Altimeter setting and terrain clearance on descent into Mulu
You are descending into Mulu (WBMU) at night in a Cessna 172. The aerodrome elevation is 80 ft, QNH 1002 hPa, temperature 30 °C. Your current indicated altitude is 7 500 ft, and the minimum safe altitude for the sector is 6 500 ft. The MSA is based on 1013 hPa and ISA temperature. You have 45 minutes of fuel remaining and a 20-minute reserve. The destination weather is reported as visibility 6 km, scattered cloud at 2 000 ft, wind calm.
1. Calculate the true altitude at your current indicated altitude and assess terrain clearance against the sector MSA.
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Pressure altitude = indicated altitude + 30 × (1013 − QNH) = 7 500 + 30 × 11 = 7 500 + 330 = 7 830 ft. True altitude differs from pressure altitude by the temperature error: approximate correction = pressure altitude × (ISA deviation) / 273. ISA temperature at 7 830 ft is about 15 − 2 × 7.83 = −0.7 °C; actual outside air temperature is not given, but at 30 °C at the surface the ISA deviation is large and positive. If the mean temperature on the column is about 20 °C above ISA, the true altitude is approximately 7 830 × (1 + 20/273) = 7 830 × 1.073 = 8 400 ft. The aircraft is therefore higher than indicated, so terrain clearance is better than the indicated 7 500 ft suggests: 8 400 ft versus the 6 500 ft MSA gives about 1 900 ft margin.
2. What are the risks if you descend to the MSA using the altimeter set to QNH, and how should you plan the descent?
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If you descend to 6 500 ft indicated on QNH, your true altitude would be about 6 500 + 330 = 6 830 ft pressure altitude, then corrected for temperature to about 7 330 ft true. The margin above terrain would still be positive but reduced compared with the MSA assumption, which is based on 1013 hPa and ISA. The main risk is that the MSA provides only 1 000 ft (or the published value) of clearance, and temperature and pressure errors erode that margin. The correct planning is to keep the altimeter on QNH, use the published MSA and approach procedure minima, and add a personal buffer above the MSA at night and in high temperatures. With 45 minutes of fuel and a 20-minute reserve, you have 25 minutes of usable flight time, so a prolonged hold or diversion is not advisable; plan a straight-in descent to arrive with the reserve intact.