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PPL · EASA 050 · The atmosphere
Pressure altitude of an aerodrome
An aerodrome has an elevation of 1 250 ft and the QNH is 998 hPa. Using 30 ft per hPa, what is the pressure altitude of the aerodrome?
Explanation
Pressure altitude (PA) is height measured from the level where the pressure is 1013 hPa, as in the standard atmosphere. When the QNH is lower than 1013 hPa, that level lies below sea level, so the pressure altitude is higher than the elevation; when the QNH is higher, it is lower.
Step by step
- Difference from standard: 1013 − 998 = +15 hPa.
- In feet: +15 × 30 = +450 ft.
- Pressure altitude: 1 250 + (+450) = 1 700 ft.
Why not the other answers
- 800 ft applies the correction the wrong way. A QNH below 1013 hPa puts the 1013 hPa level below sea level and makes the pressure altitude higher than the elevation; a QNH above 1013 hPa puts it above sea level and makes the pressure altitude lower.
- 450 ft is only the size of the correction. It still has to be applied to the aerodrome elevation.
- 1 265 ft adds the hectopascals as if they were feet. Near the ground each hPa is about 30 ft.
Tip. On the ground you can read it directly: set 1013 on the altimeter subscale (the Kollsman window) and the altimeter shows the pressure altitude.
Original questions written by FlightDesk Aero and aligned with the EASA syllabus. They are not questions from the official EASA exam bank.