Altimeter Settings (QNH, QFE, STD, and Flight Levels)

Pilots use altimeters to measure aircraft altitude. However, altimeters rely on barometric pressure, and that pressure changes with weather conditions. To ensure consistent and safe altitude readings, pilots adjust their altimeters using specific pressure settings: QNH, QFE, or Standard Pressure (STD/1013 hPa). Depending on the setting, the altimeter shows different reference heights.

• Sets the altimeter to display altitude above mean sea level (AMSL).
• When set on the ground, the altimeter shows the airport elevation.
• Used for departure, enroute (below transition altitude), and arrival procedures.
• Provided in hPa (hectopascal) by ATIS or ATC (e.g., “QNH 1018”).

Phraseology example:
"QNH one zero one eight, set and crosscheck."

• Sets the altimeter to display height above airfield elevation.
• When set on the ground, the altimeter reads zero.
• Commonly used in military and glider operations, but rarely in general aviation.
• Not used for enroute navigation or international procedures.

Phraseology example:
"QFE one zero zero five, indicating zero on the ground."

• Uses 1013.25 hPa (29.92 inHg) as a universal standard.
• Sets all aircraft on a common pressure level for separation above the transition altitude.
• Altitudes are referred to as Flight Levels (FL), e.g., FL100 = 10,000 ft on standard pressure.
• Helps ensure all aircraft in upper airspace maintain consistent vertical separation regardless of regional weather.

Phraseology example:
"Climb to flight level eight zero."

• Altitudes based on standard pressure are expressed as FLxxx (e.g., FL60 = 6,000 ft).
• Transition from QNH to STD occurs at the transition altitude, which varies by country (often 5,000 ft or 10,000 ft).
• Descending through the transition level, pilots reset the altimeter to QNH.

Conversion example:
If QNH is 1013 hPa, FL050 ≈ 5,000 ft
If QNH is lower (e.g., 990), FL050 may indicate ~4,700 ft AMSL

• Transition altitude: The altitude where you switch from QNH to STD (climbing).
• Transition level: The lowest usable flight level, where you switch from STD to QNH (descending).

The gap between them is the transition layer, ensuring separation between ascending and descending traffic.

Incorrect settings can lead to altitude deviations, loss of separation, and airspace violations. It is essential to update and crosscheck altimeter settings when cleared by ATC or as part of standard procedures.