Atmospheric Pressure and Altimetry
Altimetry questions reward precision: which reference surface is set, what the altimeter actually displays, and how temperature and pressure changes make displayed altitude differ from true altitude. Expect a numerical item on QNH or on cold-air error.
Pressure fundamentals
- Pressure is force per unit area; the unit is the hectopascal (hPa). 1 inHg ≈ 33.86 hPa.
- Pressure falls with height, more slowly in cold air (dense) and faster in warm air.
- Isobars join equal pressure; the closer they are, the stronger the pressure gradient and the stronger the wind.
The three settings
- QFE: pressure at the aerodrome reference point. Altimeter reads height above that point. Used for circuit work and touchdown.
- QNH: aerodrome pressure reduced to mean sea level using the ISA. On the ground the altimeter reads elevation, so it gives terrain clearance en route.
- QNE: the standard 1013.25 hPa set at the transition altitude; the altimeter then reads flight levels. Between transition altitude and transition level lies the transition layer.
Altimeter errors
- Instrument (mechanical) error and position/installation error (PEC).
- Pressure error: setting the wrong QNH, or failing to reset when flying from high to low pressure.
- Temperature error: the dominant one in the vertical.
- Compressibility/density error at high altitude and high speed: the altimeter under-reads.
Temperature effect on true altitude
- Cold air contracts the column, so pressure levels sit lower. At a given indicated altitude the aircraft is lower than the altimeter says: the altimeter over-reads.
- Rule of thumb: about 4 ft per 1,000 ft per °C of ISA deviation, using the mean temperature of the layer.
- Warm air does the reverse: indicated altitude is lower than true altitude. Terrain clearance is safe but performance is degraded.
- Memory aid: high to low, or hot to cold, look out below.
Worked example
An aircraft at 5,000 ft indicated; ISA at 5,000 ft is 15 − 9.9 = 5.1°C, and the outside air temperature is −10°C. Deviation = −15.1°C. Error ≈ 4 × 5 × 15 = 300 ft. True altitude is about 4,700 ft, i.e. 300 ft lower than displayed - significant over high terrain in winter.
Traps that cost marks
- Setting 1013.25 below the transition altitude, or leaving QNH set above it.
- Assuming QNH equals the aerodrome station pressure - it does not; QFE does.
- Using 27 ft per hPa at 30,000 ft.
- Reversing the sign of the cold-temperature error.