QNH-QNE-QFE , All these tree codes are a part and maybe called a subclass of Q coding system. Q coding system is standardized three letter coding system that starts with letter Q. It was firstly developed for commercial R/T (radiotelephony) communication.
QNH: QNH value is the pressure value measured at a location and reduced down to mean sea level. More clearly when an aircraft sets QNH value on altimeter while a taxi movement at an aerodrome, then it will show the aerodrome elevation above mean sea level. QNH is the widely used pressure settings in global aviation world. It is given as a regional pressure setting and should be reset with new values if you leave its area of reference into a new QNH pressure region.
QFE: QFE is the mean sea level pressure corrected for temperature and adjusted for a specific reference point of an airfield or the point needed. Generally, for aerodromes the highest point of the runway is set to be reference.
More clearly when an aircraft sets QFE value to its altimeter it will Show ZERO ,“0” when it is on the runway or taxiway (they meant to be equal to the reference point)…
When QFE set to altimeter it will indicate the HEIGHT of the aircraft from reference point.
QNE : The term refers to the indicated altitude at the landing runway threshold when 1013.2mb (29.92 inHG) is set in the altimeter. The standard and globalized mean sea level pressure value is 1013.2mb (or 29.92inHg). QNE is used when an aircraft flies into higher levels. Above transition altitude all aircraft sets 1013.2mb to provide a common datum for vertical measurement.
These could help as remainder.
- QFE: (FE) Field Elevation
- QNH: (NH) Natural Horizon
- QNE: (NE) Natural Earth
Summary of definitions
Let’s summarize all. QNH is a pressure value and when an aircraft sets it on the altimeter while on the runway. It will show the ELEVATION of the threshold above mean sea level.
QFE is another pressure value and when set as altimeter of the aircraft and when the aircraft is on the runway, it will show ZERO “0” (reference point is meant to be that runway).
QNE is an altitude 1013.2mb and it is set to aircraft at the flight above transition altitude to provide a common datum.
Effects of Temperature to the Altimeter Values
QNE is a standard value according to standard altimeter 1013.2 hpa but QNH and QFE are variable. Temperature has major effect on these altimeter values.
This depends on a very basic rule of physics. Hot air rises…
Why hot air rises ?
Hot air rises because when you heat up the air (or any other gas for that matter), it expands. When the air expands, it becomes less dense than the air around it. The less dense hot air then floats in the denser cold air much like olive oil in water because olive oil is less dense than water and when you put it into water it rises.
In different words, hot air pushes out on the air around it, and thus becomes less dense. Less dense air rises, cooler air sinks because it is heavier and gravity pulls on it more strongly.
As you see when you heat up the air it becomes less dense and it turns out as a descend on pressure values and as opposite in colder days the altimeter values turn to be higher.
Altimeter value is a vital number in aviation and it is thought that setting a wrong value will lead to lack of awareness. A wrong higher value will make you to assume that you are higher than your actual elevation. Likewise, a wrong lower value leads you to assume that you are lower than your true elevation. Both may cause serious problems like runway excursions.
I hope this article builds a strong frame about QNH-QNE-QFE and their effects on aviation.
- Annex 5: Units of Measurement,
- Annex 3: Meteorological services,
- Doc 4444: PANS-ATM,
- Doc 8168: PANS-OPS.
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