An altimeter measures air pressure and shows it as height. Because the pressure at sea level changes with the weather, you have to tell the altimeter what that pressure is. That is the altimeter setting.
The three settings
| Setting | The altimeter shows | Used for |
|---|---|---|
| QNH | Altitude above mean sea level | Departure, arrival and flight below the transition altitude |
| QFE | Height above the airfield (zero on the runway) | Some military and local operations |
| Standard (1013.25 hPa / 29.92 inHg) | Pressure altitude, flown as flight levels | Cruise above the transition altitude |
With QNH set, on the ground the altimeter reads the airport elevation. That makes it easy to check before take-off.
Transition altitude, layer and level
- The transition altitude (TA) is the altitude at or below which you fly on QNH. Climbing through it, you set standard.
- The transition level (TL) is the lowest flight level available above the TA. Descending through it, you set QNH.
- The transition layer is the airspace between the two. Nobody cruises in it.
Transition altitudes vary by country and sometimes by airport. In the United States and Canada the TA is 18,000 ft. Many other countries use much lower values, often between 3,000 and 13,000 ft. The chart or the AIP tells you.
The transition level moves with QNH: when pressure is low, flight levels sit lower, so the TL has to be higher to keep the layer safe. Controllers often announce it on the ATIS. The transition level tool works it out.
Why a wrong setting is dangerous
Flying from high pressure towards low pressure without resetting, the altimeter over-reads: the aircraft is lower than the instruments say. Pilots remember this as "high to low, look out below". Each hectopascal is roughly 27 to 30 ft; each tenth of an inch of mercury is roughly 100 ft.
Temperature matters too. In very cold air the aircraft is also lower than indicated, which is why some charts publish cold-temperature corrections.
For flight simulation and training only. Real-world procedures vary by country and operator; always follow the published charts and your local rules.
Keep reading
- Reading METAR and TAF: Decode the two weather reports you will use on every flight, group by group, with flight categories.