One altimeter, three different references. Ask any student pilot preparing for DGCA exams which topic causes the most confusion early on, and QNH, QNE and QFE will be near the top of the list. This article explains each setting in plain language, what the altimeter shows, and when pilots use it.
How an altimeter works
An aircraft altimeter is a barometer. It measures the outside air pressure and converts it to a height, using the standard rule that pressure falls as you climb. Because sea-level pressure changes every day and from place to place, the pilot sets a reference pressure in the altimeter's subscale window. That reference decides what the altimeter actually reads.
QNH: altitude above mean sea level
With QNH set, the altimeter reads altitude, the height above mean sea level. Parked on the runway, it shows the aerodrome elevation. QNH is used for take-off, landing and flight below the transition altitude, because terrain heights and obstacle heights on charts are given above mean sea level.
QFE: height above the aerodrome
With QFE set, the altimeter reads height above the aerodrome. On the runway it shows zero. Some operators and military units use QFE in the circuit, because the pilot can read height above the ground directly.
QNE: the standard setting and flight levels
QNE refers to the altimeter reading when the standard pressure of 1013.25 hPa (29.92 inHg) is set. Above the transition altitude, every aircraft uses this same setting, so all aircraft measure height against one common reference and stay safely separated even when local pressure differs along the route. The readings are given as flight levels: FL350 means 35,000 ft on the standard setting.
Watch: QNH, QNE and QFE in 60 seconds
Transition altitude, layer and level
- Transition altitude: climbing through it, pilots change from QNH to 1013.25 hPa. It is published for each aerodrome.
- Transition level: descending through it, pilots change from 1013.25 hPa back to QNH. ATC gives it with the approach clearance.
- Transition layer: the airspace between the two.
Why it matters: "high to low, look out below"
If a pilot flies from an area of high pressure into low pressure without resetting the altimeter, the aircraft will be lower than the altimeter shows. The same is true when flying into colder air. That is why pilots cross-check settings during every approach briefing and why ATC repeats the QNH in clearances.
Quick revision for exams
| Setting | Altimeter reads | On the runway |
|---|---|---|
| QNH | Altitude above mean sea level | Aerodrome elevation |
| QFE | Height above the aerodrome | Zero |
| QNE (1013.25 hPa) | Pressure altitude / flight level | Pressure altitude of the field |
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