QNH/QFE?
Thread Starter
QNH/QFE?
Excuse my ignorance if this question is blindingly out of order (I am but a simple mil/private flying person)..............
I have been trying to understand a feature of a future mil platform that has me confused. I have always been used to flying on QFE for approach, either when flying purely VFR or from the FAF. I understand (from looking at the specs for an offered civil FMS being used in a couple of future mil platforms) that you civil guys don't ever use QFE and that as a consequence civil FMS do not even include the ability to enter QFE.
First off, is it actually true that a modern civil FMS has no dedicated provision for entering QFE? (other than the "bodge" approach of entering QFE in the QNH labelled entry). I've no personal experience of flying an FMS equipped aircraft, being a strictly steam instrument person.
If this is the case, does it cause any problems when flying a manual VFR approach if the alt doesn't read height above airfield altitude? (my guess here is that it is something you adapt to, just like our us mil colleagues who fly around like this all the time).
The final question has to do with GPWS (or in this case EGPWS) height settings. I assume that a GPWS that isn't triggered by a rad alt signal must rely on an accurate pressure setting. If this is the case, how do they cope if you always fly on QNH? Don't you get a lot of false GPWS alerts? (or perhaps risk being closer to the ground than you might wish before getting one.......).
Sorry again if these seem daft questions, but I have no experience of flying heavy stuff and am guessing that this forum has more than a few experts in the subject.
Thanks in anticipation,
VP
I have been trying to understand a feature of a future mil platform that has me confused. I have always been used to flying on QFE for approach, either when flying purely VFR or from the FAF. I understand (from looking at the specs for an offered civil FMS being used in a couple of future mil platforms) that you civil guys don't ever use QFE and that as a consequence civil FMS do not even include the ability to enter QFE.
First off, is it actually true that a modern civil FMS has no dedicated provision for entering QFE? (other than the "bodge" approach of entering QFE in the QNH labelled entry). I've no personal experience of flying an FMS equipped aircraft, being a strictly steam instrument person.
If this is the case, does it cause any problems when flying a manual VFR approach if the alt doesn't read height above airfield altitude? (my guess here is that it is something you adapt to, just like our us mil colleagues who fly around like this all the time).
The final question has to do with GPWS (or in this case EGPWS) height settings. I assume that a GPWS that isn't triggered by a rad alt signal must rely on an accurate pressure setting. If this is the case, how do they cope if you always fly on QNH? Don't you get a lot of false GPWS alerts? (or perhaps risk being closer to the ground than you might wish before getting one.......).
Sorry again if these seem daft questions, but I have no experience of flying heavy stuff and am guessing that this forum has more than a few experts in the subject.
Thanks in anticipation,
VP
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Many years ago most civil operators were using QFE for approach. The problem with QFE is that it is easy to forget to reset QNH on the pressure altimeter during a go around, giving terrain avoidance safety considerations. One by one the majority of UK operators had adapted to QNH approaches by the end of the nineties. All QNH altitudes are printed in bold on most approach plates and QFE heights are usually printed but ignored.
The GPWS really doesn't come in to it as one is never lower to the ground on QNH than one would be on QFE anyway. A DH of 200 feet (QFE) on an ILS becomes a DA of 460 feet (QNH) if the threshold elevation is eg 260 feet.
I've tried not to get too far in to the depths of this to save confusing you. Hope this clears it up.
The GPWS really doesn't come in to it as one is never lower to the ground on QNH than one would be on QFE anyway. A DH of 200 feet (QFE) on an ILS becomes a DA of 460 feet (QNH) if the threshold elevation is eg 260 feet.
I've tried not to get too far in to the depths of this to save confusing you. Hope this clears it up.
Thread Starter
Thanks Ojuka.
Having worked through all the implications it seems that if everyone, civil and military, standardised on using QNH (as it seems the civil air transport world already has) life would be a bit simpler (and probably safer as a well).
I suspect that the real issue behind the question I've been asked is driven by a resistance to change an ingrained culture to use QFE when approaching/departing. This doesn't seem to be a valid argument as far as I can see. As you've pointed out. it's no great shakes to adapt to thinking in terms of altitude rather than height above a local ground reference.
Cheers,
VP
Having worked through all the implications it seems that if everyone, civil and military, standardised on using QNH (as it seems the civil air transport world already has) life would be a bit simpler (and probably safer as a well).
I suspect that the real issue behind the question I've been asked is driven by a resistance to change an ingrained culture to use QFE when approaching/departing. This doesn't seem to be a valid argument as far as I can see. As you've pointed out. it's no great shakes to adapt to thinking in terms of altitude rather than height above a local ground reference.
Cheers,
VP
Departures are normally done in QNH, QFE is reserved for arrivals and transits.
Having flown both systems a lot- as you say it's no great shakes, just requires a little more mental arithmetic.
G
Having flown both systems a lot- as you say it's no great shakes, just requires a little more mental arithmetic.
G
I agree with everything said here. I have now been flying QNH for many years without a problem.
I have always beleived that the biggest ground proximity warning a pilot will ever receive is the altimeter winding down to zero, so I like QFE.
Having said that I have flown rather too many approaches with QNH set when I should be using QFE, but have never flown one with the wrong setting on when using QNH.
I think QNH has the vote.
I have always beleived that the biggest ground proximity warning a pilot will ever receive is the altimeter winding down to zero, so I like QFE.
Having said that I have flown rather too many approaches with QNH set when I should be using QFE, but have never flown one with the wrong setting on when using QNH.
I think QNH has the vote.
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I have always beleived that the biggest ground proximity warning a pilot will ever receive is the altimeter winding down to zero, so I like QFE.
Add to that doubling the work in setting/crosschecking altis in the descent/approach, plus resetting on the go-around, doubling the possibility of mis-setting them all add up to the fact that I CANNOT understand why QFE operators (including the military) hang on to QFE.
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First off, is it actually true that a modern civil FMS has no dedicated provision for entering QFE? (other than the "bodge" approach of entering QFE in the QNH labelled entry). I've no personal experience of flying an FMS equipped aircraft, being a strictly steam instrument person.
Many years ago, I was in a glider club that used QFE for all local ops (the airport was less than 200' MSL). It made sense there (no "sensitive" altimeters), but I still do not see the sense of it for air carrier operations now.
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The "countdown to zero" on aircraft using FMS and others is given by a radalt, which works on the actual surface beneath the aircraft. This is obviously much better than a possible false reference of a barometric pressure which is incorrect as soon as the aircraft moves away from the airfield.
I was used to QFE for many years in the RAF. There was an experimental change to QNH in the 1980s but it was only temporary. I think some bigwig frightened himself in an AEF Chipmunk and decreed a return to QFE.
I was used to QFE for many years in the RAF. There was an experimental change to QNH in the 1980s but it was only temporary. I think some bigwig frightened himself in an AEF Chipmunk and decreed a return to QFE.
Thread Starter
Thanks for the comments folks.
Shy Torque: Your absolutely right (except I can't confirm the apocryphal "Airship in a Chippie tale............"). I've advised the powers that be that in this day and age it makes sense to standardise on QNH, especially given that new transport types will have civil cockpits in most other respects.
I've informally asked a few of the mil pilots in my team, all of whom have said that they can't see a problem in switching to QNH. Given that the cost of providing special-to-type cockpit software for a few military versions of what is otherwise a standard civil fit is potentially pretty high (and presents safety critical software integity issues as well), it seems that it's a no-brainer to just change procedures.
VP
Shy Torque: Your absolutely right (except I can't confirm the apocryphal "Airship in a Chippie tale............"). I've advised the powers that be that in this day and age it makes sense to standardise on QNH, especially given that new transport types will have civil cockpits in most other respects.
I've informally asked a few of the mil pilots in my team, all of whom have said that they can't see a problem in switching to QNH. Given that the cost of providing special-to-type cockpit software for a few military versions of what is otherwise a standard civil fit is potentially pretty high (and presents safety critical software integity issues as well), it seems that it's a no-brainer to just change procedures.
VP
The QFE/QNH debate does not only affect air transport aicraft.
If you are en route in a general aviation aircraft in the open FIR and want a MATZ penetration its back to the infernal QFE again and then back to QNH when leaving the MATZ.
Over the years I have seen both QFE and QNH operation and I would come firmly down on the side of QNH.
Indeed in the USA the QNH is called THE altimeter setting - lets get rid of all the Q codes (originally used when aircraft used "Wireless Telegraphy"!) and whilst we are about it lets get rid of Regional QNHs in the UK!
If you are en route in a general aviation aircraft in the open FIR and want a MATZ penetration its back to the infernal QFE again and then back to QNH when leaving the MATZ.
Over the years I have seen both QFE and QNH operation and I would come firmly down on the side of QNH.
Indeed in the USA the QNH is called THE altimeter setting - lets get rid of all the Q codes (originally used when aircraft used "Wireless Telegraphy"!) and whilst we are about it lets get rid of Regional QNHs in the UK!
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I am a firm believer in QNH having used both systems over decades. It is, however, a fallacy to say that it is "standard". As soon as you venture East of our new enlarged Fourth Reich (sorry - that should say EU) then there is no such beast as QNH. All procedures involve flying Flight Levels above transition and QFE below. Some of the larger airports also report QNH for information, but smaller ones do not. Add to that the metric altitudes and much fun ensues!
Happy flying
Happy flying
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The 747-400 FOM states that it is not cleared for QFE operations.
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The B777 FMC makes provision for QFE operations (at least the one that I fly, other customer options may vary). Thank goodness we don't go anywhere where the QFE dinasour is still rampant, but we keep it on the books and in the software in case the commercial department dreams up some new destination where it prevails.
The potential for disaster with varying Altimetry procedures is enormous, the sooner that we can rid ourselves of QFE and Metric Altitudes, and make RVSM global the better. The only lingering problem then would be varying Transition Altitudes/Levels, but that one is tougher due to enormous variation in 'worst case' terrain in various regions across the world.
The potential for disaster with varying Altimetry procedures is enormous, the sooner that we can rid ourselves of QFE and Metric Altitudes, and make RVSM global the better. The only lingering problem then would be varying Transition Altitudes/Levels, but that one is tougher due to enormous variation in 'worst case' terrain in various regions across the world.
Firefly,
why would you get rid of regional QNH settings? I'd have thought that on the occassions that you get a zebra's backside passing through the country and you want to go from one end to the other, it might be nice to stay updated with pressure changes.
One point not mentioned in the QFE argument is that there are some place in the world where the elevation precludes the QFE being settable on the altimeter. Not a problem if you always fly QNH for the approach.
VP, you asked about EGPWS; they have their own GPS which gives true height.
why would you get rid of regional QNH settings? I'd have thought that on the occassions that you get a zebra's backside passing through the country and you want to go from one end to the other, it might be nice to stay updated with pressure changes.
One point not mentioned in the QFE argument is that there are some place in the world where the elevation precludes the QFE being settable on the altimeter. Not a problem if you always fly QNH for the approach.
VP, you asked about EGPWS; they have their own GPS which gives true height.
Not necessarily so 212man – “EGPWS; they have their own GPS which gives true height.”
A). Not all EGPWS installations use or have GPS embedded; shame on those operators who don’t. Beware FMS / IRS drift!
B). Those EGPWS that do use GPS still require the ‘geometric altitude’ software update. Most updates are free, as are all revisions to the terrain and obstacle databases. See www.egpws.com Operation/pilots guides, General info/operational updates, and What’s new/new press release ‘getting the job done’.
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Unless specifically authorized everything else is forbidden.
A). Not all EGPWS installations use or have GPS embedded; shame on those operators who don’t. Beware FMS / IRS drift!
B). Those EGPWS that do use GPS still require the ‘geometric altitude’ software update. Most updates are free, as are all revisions to the terrain and obstacle databases. See www.egpws.com Operation/pilots guides, General info/operational updates, and What’s new/new press release ‘getting the job done’.
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Unless specifically authorized everything else is forbidden.
I think you're incorrect. True altitude relates to the error in the presumed height of the air column from the surface to the altimeter. Doesn't matter what setting is set on the subscale.
Even though QNH is a longer column - going all the way to sea level instead of stopping at some elevated ground reference point as with QFE - the portion from the surface down to sea level has no temperature error.
This is because there isn't a non-standard column of air from the surface to S.L., just solid rock etc so the assumed height / pressure change relationship doesn't have a somewhat elastic air column to account for.
Even though QNH is a longer column - going all the way to sea level instead of stopping at some elevated ground reference point as with QFE - the portion from the surface down to sea level has no temperature error.
This is because there isn't a non-standard column of air from the surface to S.L., just solid rock etc so the assumed height / pressure change relationship doesn't have a somewhat elastic air column to account for.
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fanfree,
Tinstaafl is correct. QNH advised by a ground station (human or automated) compensates for not only pressure variation from standard, but temperature error at ground level. Although called QNH, this is actually QFF.
Both QFF and QFE will indicate correctly when the aircraft is on the ground (Elevation or Zero respectively), and temperature error when above the ground station will be IDENTICAL when either are used. QFE offers no advantages here.
Seasons Greetings,
Old Smokey
Tinstaafl is correct. QNH advised by a ground station (human or automated) compensates for not only pressure variation from standard, but temperature error at ground level. Although called QNH, this is actually QFF.
Both QFF and QFE will indicate correctly when the aircraft is on the ground (Elevation or Zero respectively), and temperature error when above the ground station will be IDENTICAL when either are used. QFE offers no advantages here.
Seasons Greetings,
Old Smokey