Aircraft altimeters do not measure height directly. They measure air pressure and convert it into an altitude using a standard atmosphere model. Because pressure changes with weather and with height, pilots must set the correct reference pressure before using an altimeter.
QNH, QFE, and standard pressure let pilots describe vertical position in a consistent and safe way.
QNH makes the altimeter read elevation above mean sea level when the aircraft is on the ground at an airport. QFE makes the altimeter read zero on a chosen runway or airfield reference point, so it shows height above that field. Above the transition altitude, aircraft use the same standard pressure setting, 1013.25 hPa or 29.92 inHg, and report flight levels instead of local altitudes.
A shared pressure datum keeps aircraft vertically separated even when flying across regions with different weather pressure.
Key Facts
- Pressure decreases with altitude, so an aneroid altimeter estimates altitude from static air pressure.
- QNH setting: altimeter reads altitude above mean sea level when correctly set.
- QFE setting: altimeter reads height above a selected airfield reference point.
- Standard pressure setting: 1013.25 hPa = 29.92 inHg.
- Flight level formula: FL number = pressure altitude in feet / 100, so 35,000 ft is FL350.
- Rule of thumb near sea level: 1 hPa pressure error is about 27 ft of altitude error.
Vocabulary
- Altimeter
- An instrument that uses static air pressure to estimate and display an aircraft's altitude.
- QNH
- A pressure setting that makes an altimeter show altitude above mean sea level.
- QFE
- A pressure setting that makes an altimeter show zero at a selected runway or airfield reference point.
- Transition altitude
- The altitude at or above which pilots change from a local altimeter setting to the standard pressure setting.
- Flight level
- A vertical position based on standard pressure, expressed in hundreds of feet, such as FL180 for 18,000 ft pressure altitude.
Common Mistakes to Avoid
- Using QFE when altitude above sea level is required. This is wrong because QFE shows height above the airfield, not clearance above terrain or obstacles referenced to mean sea level.
- Forgetting to change to standard pressure above the transition altitude. This is wrong because aircraft at higher levels must share the same datum for reliable vertical separation.
- Treating flight levels as true height above the ground. This is wrong because a flight level is based on pressure altitude, not terrain height or true geometric altitude.
- Ignoring pressure changes during flight. This is wrong because a falling or rising pressure system can make the indicated altitude differ from the aircraft's actual height above sea level.
Practice Questions
- 1 An aircraft is on a runway with elevation 620 ft above mean sea level. With QNH set correctly, what should the altimeter read? With QFE set for that runway, what should it read?
- 2 A pilot accidentally sets an altimeter 4 hPa too high. Using 1 hPa = 27 ft, about how many feet too high or too low will the indicated altitude be?
- 3 Two aircraft are cruising in the same region above the transition altitude. Explain why both should use 1013.25 hPa instead of their departure airport's QNH.