Pressure, Density and Altimetry
Pressure, Density and Altimetry questions test the pressure settings (QNH, QFE, QNE), the transition between them, density altitude, and the errors that make an altimeter read falsely high or low.
Pressure basics
- 1 hPa ≈ 27 ft near sea level (rule of thumb 30 ft); pressure halves roughly every 18,000 ft.
- QFE: pressure at aerodrome level - the altimeter reads height above the aerodrome.
- QNH: with it set, the altimeter reads aerodrome elevation on the ground.
- Standard setting 1013.25 hPa gives pressure altitude; used at and above the transition level.
Altimeter errors
- Temperature: in air colder than ISA the pressure column contracts, so the altimeter over-reads - indicated altitude is higher than true altitude. The error grows with height above the setting source and with the temperature deviation.
- Pressure: flying from high to low pressure without resetting makes the altimeter over-read. High to low, look out below.
- Density altitude: high temperature, high humidity and low pressure raise density altitude and degrade take-off, climb and engine performance.
Worked example
Aerodrome elevation 1000 ft, QNH 1003 hPa. With 1013.25 hPa set, the altimeter reads 1000 + 27 × (1013 − 1003) ≈ 1000 + 270 = 1270 ft pressure altitude. If the temperature were 15 °C below ISA, true altitude would be lower than 1270 ft - the altimeter is optimistic, which matters for terrain clearance.
Traps
- Assuming the transition level equals the transition altitude; the TL is at or above the TA and varies with QNH.
- Applying the temperature correction only at high altitude; it also matters at low temperature and high elevation.
- Reversing the direction of the pressure error.