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-   -   QNH or QFE ? (https://www.pprune.org/private-flying/611731-qnh-qfe.html)

double_barrel 1st Aug 2018 09:04

QNH or QFE ?
 
Excuse the probably daft question, but. When I hear 'altimeter nnnn' or read 'Qnnnn' in a METAR can I always assume that is QNH?

I have not heard a value being explicitly stated as QNH by ATIS, METAR or ATC. I read that QFE may be provided, but I have never encountered it. How would QFE be described in an ATIS transmission? Presumably it is just a fixed offset from QNH?


Thanks

Discorde 1st Aug 2018 09:57

Most countries (and most pilots) do not use QFE. METAR, TAF, VOLMET and ATIS data refer to QNH (or simply 'altimeter') in the US.

Fl1ingfrog 1st Aug 2018 10:06

I'm not sure from where you are obtaining your information. QFE is no longer the standard (with the exception of the UK military). All pressures provided in official reports by the Met Office or ATC will be QNH and this is the case in a METAR. In addition to the qualification "QNH" the associated term "altitude" is used.

Within the UK and France the pilot may request a QFE from ATC which will be provided and the associated term "height" will be used. Many small UK airfields that do not have ATC but provide for an A/G service QFE is still commonly used but this is slowly changing. UK Helicopter operators usually prefer QFE because it is particularly valuable for setting down on platforms aboard ships and oil rigs etc. I don't know if this is always the case within the UK or elsewhere.

Where pressure is given in a report or passed by ATC the qualification "QNH" or "QFE" always precedes the actual pressure.

xrayalpha 1st Aug 2018 11:22

Double Barrel,

You don't say which country you are in.

Basically, only the UK - as far as I am aware - really uses QFE.

For one thing, if you were on high ground you won't be able to wind the altimeter down far enough to get it to read QFE! Even in the UK, with Strathaven being 847ft AMSL, we can have this problem. I have even heard that Loganair one time in a deep winter low couldn't even set QNH flying out of Orkney or Shetland one winter, but that is another story!

The Glasgow ATIS, for example - you can listed to it on 0141 887 7449 - specifically gives QNH. There is no mention of QFE. As you say, it is a fixed offset for QFE and is easy to calculate.

The British Microlight Aircraft Association PPL syllabus suggests - but doesn't mandate - the use of QFE on General Skills Tests. Perhaps not a surprise since, as mentioned, it can be impossible to select at times here at Strathaven! So even in the UK it is optional - although you should learn what it is and be able to calculate it for ground exams.

double_barrel 1st Aug 2018 12:37

Thanks all. That makes perfect sense. I am not in the UK, but I am using UK-centric reading material. I will stop worrying about QFE! I mis-remembered what I had heard on the local ATIS - it does indeed specify QNH. The various local 'controllers' - towers and approaches seem to use a mix of QNH and the word altimeter

Jan Olieslagers 1st Aug 2018 21:06

Forget about QFE, it is indeed one more UK peculiarity. OTOH "altimeter setting" is something I never heard (which does not mean all that much, I am not a veteran at all), I think it is US'an parlance. In Belgium, France, Germany, I never heard anything but "QNH is so and so much". Perfectly useable.

India Four Two 2nd Aug 2018 02:00

“Altimeter setting” or just “Altimeter” is universally used in the US and Canada. Most pilots here have never heard the term QNH or any other of the Q codes.

MarkerInbound 2nd Aug 2018 03:25


Originally Posted by xrayalpha (Post 10211849)
Basically, only the UK - as far as I am aware - really uses QFE.

For one thing, if you were on high ground you won't be able to wind the altimeter down far enough to get it to read QFE! Even in the UK, with Strathaven being 847ft AMSL, we can have this problem. I have even heard that Loganair one time in a deep winter low couldn't even set QNH flying out of Orkney or Shetland one winter, but that is another story!
.

Russia, China and some of the -stans still use QFE. Will normally be able to provide QNH to non-local aircraft.

American Airlines used QFE until the late 80s/early 90s. The crews converted QNH to QFE. I heard one Captain had converted a power drill to set the altimeter going into Mexico City.

double_barrel 2nd Aug 2018 05:40


Originally Posted by MarkerInbound (Post 10212374)
...... I heard one Captain had converted a power drill to set the altimeter going into Mexico City.

Spluttered my coffee onto keyboard.....

Pontius 2nd Aug 2018 05:49


“Altimeter setting” or just “Altimeter” is universally used in the US and Canada. Most pilots who are too lazy to read the AIM here have never heard the term QNH or any other of the Q codes
Adjusted at no cost :ok:


Russia, China and some of the -stans still use QFE.
China uses QNH (in Hectopascals).


I heard one Captain had converted a power drill to set the altimeter going into Mexico City.
That's funny :}

Thud105 2nd Aug 2018 09:32

Q-Codes;- a brilliant, ground-breaking idea - in 1909. Possibly not that relevant, or even useful, more than a century later? Discuss.
QFE? Ridiculous.

Discorde 2nd Aug 2018 09:42

Here's a thread which started almost 10 years ago:

QFE who needs it?

BackPacker 2nd Aug 2018 09:44


Originally Posted by India Four Two (Post 10212351)
“Altimeter setting” or just “Altimeter” is universally used in the US and Canada.

"Altimeter bla bla bla" is similar to QNH in the sense that it refers to mean sea level pressure. The way it is calculated (non-temp corrected) is the same.

But in the US at least, it's expressed in inches (actually 1/100 inches) of mercury, instead of hPa. So "QNH 1013" is identical to "altimeter 2992". Yes, there are conversion charts and any modern altimeter intended for use worldwide has both subscales.

Discorde 2nd Aug 2018 09:51

It would be helpful if the US adopted hPa, to avoid situations such as:

'American Jet, descend altitude two thousand, QNH nine nine eight.'
'Amjet, descend altitude two thousand . . . er . . . altimeter . . . QNH . . . two nine nine eight.'

spekesoftly 2nd Aug 2018 11:05


Originally Posted by Discorde (Post 10212548)
It would be helpful if the US adopted hPa, to avoid situations such as:

'American Jet, descend altitude two thousand, QNH nine nine eight.'
'Amjet, descend altitude two thousand . . . er . . . altimeter . . . QNH . . . two nine nine eight.'

A good example of why, in the UK at least, the correct phraseology is:-

"American Jet descend to altitude 2000 feet QNH 998 hectopascals"

India Four Two 2nd Aug 2018 11:15


It would be helpful if the US adopted hPa ...
That would require changing a lot of altimeters - over 200,000. It's not going to happen.

2 sheds 2nd Aug 2018 12:53


The Glasgow ATIS, for example - you can listed to it on 0141 887 7449 - specifically gives QNH. There is no mention of QFE. As you say, it is a fixed offset for QFE and is easy to calculate.
No it isn't - it depends on prevailing temperature and pressure range.

2 s

BackPacker 2nd Aug 2018 13:48


Originally Posted by 2 sheds (Post 10212670)
No it isn't - it depends on prevailing temperature and pressure range.

(my bold)

Actually, QNH does NOT depend on the prevailing temperature. It is the airfield level pressure reduced to MSL based on ISA conditions. So in the calculation from airfield-level pressure (which is measured) to QNH, an ISA temperature and temperature profile is assumed. Why? Because your altimeter is also not temperature aware/compensated. So these two errors cancel each other out, and with the correct QNH set, your wheels touch the runway when the altimeter reads airfield/threshold elevation.

There is another Q-code, QFF, where the actual temperature - and even the actual humidity - is taken into account when converting the observed airfield level pressure to the MSL level pressure. So QFF would be the pressure at the bottom of the hole, when you were to dig a hole at the airfield down to MSL. But QFF is fortunately not used in aviation, since the altimeters would need to be temperature compensated: You would not only need to enter the QFF on the subscale, but also the airfield temperature and humidity to ensure your altimeter reads threshold elevation when you touch down. (And note that even QFF makes a few assumptions that may not be valid. One of which is that the temperature lapse rate from airfield elevation to MSL is according to ISA.) https://en.wikipedia.org/wiki/QFF

This means that the difference between QNH and QFE only depends on the airfield elevation. In that sense, it is a fixed value per airfield (although the way the rounding works, there may occasionally be a +/- 1 hPa difference, I guess).

MarkerInbound 2nd Aug 2018 14:50


China uses QNH (in Hectopascals).

Some of the larger airports with western traffic use QNH, I got QFE from the tower at Wuxi last month. Since the airport elevation is 16 feet and the altimeter is marked off in 20 foot chunks we didn't worry about it.

BossEyed 2nd Aug 2018 16:20


Originally Posted by MarkerInbound (Post 10212374)
I heard one Captain had converted a power drill to set the altimeter going into Mexico City.

That would require the altimeter sub-scale to go down to below 770 hPa / 23 inHg. This is my sceptical face. :8


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