Air Law Question
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From: London
Air Law Question
I understand QFE is the height above an airfield. QNH is an altitude above the sea level datum. While studying my air law book, I was reading on the chapter regarding what datum to use in various stages of flight. I came across a term called aerodrome QNH?? 
...could someone please enlighten me....what is the difference between aerodrome QNH and Aerodrome QFE?
Thanks
...could someone please enlighten me....what is the difference between aerodrome QNH and Aerodrome QFE?Thanks
Joined: May 2006
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From: UK
Air traffic will be able to report either QNH or QFE and the choice is yours as to which you will use.
Quite simply, the aerodrome QNH would show you the aerodrome elevation (amsl) on the altimeter when sat on the runway, whereas when set to aerodrome QFE the altimeter will read 0 feet.
You therefore have a choice of using aerodrome QNH or aerodrome QFE but more importantly remember which one you have actually set. This will hopefully be repeated over and over during your training so I wouldn't be overly concerned.
IFR flights normally use aerodrome QNH and the pilots factor the aerodrome elevation into their calculations whereas most VFR flights tend to use aerodrome QFE within the vicinity of the aerodrome, which will show the height above the aerodrome.
Hope this helps
Quite simply, the aerodrome QNH would show you the aerodrome elevation (amsl) on the altimeter when sat on the runway, whereas when set to aerodrome QFE the altimeter will read 0 feet.
You therefore have a choice of using aerodrome QNH or aerodrome QFE but more importantly remember which one you have actually set. This will hopefully be repeated over and over during your training so I wouldn't be overly concerned.
IFR flights normally use aerodrome QNH and the pilots factor the aerodrome elevation into their calculations whereas most VFR flights tend to use aerodrome QFE within the vicinity of the aerodrome, which will show the height above the aerodrome.
Hope this helps
Joined: Jan 2008
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From: The Smaller Antipode
QFE is almost certainly only a UK phenomenon nowadays - but I could be wrong ?
My airfield is some 150 nm from the place where the pressure is read, and can be significantly different, so it would seem to make sense to use QFE in the local circuit, but travelling to another airfield - and most in NZ have no control tower - it would be impossible to know what QFE to set for the different pressure exerienced there, so we all use an "Area " QNH, and apply the field height from the aerodrome plate to ensure that we fly the circuit at 1,000 agl.at any airfield. That also ensures that we all fly around a fairly vast area of NZ using the same reference, for the same reason that commercial aircraft change to 'flight levels' based on them all having 1013.2 mb set on their individual altimeters.
If remaining in the circuit at my local field I will adjust the altimeter to read field elevation, i.e. 496 ft. and fly my circuits at 1500 ft. on the altimeter, as I can expect locally based non-radio aircraft to be doing just that. This usually results in an altimeter setting a few Mbs. removed from the Area QNH that I get when I call for a radio check from a controller who is actually hundreds of miles away, I guess that the setting I apply could be called my 'airfield QNH' although that term is not actually used. If my altimeter is within a couple of hundred feet of that shown by setting the Area QNH I know at least that I don't have a gross error in the instrument.
The Americans wouldn't know what you mean by QFE, they refer to 'Altimeter setting ' which is your QNH.
My airfield is some 150 nm from the place where the pressure is read, and can be significantly different, so it would seem to make sense to use QFE in the local circuit, but travelling to another airfield - and most in NZ have no control tower - it would be impossible to know what QFE to set for the different pressure exerienced there, so we all use an "Area " QNH, and apply the field height from the aerodrome plate to ensure that we fly the circuit at 1,000 agl.at any airfield. That also ensures that we all fly around a fairly vast area of NZ using the same reference, for the same reason that commercial aircraft change to 'flight levels' based on them all having 1013.2 mb set on their individual altimeters.
If remaining in the circuit at my local field I will adjust the altimeter to read field elevation, i.e. 496 ft. and fly my circuits at 1500 ft. on the altimeter, as I can expect locally based non-radio aircraft to be doing just that. This usually results in an altimeter setting a few Mbs. removed from the Area QNH that I get when I call for a radio check from a controller who is actually hundreds of miles away, I guess that the setting I apply could be called my 'airfield QNH' although that term is not actually used. If my altimeter is within a couple of hundred feet of that shown by setting the Area QNH I know at least that I don't have a gross error in the instrument.
The Americans wouldn't know what you mean by QFE, they refer to 'Altimeter setting ' which is your QNH.
Joined: Aug 2000
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From: UK
The Americans wouldn't know what you mean by QFE, they refer to 'Altimeter setting ' which is your QNH.
Joined: Feb 2007
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From: Amsterdam
If I read the OP correctly, I think the context would be different and the question would not be so much the difference between (aerodrome) QNH and QFE, but between aerodrome and area QNH.
The aerodrome QNH and QFE have been answered sufficiently. I would only add that these numbers are updated anytime that that's required, multiple times per hour if necessary.
The area QNH is the lowest QNH that would be predicted over the course of an hour, somewhere in the area concerned. Why the lowest and not an average or the highest predicted? Simply because using the lowest QNH gives the most safety buffer over terrain.
So on any given flight you would use at least three different datums:
- The aerodrome QNH or QFE of the departing aerodrome.
- One or more area QNHs once leaving the immediate vicinity of the departure aerodrome, OR the standard setting of 1013.2 if flying above the TA.
- And lastly the aerodrome QNH or QFE of the destination aerodrome.
Note that normally an area QNH will be the same as, or lower than the aerodrome QNH of an aerodrome in that area. In any case, it can never be higher unless there is a major prediction error.
It is important to understand the differences between all these datums and to understand why they change and may differ from each other, and even calculate one from the other. And then you can even elect to set a different datum than what what is normally required if that suits your style of flying. The NZ example above was very interesting (what do you do if the only "official" QNH is one that's taken 150 nm away?) but I'll give you two more:
We have an annual spot landing competition. As part of this competition, the instructor in the airplane will also look at the neatness and tightness of your circuit, including altitude keeping in the circuit. As the circuit is defined at 700' or 1000' AGL, we set QFE on the altimeter instead of QNH, which is normally used when flying circuits in the Netherlands.
My aerobatics training area is in the Rotterdam TMA which goes from 1500' to FL55. When flying there I keep the altimeter set on the area QNH and read the FL from the transponder. It is very awkward to adjust the altimeter from QNH to 1013 and back every time you spin or loop or zoom through the TA/TL (which is at 3000' over there).
The aerodrome QNH and QFE have been answered sufficiently. I would only add that these numbers are updated anytime that that's required, multiple times per hour if necessary.
The area QNH is the lowest QNH that would be predicted over the course of an hour, somewhere in the area concerned. Why the lowest and not an average or the highest predicted? Simply because using the lowest QNH gives the most safety buffer over terrain.
So on any given flight you would use at least three different datums:
- The aerodrome QNH or QFE of the departing aerodrome.
- One or more area QNHs once leaving the immediate vicinity of the departure aerodrome, OR the standard setting of 1013.2 if flying above the TA.
- And lastly the aerodrome QNH or QFE of the destination aerodrome.
Note that normally an area QNH will be the same as, or lower than the aerodrome QNH of an aerodrome in that area. In any case, it can never be higher unless there is a major prediction error.
It is important to understand the differences between all these datums and to understand why they change and may differ from each other, and even calculate one from the other. And then you can even elect to set a different datum than what what is normally required if that suits your style of flying. The NZ example above was very interesting (what do you do if the only "official" QNH is one that's taken 150 nm away?) but I'll give you two more:
We have an annual spot landing competition. As part of this competition, the instructor in the airplane will also look at the neatness and tightness of your circuit, including altitude keeping in the circuit. As the circuit is defined at 700' or 1000' AGL, we set QFE on the altimeter instead of QNH, which is normally used when flying circuits in the Netherlands.
My aerobatics training area is in the Rotterdam TMA which goes from 1500' to FL55. When flying there I keep the altimeter set on the area QNH and read the FL from the transponder. It is very awkward to adjust the altimeter from QNH to 1013 and back every time you spin or loop or zoom through the TA/TL (which is at 3000' over there).
