The Climbing Departure and the Cold-Weather Altimeter
You are planning a departure from a hill station aerodrome. Field elevation is 2,400 ft AMSL. The current QNH is 1003 hPa, and the outside air temperature at the field is 32°C. Your aircraft's altimeter subscale is set to 1013.2 hPa (standard pressure), because you intend to level off at FL 075 shortly after departure for terrain clearance. The minimum safe altitude along your initial route is 6,800 ft AMSL. You have been taught that the altimeter over-reads in cold air and under-reads in warm air relative to standard, and that pressure error behaves the opposite way. The aircraft's true altitude on the standard setting differs from indicated altitude by roughly 30 ft per hPa of pressure error plus a temperature correction of about 4 ft per 1,000 ft per °C of ISA deviation.
1. Compute the pressure error and the temperature error at FL 075, and state the true altitude you will actually have when the altimeter indicates 7,500 ft.
Read the worked answer
Pressure error: The altimeter is set to 1013.2 hPa but the local QNH is 1003 hPa. The pressure error is 1013 − 1003 = 10 hPa. With a standard setting lower than actual pressure, the altimeter over-reads; here the true altitude is lower than indicated by roughly 10 × 30 = 300 ft.
Temperature error: At 7,500 ft the ISA temperature is 15 − (2 × 7.5) = 0°C. Actual temperature at that level (lapsing at ~2°C/1,000 ft from 32°C) is about 32 − 15 = 17°C. ISA deviation ≈ +17°C. Temperature error ≈ 4 ft × 7.5 × 17 ≈ 510 ft, and warm air makes the true altitude higher than indicated.
Net: True altitude ≈ 7,500 − 300 + 510 ≈ 7,710 ft. That is above the 6,800 ft minimum, so the flight is safe - but only because the warm-air error more than offsets the pressure error. If the temperature were standard, the true altitude would be about 7,200 ft, still safe but with far less margin.
2. A colleague argues you should just set QNH and fly the same indicated altitude. Explain whether that changes the true altitude and whether it changes your terrain clearance.
Read the worked answer
Setting QNH changes what the altimeter displays for a given true altitude, not the true altitude itself. With QNH set, the altimeter reads field elevation on the ground and reads approximately true altitude at low levels, so the 300 ft pressure error disappears from the indication. However, the temperature error does not disappear: in warm air the altimeter still under-reads true altitude (true altitude is higher than indicated), and in cold air it over-reads. The practical effect is that with QNH set you can compare indicated altitude directly with published minimum safe altitudes, which are themselves referenced to QNH. The real trap is flying a flight level (standard setting) in an area where the QNH differs significantly from 1013 hPa, because then the indicated level no longer corresponds to the published altitude. The correct decision is: use QNH below the transition altitude, and when using flight levels, apply the cold-temperature altitude correction published for the procedure - never assume the indicated level equals the true level.