What is the standard pressure setting in aviation?

The standard pressure setting is 1013.25 hPa (or 29.92 inHg), used above the transition altitude for flight levels.

How does QNH differ from QFE?

QNH sets the altimeter to show altitude above mean sea level, while QFE sets it to show height above the airfield reference point.

Why is the transition altitude important for pressure settings?

Transition altitude marks where pilots switch from local QNH to the standard pressure setting to ensure uniform flight level separation.

What happens if you fly from high to low pressure without adjusting your altimeter?

Your altimeter will over-read, making you lower than indicated and increasing the risk of terrain collision.

Why are barometric pressure settings critical for safe flight?

Correct pressure settings ensure accurate altitude indication, which is essential for terrain clearance and separation from other aircraft.

Barometric Pressure Settings

Medium4 min readInstrumentation

Moderately Examined

Why this matters

Correct barometric pressure settings are vital for maintaining safe terrain clearance, accurate approach profiles, and proper separation from other aircraft. Misunderstandings or errors can directly lead to altitude busts or controlled flight into terrain.

Barometric pressure settings are fundamental to accurate altitude indication in aviation. Pilots use settings like QNH, QFE, and the standard pressure (1013.25 hPa) to ensure the altimeter displays the correct height above a chosen reference, whether that's sea level, the airfield, or the standard datum. Understanding the difference between these settings and when to use each is crucial for safe flight operations and correct altitude separation.

Quick Check

Which barometric pressure setting should be used to indicate altitude above mean sea level during flight in the vicinity of an aerodrome?

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In depth

Explanation

Types of Barometric Pressure Settings

QNH: Sets the altimeter to indicate altitude above mean sea level (AMSL). When QNH is set, the altimeter reads the true elevation of the aircraft above sea level, matching published aerodrome elevations.
QFE: Sets the altimeter to read zero when on the airfield reference point. The altimeter then shows height above that specific airfield, useful for circuit and approach operations at that location.
Standard Pressure Setting (1013.25 hPa): Used above the transition altitude, this setting allows all aircraft to reference the same pressure datum, ensuring vertical separation (flight levels) regardless of local atmospheric pressure variations.

Transition Altitude and Pressure Reference

Below the transition altitude, pilots use QNH (or QFE in some local procedures) for terrain clearance and approach accuracy.
Above the transition altitude, all aircraft set 1013.25 hPa to maintain standardised flight levels, preventing conflicts due to regional pressure differences.

Altimeter Setting Procedures

Pilots must regularly update the altimeter setting with the latest QNH or QFE, especially when flying between regions with different pressures.
Air traffic control provides updated pressure settings during flight, and pilots must adjust their altimeter subscale accordingly.

QNH vs QFE: Key Differences

QNH gives altitude above mean sea level (used for enroute and approach phases).
QFE gives height above a specific airfield (mainly for local circuit or military operations).

Barometric Errors and Corrections

Incorrect settings can cause the altimeter to over-read or under-read true altitude, leading to potential safety hazards, especially during approach or in mountainous terrain.
Temperature deviations from ISA also affect altimeter accuracy, but only barometric error is corrected by adjusting the subscale.

Pitot-Static System Context

The altimeter uses static pressure only, not dynamic pressure, to determine altitude. Any blockage, leakage, or position error in the static system can affect the reading and must be understood for troubleshooting and safe operation.

The essentials

Key Points

QNH sets the altimeter to indicate altitude above mean sea level (AMSL).

QFE sets the altimeter to show height above a specific airfield reference point.

Standard pressure (1013.25 hPa) is used above the transition altitude for flight levels.

Altimeter uses static pressure only—not dynamic pressure.

Incorrect barometric setting leads to altitude errors and potential safety hazards.

Transition altitude is the point where pilots switch from QNH/QFE to standard pressure.

Barometric error is corrected by adjusting the altimeter subscale to the correct local pressure.

Watch out

Exam Traps & Typical Mistakes

Confusing QNH (altitude above sea level) with QFE (height above airfield).

Believing the altimeter uses both static and dynamic pressure (it uses static only).

Forgetting to set standard pressure above the transition altitude or below the transition level.

Assuming temperature error can be corrected by barometric setting (it cannot).

Mixing up when to use QNH, QFE, or standard pressure during different flight phases.

Test yourself

Example Exam Questions

Question 2Medium

What is the main difference between QNH and QFE pressure settings?

Question 3Medium

When climbing through the transition altitude, what barometric pressure setting must be set on the altimeter?

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