What is the difference between QNH and QFE?

QNH sets the altimeter to show elevation above mean sea level, while QFE sets it to read zero at the airfield reference point.

When do pilots switch from QNH to standard pressure (1013.2 hPa)?

Pilots change from QNH to standard pressure at the transition altitude when climbing, and revert to QNH at the transition level when descending.

What does the altimeter indicate when set to 1013.2 hPa?

It shows pressure altitude, which is used to define flight levels.

Why is correct altimeter setting crucial for safe flight?

Incorrect settings can lead to altitude errors, risking terrain collision or loss of separation from other aircraft.

How does temperature affect true altitude compared to indicated altitude?

In colder than ISA conditions, true altitude is lower than indicated, requiring temperature correction for safety near terrain.

Altimeter Setting Procedures

Hard4 min readMeteorology

Moderately Examined

Why this matters

Accurate altimeter setting is vital for maintaining safe vertical separation from terrain and other aircraft, especially during transitions between different airspace structures and when operating near minimum altitudes.

Altimeter setting procedures ensure that the aircraft's altimeter displays the correct reference—height above ground, altitude above mean sea level, or flight level—depending on the phase of flight and the pressure setting used. Pilots use QNH, QFE, or standard pressure (1013.2 hPa) to align their altimeter with local or standard atmospheric pressure, which is critical for accurate vertical navigation and safe separation from terrain and other aircraft.

Quick Check

When QFE is set on the altimeter at an airfield, what will the instrument indicate on the ground?

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In depth

Explanation

Key Altimeter Settings Explained

QNH: Set this to display altitude above mean sea level (AMSL). On the ground at an airfield, the altimeter will read the airfield's elevation. QNH is used for most phases of flight below the transition altitude.
QFE: When set, the altimeter reads zero at the reference airfield. This setting shows height above the airfield surface, useful for circuits or local flights.
Standard Pressure (1013.2 hPa): Above the transition altitude, all aircraft set their altimeters to standard pressure. This ensures everyone operates on the same reference—pressure altitude—expressed as flight levels (FL).

Transition Altitude and Level

Transition Altitude: The altitude at which you switch from QNH (or QFE) to standard pressure during climb.
Transition Level: The lowest flight level available for use during descent, where you switch from standard pressure back to QNH.

Altimeter Reading Changes

When climbing through the transition altitude, setting the subscale from QNH to 1013.2 hPa may cause the indicated altitude to jump up or down, depending on the difference between QNH and standard pressure.
Example: If QNH is higher than 1013.2 hPa, switching to standard pressure will decrease the indicated altitude; if QNH is lower, the indicated altitude increases.

Calculating Altimeter Readings

On the ground with QNH set: Altimeter shows airfield elevation.
On the ground with QFE set: Altimeter reads zero.
On the ground with 1013.2 hPa set: Altimeter shows the pressure altitude of the airfield.

Related Terms

Height: Vertical distance above a specified datum (often the airfield, using QFE).
Altitude: Vertical distance above mean sea level (using QNH).
Pressure Altitude: Indicated altitude with 1013.2 hPa set.
Flight Level (FL): Pressure altitude in hundreds of feet above 1013.2 hPa.
True Altitude: Actual height above mean sea level, corrected for temperature and pressure deviations.

Temperature Corrections

In cold conditions, indicated altitude may overestimate true altitude—apply corrections when close to terrain.

The essentials

Key Points

QNH sets the altimeter to show altitude above mean sea level; QFE sets it to show height above the airfield.

Standard pressure (1013.2 hPa) is used above the transition altitude for flight levels.

Transition altitude is where you switch from QNH/QFE to standard pressure during climb.

Transition level is where you switch back to QNH during descent.

Setting errors or incorrect procedures can result in significant altitude misreadings.

Temperature deviations from ISA can cause true altitude to differ from indicated altitude.

Pressure altitude is the reading with 1013.2 hPa set, forming the basis for flight levels.

Watch out

Exam Traps & Typical Mistakes

Confusing QNH (altitude above MSL) with QFE (height above airfield).

Forgetting that the altimeter reading may increase or decrease when switching to standard pressure, depending on the QNH value.

Assuming the altimeter always reads true altitude, ignoring temperature and pressure errors.

Mixing up transition altitude (for climb) and transition level (for descent).

Believing that QNH always gives accurate altitude in flight, when temperature corrections may be needed in cold weather.

Test yourself

Example Exam Questions

Question 2Medium

During climb, you reach the transition altitude and change the altimeter setting from QNH 1020 hPa to 1013 hPa. What happens to the indicated altitude?

Question 3Easy

Which altimeter setting should be used to ensure separation from other aircraft above the transition altitude?

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