(Altimeter-Setting Procedures)
The altimeter is a barometer reading in feet; what it tells you depends entirely on the subscale setting:
- QNH: mean-sea-level pressure; the altimeter then reads altitude (height above MSL). On the ground it reads aerodrome elevation.
- QFE: aerodrome-level pressure; the altimeter reads height above the aerodrome. On the ground it reads zero.
- Standard setting, 1013.2 hPa (QNE): the altimeter reads pressure altitude, expressed as a flight level (FL50 = 5,000 ft on 1013).
Two derived ideas. The Regional Pressure Setting (RPS) is the lowest forecast QNH in an Altimeter Setting Region for each hour: the UK has 20 such regions, and the RPS is updated hourly. Because it is the lowest forecast value, using it keeps you at or slightly above the indicated altitude (safe for terrain). The transition altitude (3,000 ft over most of the UK) is where you switch from QNH (below) to the standard setting (above); the lowest usable level above it is the transition level, and the gap between is the transition layer.
A vital rule of thumb: below 5,000 ft, 1 hPa ≈ 30 ft. And the mnemonic that catches people in the exam and in the air alike, "from high to low, beware below": applies to both falling pressure and falling temperature; in a very cold air mass you can be as much as 10% lower than indicated.
Worked example, non-standard QNH and a flight level. The regional QNH is 996 hPa. Standard is 1013, so the 1013 datum sits about 17 hPa, almost 500 ft, below mean sea level: meaning FL40 is only about 3,500 ft amsl. To stay 1,000 ft above terrain or traffic at 3,000 ft amsl you need the next level up: FL45 (which equates to about 4,000 ft amsl in that pressure). This is exactly why ATC may assign an apparently high flight level on a low-pressure day.