QNH/QFE?
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Old Smokey and my old friend Tinny have it right. Sat on the ground with your subscale set to QNH it WILL show elevation above sea level (+15 feet or so - unless the alti is faulty in which case QFE will be no more accurate).
So, can anyone come up with a sensible reason still to use QFE? Unless you don't like having airfield elevation showing when you land 'cos you've lost all the bugs on the outside rim of the alti?
So, can anyone come up with a sensible reason still to use QFE? Unless you don't like having airfield elevation showing when you land 'cos you've lost all the bugs on the outside rim of the alti?
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Within the RAF, the main users of QFE are the fast jet boys, who normally take off/land from the same mil base, which uses QFE. Always landing at 0' is one less thing to think about for them after a v.v.v busy sortie with your brain melting.
When they are outside the circuit, they use QNH, so the terrain clearance argument does not wash.
RAF multi fleet generally use QNH, and we will try to fit in with local procedures, so as to avoid confusion. QNH is undoubtably the way ahead whenever terrain is a factor. For example, operating into Kabul, with high ground 13000' AAL, within 10 miles, and no approach aids/radar, knowing your ground clearance is vital.
A standard military T/O or app brief covers the use of QFE/QNH, airfield elevation and safety altitude, so this covers all bases.
When they are outside the circuit, they use QNH, so the terrain clearance argument does not wash.
RAF multi fleet generally use QNH, and we will try to fit in with local procedures, so as to avoid confusion. QNH is undoubtably the way ahead whenever terrain is a factor. For example, operating into Kabul, with high ground 13000' AAL, within 10 miles, and no approach aids/radar, knowing your ground clearance is vital.
A standard military T/O or app brief covers the use of QFE/QNH, airfield elevation and safety altitude, so this covers all bases.
Two points:
First,
Beware tunnel vision, Catain S - many of us don't have any of those things to start with.
Second point, as I've said in other discussions, I don't know why so many people get so excited about which is "best". In the RAF I have often taken off from home base using QFE, flown the route on SPS and landed somewhere like Nairobi, or the USA, on QNH. There's quite a good argument to be made that such practice makes you very very careful about what you are doing. Which is good.
First,
Unless you don't like having airfield elevation showing when you land 'cos you've lost all the bugs on the outside rim of the alti?
Second point, as I've said in other discussions, I don't know why so many people get so excited about which is "best". In the RAF I have often taken off from home base using QFE, flown the route on SPS and landed somewhere like Nairobi, or the USA, on QNH. There's quite a good argument to be made that such practice makes you very very careful about what you are doing. Which is good.
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QNH = altimeter setting
I teach meteorology and flight ops in Canada, where we always use QNH below FL180, and call it "the altimeter setting."
If I'm asked why we don't use QFE, I explain that it isn't physically possible to set the altimeter to zero at many airports, and that the British only use it because they live on a small flat island and don't know how to subtract.
Honestly, it quickly becomes second nature to subtract the aerodrome elevation from the indicated altitude. That winding down to zero bit won't help you much if you turn the wrong way in the missed, and hit a two thousand foot hill, anyway.
If I'm asked why we don't use QFE, I explain that it isn't physically possible to set the altimeter to zero at many airports, and that the British only use it because they live on a small flat island and don't know how to subtract.
Honestly, it quickly becomes second nature to subtract the aerodrome elevation from the indicated altitude. That winding down to zero bit won't help you much if you turn the wrong way in the missed, and hit a two thousand foot hill, anyway.
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QFE for field elevation, so the altimeter reads the height above the airfield and, ideally, ought to read zero at touchdown.
But what is this 'field elevation'? There's a 75 foot difference between the thresholds of our single runway and the tower's monitoring station is different from both thresholds. I've worked in places where the difference between the thresholds is as much as 180 feet. Then there's the question of how often the reading is updated,
GPWS/EGPWS calls out "Minimums" then the actual height during the final moments before touchdown anyway - "Fifty - Forty - Thirty - Twenty - Ten" - and can be pin-programmed to also call-out various other significant heights: there's around fifty different choices available to whoever sets the policy in your Flight Operations Department.
Not to mention "Bank Angle" and other choice criticisms of your approach technique...
[You did remember to put the gear down didn't you?]
But what is this 'field elevation'? There's a 75 foot difference between the thresholds of our single runway and the tower's monitoring station is different from both thresholds. I've worked in places where the difference between the thresholds is as much as 180 feet. Then there's the question of how often the reading is updated,
GPWS/EGPWS calls out "Minimums" then the actual height during the final moments before touchdown anyway - "Fifty - Forty - Thirty - Twenty - Ten" - and can be pin-programmed to also call-out various other significant heights: there's around fifty different choices available to whoever sets the policy in your Flight Operations Department.
Not to mention "Bank Angle" and other choice criticisms of your approach technique...
[You did remember to put the gear down didn't you?]
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Blacksheep, in the UK the QFE will be calculated to take account of the difference between the observation point and the aerodrome.
Without getting the books out, as I recall the QFE will be calculated for the highest point on the aerodrome and if the elevation of any threshold is 7ft or more less than the highest point on the aerodrome a threshold QFE will be calculated and passed by ATC if that runway is used.
The QFE will be updated at the time of a routine observation (typically every hour at least) or if a significant change in the ambient pressure occurs (i.e. more than 1.0 hPa) before the next routine observation is due.
As I say, this is UK practice.
Speaking as a controller, I don't really care which setting a pilot choses to use - I'll use the relevant procedures whatever. I recall when QNH landing was introduced by some operators - it seemed like a bit of a nuisance to have to remember more procedures but after a couple of weeks it became second nature.
Without getting the books out, as I recall the QFE will be calculated for the highest point on the aerodrome and if the elevation of any threshold is 7ft or more less than the highest point on the aerodrome a threshold QFE will be calculated and passed by ATC if that runway is used.
The QFE will be updated at the time of a routine observation (typically every hour at least) or if a significant change in the ambient pressure occurs (i.e. more than 1.0 hPa) before the next routine observation is due.
As I say, this is UK practice.
Speaking as a controller, I don't really care which setting a pilot choses to use - I'll use the relevant procedures whatever. I recall when QNH landing was introduced by some operators - it seemed like a bit of a nuisance to have to remember more procedures but after a couple of weeks it became second nature.
Cunning Artificer
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I do remember that while I was still employed (in the UK) as a hands-on basher of instruments, the QFE that you got from the tower seldom, if ever, bore any relation to the altimeter reading obtained. The surface pressure changes constantly and may vary by a millibar or two from one part of the field to another. If QFE only gives a more-or-less figure for altitude at touch-down why bother?
As to procedures, isn't it better to have everyone working to the same standard? An altimeter is calibrated to a 'standard' atmosphere and is supposed to indicate your barometric height above a standard pressure for MSL. Not your real height, mark you. A notional standard. Terrain clearance using the standard is then obtained by nominating a minimum safe clearance to allow for instrument errors and the difference between the standard atmosphere and the current version of the real world. So, why override these wise precautions when you descend and fly closer to the lumpy stuff?
I know its an extreme example, but when I worked in Kathmandu I knew that the lumpy bit on the regular approach from the south was around nine thousand feet above MSL and the threshold was around five thousand above MSL. If the altimeter were set to QFE and your minimum clearance is, say, two thousand feet, what should be on the clock five miles from touch down? OK, now we've worked it out, to avoid bumping into the nine thousand foot mountain that the radio altimeter cannot see until its too late, we'll fly at six thousand feet. Logical eh? Or would you prefer QNH and have eleven thousand on the clock until you can see KTM?
Human Factors. Why ask for trouble?
As to procedures, isn't it better to have everyone working to the same standard? An altimeter is calibrated to a 'standard' atmosphere and is supposed to indicate your barometric height above a standard pressure for MSL. Not your real height, mark you. A notional standard. Terrain clearance using the standard is then obtained by nominating a minimum safe clearance to allow for instrument errors and the difference between the standard atmosphere and the current version of the real world. So, why override these wise precautions when you descend and fly closer to the lumpy stuff?
I know its an extreme example, but when I worked in Kathmandu I knew that the lumpy bit on the regular approach from the south was around nine thousand feet above MSL and the threshold was around five thousand above MSL. If the altimeter were set to QFE and your minimum clearance is, say, two thousand feet, what should be on the clock five miles from touch down? OK, now we've worked it out, to avoid bumping into the nine thousand foot mountain that the radio altimeter cannot see until its too late, we'll fly at six thousand feet. Logical eh? Or would you prefer QNH and have eleven thousand on the clock until you can see KTM?
Human Factors. Why ask for trouble?
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Without getting the books out, as I recall the QFE will be calculated for the highest point on the aerodrome and if the elevation of any threshold is 7ft or more less than the highest point on the aerodrome a threshold QFE will be calculated and passed by ATC if that runway is used
The elevation for which the QFE is given is the highest point on the runway(s). Thus landing on an undulating aerodrome would have the altimeter reading zero or less depending on what part of the runway(s) was the highest.
The use of a threshold QFE only applies to instrument runways and for straight in approaches.
This complicates things further because there will be a different QFE required if one cancells a straight in approach late in favour of a circling (say a wind shift)!.....more cheese holes lining up!
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Blacksheep, it is impossible to set the Kathmandu QFE on most altimeters so the argument does not apply.........other than the posibility of having a pilot who normally lands with the alitimeter reading zero forgetting that that arrival will happen at 5000ft.....but I guess that the nearby mountains would prevent wreckage from reaching the runway!
Regards,
DFC
The elevation for which the QFE is given is the highest point on the runway(s). Thus landing on an undulating aerodrome would have the altimeter reading zero or less depending on what part of the runway(s) was the highest.
The use of a threshold QFE only applies to instrument runways and for straight in approaches.
This complicates things further because there will be a different QFE required if one cancells a straight in approach late in favour of a circling (say a wind shift)!.....more cheese holes lining up!
---
Blacksheep, it is impossible to set the Kathmandu QFE on most altimeters so the argument does not apply.........other than the posibility of having a pilot who normally lands with the alitimeter reading zero forgetting that that arrival will happen at 5000ft.....but I guess that the nearby mountains would prevent wreckage from reaching the runway!
Regards,
DFC
In the UK the Aerodrome QFE is based on the Aerodrome elevation - ie the highest point on the manoeuvring area.
A threshold QFE is passed, in the case of an instrument runway, when the touchdown elevation is 7 feet or more below the aerodrome elevation.
Other States may have different criteria. Last time I looked in Greece the QFE is based on the elevation of the aerodrome reference point (usually about the middle of the main runway).
Regarding the QFE/QNH debate which is almost as old as aviation itself, having used both I definitely prefer QNH - far less winding on the altimeter and it breeds a lot more awareness of terrain elevation. Having seen many an altitude bust in the sim on Go Arounds when QNH has been set late (especially at higher elevation airports) or not at all, I think this is yet another reason for QNH operation.
That said what is really important is that the altimeters are being managed correctly whatever system you are using.
Also the altimeter tolerance check prior to flight should always be conducted on QNH (and not QFE) - the UK AIP lists apron elevations specifically for this purpose. At the general aviation level I have seen many a pilot attempt to conduct the altimeter tolerance check on QFE only to discover it is outside the prescribed limits because of the difference in elevation between the aerodrome elevation (assuming aerodrome QFE in this case) and the actual elevation of where the aircraft is standing. This is further compounded by some AG/FIS who only pass QFE when pilots ask for taxi and state their detail is circuits. They dont seem to understand that you need the QNH to check the altimeter for tolerance even if you are subsequently going to set QFE whilst in the circuit!!
A threshold QFE is passed, in the case of an instrument runway, when the touchdown elevation is 7 feet or more below the aerodrome elevation.
Other States may have different criteria. Last time I looked in Greece the QFE is based on the elevation of the aerodrome reference point (usually about the middle of the main runway).
Regarding the QFE/QNH debate which is almost as old as aviation itself, having used both I definitely prefer QNH - far less winding on the altimeter and it breeds a lot more awareness of terrain elevation. Having seen many an altitude bust in the sim on Go Arounds when QNH has been set late (especially at higher elevation airports) or not at all, I think this is yet another reason for QNH operation.
That said what is really important is that the altimeters are being managed correctly whatever system you are using.
Also the altimeter tolerance check prior to flight should always be conducted on QNH (and not QFE) - the UK AIP lists apron elevations specifically for this purpose. At the general aviation level I have seen many a pilot attempt to conduct the altimeter tolerance check on QFE only to discover it is outside the prescribed limits because of the difference in elevation between the aerodrome elevation (assuming aerodrome QFE in this case) and the actual elevation of where the aircraft is standing. This is further compounded by some AG/FIS who only pass QFE when pilots ask for taxi and state their detail is circuits. They dont seem to understand that you need the QNH to check the altimeter for tolerance even if you are subsequently going to set QFE whilst in the circuit!!
Last edited by fireflybob; 11th Jan 2005 at 23:22.
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QFE is nice for student pilots doing visual circuits, and not venturing more than 4 or 5 miles from their base airport (I used it as an ab-initio student pilot at an 800 ft AMSL aerodrome).
Beyond that, all elevations, MSAs, MEAs, LSALTs, obstacle elevations, every damned thing, is published as elevation AMSL, and QFE is totally innapropriate.
QFE is an archaic dinasour, and belongs with all of the other dinasours, in the museum.
A pox on QFE!
Beyond that, all elevations, MSAs, MEAs, LSALTs, obstacle elevations, every damned thing, is published as elevation AMSL, and QFE is totally innapropriate.
QFE is an archaic dinasour, and belongs with all of the other dinasours, in the museum.
A pox on QFE!
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Don't tell me the QFE is still in use! Combined with Human Factors, QFE is a killer. I have personally observed, at an aerodrome 600ft amsl, with a circuit height of 800ft aal (QFE), an aircraft make a standard circuit rejoin at 800ft QNH. Result? Fatal mid-air with a departing aircraft just airborne, one dead pilot, and two others injured.
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I did say the example was extreme DFS - the air is certainly a bit thin in KTM. Perhaps that's why the 'grass' always seems stronger up there and it remains a popular place with aged sky-pilots? And yes, even with QNH the big lumpy bits do stop the wreckage reaching the runway - there's tons of aluminium confetti scattered on the far side.
Nevertheless, my example does illustrate why QFE isn't suitable as a standard; and even for students going round and round the field in circuits, there are hidden dangers - as WITCH just pointed out.
Old Smokey says it all - "A pox on QFE!"
Nevertheless, my example does illustrate why QFE isn't suitable as a standard; and even for students going round and round the field in circuits, there are hidden dangers - as WITCH just pointed out.
Old Smokey says it all - "A pox on QFE!"
