure Levels

31. An aircraft cruising at FL80, flying from low to high pressure maintains a constant true altitude. Why?
FL100
COLD
WARM
A hPa
B hPa
1013 hPa