Altimeter setting procedures define how the altimeter barometric sub-scale must be set during the different phases of the flight.
Types of Altimeter Settings
There are three primary altimeter settings used in aviation:
- QNH: This setting adjusts the altimeter to read the altitude above mean sea level (MSL). It is the most commonly used setting and is essential for ensuring that all aircraft in a given area are using the same reference point for altitude.
- QNH: Q-Normal-Height
- Altitude Correction: This is calculated based on the altitude above sea level and the standard lapse rate of atmospheric pressure (approximately 1 inch of mercury per 1000 feet).
- Formula: QNH = Barometer Reading + Altitude Correction
- QFE: This setting adjusts the altimeter to read zero when the aircraft is on the ground at a specific airfield. It indicates the height above the airfield elevation and is particularly useful for landing operations.
- QFE: Q-Field-Elevation
- No correction: Since QFE is already at the airfield’s elevation, no altitude correction is needed.
- Formula: QFE = Barometer Reading
- QNE: This is the standard pressure setting of 1013.25 hPa (29.92 inHg) used above the transition altitude. It is used to ensure uniformity in altitude readings when aircraft are flying at higher altitudes, typically above 18,000 feet in many regions.
- QNE: Q-Normal-Elevation
- Altitude Correction: This is calculated based on the difference between the actual altitude and 5000 feet, using the standard lapse rate.
- Formula: QNE = Barometer Reading + (5000 feet – Actual Altitude) * Lapse Rate
Note: The specific values for the standard lapse rate and altitude correction factors may vary slightly depending on the region and aviation authority.
Importance of Correct Altimeter Settings
Using the correct altimeter setting is critical for several reasons:
- Safety: Accurate altitude readings prevent collisions with terrain and other aircraft. Incorrect settings can lead to altitude deviations, which can be dangerous, especially in congested airspace.
- Navigation: Pilots rely on precise altitude information for navigation and to comply with air traffic control (ATC) instructions. This ensures smooth and efficient flight operations.
- Standardization: Standard altimeter settings like QNH and QNE ensure that all aircraft in a given airspace are using the same reference points, reducing the risk of misunderstandings and errors.
The aircraft altimeter barometric sub-scale must be set to the appropriate setting for the phase of flight.
Setting Procedures
Pilots must adjust their altimeters according to specific procedures during different phases of flight:
- Pre-Flight: Before takeoff, pilots set the altimeter to the local QNH provided by ATC or the airport’s weather station.
- Climb: During the climb, the altimeter is set to QNE once the aircraft passes the transition altitude.
- Cruise: At cruising altitude, the altimeter remains set to QNE to maintain a standard reference.
- Descent: During descent, the altimeter is reset to the local QNH as the aircraft passes below the transition level.
- Landing: For landing, the altimeter may be set to QFE if the airfield uses this setting, ensuring accurate height readings above the runway.
Transition Altitude
Transition Altitude: The altitude at which the pilot changes the aircraft’s altimeter setting (usually from QNH) to standard pressure (1013.25 hPa). This altitude varies by country and region. For example, in the United States, the transition altitude is fixed at 18,000 feet, and the transition level is usually Flight Level 180 (FL180) or higher.
- Transition Level: The lowest flight level above the transition altitude where the standard pressure setting is used. It is typically a few hundred feet above the transition altitude to create a buffer zone known as the transition layer.
- Transition Layer: The space between the transition altitude and the transition level is known as the transition layer. This buffer zone helps prevent conflicts between aircraft climbing and descending through these critical altitudes.
Note: Air Traffic Control (ATC) provides instructions to pilots regarding when to change their altimeter settings, ensuring consistent altitude reporting and safe separation between aircraft.
