Post by Jonathan Beckett on Apr 21, 2022 9:36:13 GMT
Although I've always had some idea what the transition altitude was about, I've never really known for sure - so set off last night down an internet rabbit hole - reading various official descriptions, and trying to make sense of what I read.
Here's what I learned:
Transition Altitude
This is the altitude at or below which your altitude is measured in feet above sea level - which is dependent upon knowing the "QNH" communicated by the airfield to calibrate your altimeter (see the description of QNH below).
Below the "Transition Altitude", altitude is communicated in feet (e.g. "seven thousand feet").
Above the Transition Altitude, altitude is communicated as "Flight Level" - expressed as FLn where n represents multiples of 100 feet (e.g. FL360 equals 36,000 feet)
In the USA, the Transition Altitude is set at 18,000 feet across most of the country. Throughout the rest of the world the Transition Altitude varies from airport to airport and will be communicated on charts, and by air traffic control.
Transition Level
This is the lowest assignable flight level above the Transition Altitude. The area between the Transition Altitude and Transition Level is known as the Transition Layer. Aircraft should only climb or descend through the Transition Layer. The layer accounts for aircraft switching between airfield and standard QNH settings - which will affect their indicated altitude during the period they are switching over. For this reason, QNH should only be adjusted while above the Transition Altitude.
QNH
QNH is the atmospheric pressure at sea level. In order to calibrate an altimeter to accurately display the altitude of the aircraft, it's QNH will need to be set (all altimeters allow calibration either through digital or analogue control). QNH is not an acronym - it is a "Q Code" (Q Codes date back to early radio telegraphy standards). QNH can be expressed in Hectopascals, or Inches.
"Standard" QNH at or above the transition altitude is 1013.25 hectopascals, or 29.92 inches.
QFE
QFE is the atmospheric pressure at the airfield. Often the Q Codes for QNH and QFE are associated with the terms "Natural Height", and "Field Elevation" to aid in differentiating them.
Here's what I learned:
Transition Altitude
This is the altitude at or below which your altitude is measured in feet above sea level - which is dependent upon knowing the "QNH" communicated by the airfield to calibrate your altimeter (see the description of QNH below).
Below the "Transition Altitude", altitude is communicated in feet (e.g. "seven thousand feet").
Above the Transition Altitude, altitude is communicated as "Flight Level" - expressed as FLn where n represents multiples of 100 feet (e.g. FL360 equals 36,000 feet)
In the USA, the Transition Altitude is set at 18,000 feet across most of the country. Throughout the rest of the world the Transition Altitude varies from airport to airport and will be communicated on charts, and by air traffic control.
Transition Level
This is the lowest assignable flight level above the Transition Altitude. The area between the Transition Altitude and Transition Level is known as the Transition Layer. Aircraft should only climb or descend through the Transition Layer. The layer accounts for aircraft switching between airfield and standard QNH settings - which will affect their indicated altitude during the period they are switching over. For this reason, QNH should only be adjusted while above the Transition Altitude.
QNH
QNH is the atmospheric pressure at sea level. In order to calibrate an altimeter to accurately display the altitude of the aircraft, it's QNH will need to be set (all altimeters allow calibration either through digital or analogue control). QNH is not an acronym - it is a "Q Code" (Q Codes date back to early radio telegraphy standards). QNH can be expressed in Hectopascals, or Inches.
"Standard" QNH at or above the transition altitude is 1013.25 hectopascals, or 29.92 inches.
QFE
QFE is the atmospheric pressure at the airfield. Often the Q Codes for QNH and QFE are associated with the terms "Natural Height", and "Field Elevation" to aid in differentiating them.
