QNH/QFE?

[VP959]

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

[idg]

VP Check your PM.

[VP959]

Thanks idg, much appreciated.

[Ojuka]

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.

[VP959]

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

[Genghis the Engineer]

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

[Miles Magister]

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.

[Captain Stable]

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.And if, as you go on to say, you fly the approach with QNH still set and you think you are looking at QFE, you don't get that ground proximity warning. Which is precisely why QNH approaches have to be standard. I think you have successfully argued for a preference for QNH.

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.

[Intruder]

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.
The 744 has a QFE entry in the FMS. However, when using QFE, VNAV and GPWS are not accurate, so VNAV must not be used and the GPWS overrides must be utilized.

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.

[ShyTorque]

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.

[VP959]

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

[fireflybob]

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!

[BizJetJock]

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:D

[Dan Winterland]

The 747-400 FOM states that it is not cleared for QFE operations.

[Intruder]

The 747-400 FOM states that it is not cleared for QFE operations.
FOMs and FHBs are airline-specific. Ours are cleared for QFE operation (though they weren't a couple years ago).

[Old Smokey]

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.

[212man]

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.

[alf5071h]

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’.
--------------------
Unless specifically authorized everything else is forbidden.

[Tinstaafl]

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.

[Old Smokey]

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

[Captain Stable]

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?

[P-T-Gamekeeper]

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.

[keithl]

Two points:
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?
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.

[Luftwaffle]

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.

[Blacksheep]

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?]

[Spitoon]

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.

[Genghis the Engineer]

Also without getting the books out, isn't QFE calculated for the centre of the primary runway?

G

[Blacksheep]

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?

[DFC]

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!

---

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

[fireflybob]

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!!

[Old Smokey]

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!

[WITCH]

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.

[Blacksheep]

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!"