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An example explanation

We pride ourselves on clear teaching. That's why our tests and exams explain their answers, and if you still don't understand (even with an explanation), you can Ask an Instructor. We use pictures and colour wherever we can to make things easier to follow.

Question

The QNH is 999hPa. An aircraft is flying over terrain at an altitude of 4500ft. The OAT is minus 10°C. What is the true altitude of the aircraft (assume 1hPa = 30ft)?

  1. A4230ft Right answer
  2. B4555ft
  3. C4720ft
  4. D4900ft

Explanation

This is a straightforward true altitude question, using the altitude window on the whizz-wheel.

In the altitude window of the whizz-wheel, you'll see that the inputs are air temperature and pressure altitude.

The pressure altitude is what the altimeter would read if the pilot set a sub-scale pressure setting of 1013hPa. The aircraft is at 4500ft, but on a sub-scale setting of 999hPa.

To work out the pressure altitude, we turn the sub-scale up from 999 to 1013, an increase of 14hPa. The question says to assume 1hPa = 30ft, so the altimeter reading increases by 14 × 30ft = 420ft, from 4500ft to 4920ft. Our pressure altitude is therefore 4920ft.

The temperature is −10°C. With both pressure altitude and actual temperature, we use the whizz-wheel to convert indicated altitude to true altitude:

  1. In the altitude window, set −10°C against 4920ft.
  2. Against the actual altitude (inner scale) of 4500ft, read the true altitude of 4230ft on the outer scale.
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