An altimeter does not measure height directly. It measures air pressure and converts it into feet, so the number it shows depends entirely on the pressure you set on its subscale. That is why QNH, QFE and QNE come up again and again in the DGCA Aviation Meteorology paper. In this guide you will learn what each altimeter setting means, when it is used, and how to convert between them with confidence.
Why altimeter settings in aviation matter
A pressure altimeter shows the vertical distance between the aircraft and whatever pressure level is set on its subscale. Change the setting and you change the reference, even though the aircraft has not moved.
- Set the pressure at the airfield, and the altimeter reads height above the airfield.
- Set the pressure reduced to sea level, and it reads altitude above mean sea level (MSL).
- Set the standard pressure of 1013.25 hPa, and it reads pressure altitude, which pilots express as a flight level.
Pressure is measured in hectopascals (hPa) in India, the same as millibars.
QNH, QFE and QNE: the three settings
Each Q code is a short name for one altimeter reference. Here is what each one gives you.
QFE: height above the aerodrome
QFE is the atmospheric pressure at the aerodrome elevation (or at the runway threshold). With QFE set, the altimeter reads zero on the ground at that airfield and shows height above it in the air. It is useful in the circuit, but it tells you nothing about the height of terrain away from the airfield.
QNH: altitude above mean sea level
QNH is the aerodrome pressure reduced to mean sea level using the International Standard Atmosphere (ISA). With QNH set, the altimeter shows altitude above MSL, and on the ground it reads approximately the aerodrome elevation. Charts show terrain and obstacle elevations above MSL, so QNH is the setting that makes terrain clearance easy to check.
QNE: the standard setting and flight levels
In everyday use, QNE refers to the standard pressure setting of 1013.25 hPa (often rounded to 1013). With it set, every aircraft's altimeter uses the same reference, and the reading is called pressure altitude. Flight levels are pressure altitude in hundreds of feet, so FL 100 is a pressure altitude of 10,000 ft. Strictly, QNE is the altimeter reading on the runway when 1013.25 hPa is set, so read exam questions carefully.
Transition altitude and transition level
Pilots switch between QNH and the standard setting at fixed points, so that aircraft near the ground use altitude and aircraft higher up use flight levels.
- Transition altitude: the altitude at or below which the vertical position is given as altitude, on QNH.
- Transition level: the lowest flight level available for use above the transition altitude.
- Transition layer: the airspace between the transition altitude and the transition level.
When climbing, you set 1013.25 hPa as you pass the transition altitude. When descending, you set QNH as you pass the transition level. Transition altitudes are published for each aerodrome in India's Aeronautical Information Publication (AIP), so check the current AIP and charts rather than memorising one figure.
Pressure altitude calculation: worked example
Near sea level, 1 hPa of pressure change is equal to about 27 ft of height. Some textbooks and questions round this to 30 ft. Use the figure the question gives you.
An aerodrome has an elevation of 540 ft and a QNH of 1010 hPa. Find the QFE and the pressure altitude of the aerodrome, using 27 ft per hPa.
Elevation = 540 ft
Pressure change = 540 ÷ 27 = 20 hPa
QFE = QNH − 20 = 1010 − 20 = 990 hPa
Difference between standard and QNH = 1013 − 1010 = 3 hPa
3 hPa × 27 ft = 81 ft
Pressure altitude = 540 + 81 = 621 ft
The QFE is lower than the QNH because the airfield is above sea level, and pressure falls as you go up. The pressure altitude is higher than the elevation because the QNH is below standard.
The rule to remember: when QNH is below 1013, pressure altitude is higher than true elevation; when QNH is above 1013, pressure altitude is lower.
Common exam mistakes with altimeter settings
- Mixing up height and altitude. QFE gives height above the aerodrome; QNH gives altitude above MSL.
- Adding instead of subtracting. Higher places have lower pressure, so QFE at an elevated aerodrome is less than QNH.
- Setting the wrong value at the wrong point. Standard setting on the climb at transition altitude; QNH on the descent at transition level.
- Forgetting the high-to-low trap. If you fly from high pressure towards low pressure without resetting, the altimeter over-reads: you are lower than it shows. The saying "high to low, look out below" helps.
The altimeter only ever tells you how far you are from the pressure level set on its subscale, so always ask which setting is in use.
Frequently asked questions
What is the difference between QNH and QFE?
QFE is the actual pressure at the aerodrome, so the altimeter reads zero on the ground and shows height above the airfield. QNH is that pressure reduced to mean sea level, so the altimeter shows altitude above sea level and reads the aerodrome elevation on the ground. QNH helps with terrain clearance.
What does QNE mean in aviation?
QNE is commonly used to mean the standard altimeter setting of 1013.25 hPa. With it set, the altimeter shows pressure altitude, which is used to fly flight levels. Strictly, QNE is the reading on the altimeter at the aerodrome with 1013.25 hPa set, so read DGCA questions carefully to see which meaning is intended.
When do pilots set 1013 hPa?
Pilots set the standard setting of 1013.25 hPa when climbing through the transition altitude, and then fly flight levels. On descent, they change back to QNH when passing the transition level. Transition altitudes differ between aerodromes, so pilots check the published value in the AIP and on approach charts for each airfield.
How do you convert QNH to QFE?
Divide the aerodrome elevation by the height per hectopascal, about 27 ft near sea level, to find the pressure difference. Subtract that from QNH to get QFE. For example, an aerodrome at 270 ft with a QNH of 1012 hPa has a QFE of about 1002 hPa.
Why does the altimeter over-read when flying from high to low pressure?
If the subscale setting is not updated, the altimeter still measures from the old, higher pressure level. As the actual pressure falls, the aircraft descends to keep the same reading, so it ends up lower than indicated. This is why pilots update QNH regularly and remember "high to low, look out below".
Practise QNH, QFE and QNE questions
Altimeter settings are easy marks once you have practised a few conversions. Try the free DGCA practice questions, and read real reports with the METAR decoder to spot the QNH in every METAR. Fly with Sudharshan Aviation Academy (FSAA) covers QNH, QFE and QNE, the ISA and all pressure topics in the DGCA Aviation Meteorology course.