QFE/QNH and ILS

[ukhomerj]

Why do commercial aircraft obtain the QNH on a final approach, yet when I am flying for my PPL, we ask for a QFE?

Also, why do the ATIS give warnings of posible problems with the ILS approach during times of high pressure?

I am sure there is an obvious reason for this.

Thanks

[READY MESSAGE]

It's to do with missed approaches, on the go around the published procedures (and/or ATC) will give you an altitude and not a height. It is perfectly acceptable to fly an ILS on QFE so your altimeter reads zero on touchdown but should you need to execute a go around then resetting the altimeter from QFE to QNH is one more thing to think about when you are already working your a**e off. It's very easy to forget it too and we all know the risks of flying around with the wrong pressure setting set.

Just remember when flying an approach on QNH that you'll hit the runway before the altimeter reads zero!!

That's prob just one small reason, but the one I always use!!

Happy ILSs whether QNH or QFE!!

RM

[Notso Fantastic]

Homer- we gave up using QFE umpteen years ago. I want to know how high I am and how high the terrain is around me! Once you are away from the runway, how high you are above the runway is of academic interest only! When you fly into places like Addis Ababba (8000'+), winding to QFE took a pilot out for too long! Nairobi and Joburg (4500'), even Madrid-2000'. Just not worth it. Official procedure is right to stick to QNH for airline ops.

[oxford blue]

QFE is often used when students are just starting flying because at PPL level you get shown how to do a circuit at your home base and you learn the standard circuit heights (ususally something like - start the climbing turn at 700 feet, fly 1000 feet downwind, start reducing height during the baseleg turn to about 700 feet and be lined up at about 500 feet on finals (OK, different instructors may have different figures to these, but they're all roughly the same).

Once you've learnt these for your home airfield for QFE, if you then go somewhere else for some circuit flying or a landaway, you can use the same figures at other airports, whereas if you use QNH you have to add the airfield elevation to these figures and so they will be different for airfields which are not at the same elevation. When you are just beginning, this is a bit more to remember.

The disadvantage of QFE is that hazards and obstacles in the area surrounding the airport are given on area maps and are referenced to Mean Sea Level, ie, QNH. So once you get out of the circuit, you need to be on QNH for terrain avoidance. With light training aircraft, there's usually plenty of time to re-set from QFE to QNH on climbout from the circuit, but in an airliner, where you are more often flying in bad weather and where things happen more quickly, there is a danger of forgetting to reset to QNH, so they stay on QNH the whole time. They don't very often fly visual circuits, so they don't need standard heights and with 2 pilots, they can brief on the particular altitudes required at individual airports.

As for the ATIS warning, I don't know unless you tell me which airport we are talking about. I doubt if it's an altimetry problem. It may well be something to do with RF propagation.

[Tinstaafl]

QFE usage seems to be UK thing. Oz & USA use QNH (& area QNH but same principle).


QFE provides altitude information around a single geographic point but fails to provide a means of comparing terrain to a/c altitude. Handy (I suppose) for circuits, but certainly not necessary.

QNH provides a common reference point for all purposes: terrain separation, circuit operations, transitting traffic, missed approach requirements etc etc. The only downside is having to remember aerodrome elevation & adding the appropriate circuit altitude to it. Even that's usually rounded to the nearest 100' so hardly onerous.

[Lucifer]

Note that although QNH is used for all commercial airlines, the RAF still use QFE - justifications behind both vary.

[saudipc-9]

I've flown into airports where the slope of the runway resulted in over 50' difference between one end and the other. QNH in this case is clearly the better option if flying an ILS. One end might be 200' AGL but the other would be 150' AGL but both would read 200' on the altimeter.
The pressure problems I think go to the limits of the altimeter. In very high pressure situations you might not be able to set the altimeter to read zero when flying QFE. I'll post what that limit is later I cannot think of it right now. Anyone else pull that number out? It exsists for minimum enroute altitudes above a certain altimeter settings.

[oxford blue]

As Lucifer says, the RAF prefer QFE (for training, fighter, and bomber aircraft), though their transport fleet uses QNH for compatibility with airline operations. I personally prefer QFE but, being ex-RAF, I'm used to it. I do agree that QNH is preferable for civilian airport line-orientated operations.

Having a different runway elevation isn't a problem because ATC will give you a different QFE for each end. If the elevations were 50 feet apart, there'd be 2 hectopacals difference in the QFEs.

[Dufwer]

I don't have the books in front of me to confirm, which means I'm probably wrong, but isn't there something in the Air Law books (CAA) that states IFR flights have to have at least one altimeter set to QNH while in controlled airspace?

[eyeinthesky]

Further to Saudipc's note about low pressure:
There is also QNE. This is what the altimeter will read in feet on touchdown on the relevant threshold with the altimeter set to 1013.2 (standard pressure setting) and can be given out when the QFE is so low that it cannot be dialled up on an altimeter. It is calculated from the QFE (threshold or aerodrome) and then a table is used to make the conversion.

Seems simpler to land QNH (unless of course that is also too low to be selected).

[Sonia767]

Hey Ready Message,


Just remember when flying an approach on QNH that you'll hit the runway before the altimeter reads zero!!

Absolutely correct except in AMSTERDAM :ok:

Sonia

[Brit312]

When the VC-10 was operated by BOAC it was a QFE operation, but when landing at places such as Nairobi it was impossible to set the QFE as the millibar scale did not go that low, so you had to use QNH for high level airfields . OH dear
Reading an accident report recently on the IL-76 mid air collision near Delhi the QNH was given as 1011, and the Russian navigator converted it to a setting of 758. Has anybody got any idea what units they are using for this setting

[ETOPS]

ukhomerj

One reason the ATIS might warn of ILS problems in high pressure weather is a VHF "quirk" called transducting .

This allows normally shortrange VHF signals to travel unexpectedly long distances as they can be refracted in the atmosphere and bounced along like HF signals. A common example was the old Manchester app freq of 119.4 shared with Dusseldorf. On normal days there were no problems, but if a large high settled over Europe you could clearly hear MAN app whilst descending over Botrop near Essen!

The ILS (VHF loc signal) you refer to might share it's frequency with another distant airfield - you can guess the rest..........

[Brit312]

Thanks Mike, I coudn't agree more. Anyway thanks again

[Georgeablelovehowindia]

Brit312, to my recollection BOAC's VC-10s (and possibly everyone elses) had silly altimeters where it was indeed possible to set the QFE for NBO and JNB. There was a fold-out handle on the baro knob to allow the setting to be wound off rapidly. I have spoken to a friend of mine tonight who thinks he landed a VC-10 on the QFE going into Mexico City (7350 ft. or approx. 740 mb.). The highest airfield they operated into was Bogota (8370 ft. or approx. 700 mb.) I wonder if the altimeter could cope with this; they certainly had to fit special high speed tyres, I do remember that.

BOAC had two incidents at NBO where there was the classic QFE/QNH fankle. The first was a Comet which actually grazed the ground several miles out. Decades later, a 747 under the command of a highly-respected training captain, was experiencing problems coupling on to the ILS. (The bushes and sundry wild animals were in the way.) The flight engineer glanced out and with huge presence of mind said the correct thing: 'PULL !!' or something like that. History then relates that having recovered the situation, landed, taxied in and shut-down, the captain then said 'Gentlemen, I hereby suspend all of us.' They subsequently flew home as passengers to face the music and I think in the captain's case it was a sad end to an illustrious career.

This latter incident may have brought about the use of QNH for takeoff and landing in BOAC and then BA.