According to the
Australian Bureau of Meteorology:
In Australia, Area QNH areas are defined airspace for which QNH forecasts are prepared routinely. The areas correspond to the briefing area/QNH area boundaries found on Airservices Australia’s Planning Chart Australia (PCA). An altimeter set to Area QNH will be representative to within ±5 hPa of any actual QNH of any location within the defined area.
You are a diligent pilot and have set your altimeter to the current forecast Area QNH for the area in which you are flying. It happens to be 1013 hPa.
Assume, for the purposes of the first question, that:
- The forecast QNH is representative in accordance with the BOM’s explanatory material quoted above.
- 1013 hPa is exactly sea level pressure in the ISA.
- 1 hPa = exactly 30’ in the ISA.
- The temperature is exactly ISA.
- Your altimeter is perfectly accurate and you are flying over the sea.
- Your aircraft’s static source is at the same height above the sea as the altimeter.
- Your altimeter is indicating 500’.
Question: What is your altimeter’s actual height above the sea below?
Is it:
(a) 500’.
(b) It could be anywhere between about 350’ and 650’.
(c) It could be anywhere between about 425’ and 575’.
(d) None of the above, the correct answer is [explain].