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Given the following information, calculate the true altitude of the aircraft to ensure that the Minimum Obstacle Clearance (MOCA) is available:

MOCA: 4 870 ft
Cruise Altitude: FL 60
Local aerodrome QNH: 995 hPa
Local aerodrome elevation: 2 430 ft
OAT: ISA -15°C
(Assume: 1 hPA = 30 ft)

  • A
    5 642 ft
  • B
    5 132 ft
  • C
    5 278 ft
  • D
    6 293 ft

To calculate the True Altitude, the Pressure and Temperature Corrections are required:

1. Find the Indicated Altitude with Local QNH:

  • Pressure Correction = (1013 - 995) hPa x 30 ft/hPa = 18 hPa x 30 ft/hPa = 540 ft
  • Indicated Altitude with Local QNH = Pressure Altitude - Pressure correction = 6 000 ft - 540 ft = 5 460 ft (from higher to lower pressure setting, Indicated altitude decreases).
2. Find the True Altitude due to OAT other than ISA:
  • Temperature Correction = 4 x (5 460 ft - 2 430 ft)/1000 x 15ºC = 182 ft.
  • True Altitude = 5 460 ft - 182 ft = 5 278 ft (In cold atmosphere, the Indicated is higher than the True altitude).

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