An aircraft is in level flight at FL100 over a mountain range, which extends up to 2400 metres AMSL. Determine the approximate terrain clearance, if the regional QNH is 998 hPa.
(Barometric lapse rate: 30 ft / hPa)
To calculate the terrain clearance, we must determine the aircraft’s true altitude.
If the QNH value is higher than the standard pressure of 1013 hPa, the altimeter reading will increase.
If the QNH value is lower than the standard pressure of 1013 hPa, the altimeter reading wil decrease.
If the local QNH is 998 hPa, the difference to standard pressure will be: 1013 hPa – 998 hPa = 15 hPa
Assuming a barometric lapse rate of 30 ft/hPa:
30 ft/hPa x 15 hPa = 450 ft
Since the local QNH is lower than standard, we can conclude that the altimeter indication will decrease by 450 ft.
10 000 ft – 450 ft = 9550 ft
The ground elevation is 2 400 m, which corresponds to 2400 m x 3.28 ft = 7872 ft
The ground clearance is: 9550 ft – 7 872 ft = 1 678 ft (approx. 1700 ft)
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