Difference between QFF and QNH

[ptwaugh]

Hi,

I'm in the US where these 'Q' codes are not common, and I want to better understand them.

I understand (I think):

QFE : Absolute pressure locally (usually at airport threshold).
ALT set with this pressure displays 0 on the field
(at the reference point, in ISA temp?)

QNH : Local barometric pressure at MSL (reported in METAR)
ALT set to this pressure displays Indicated Altitude
(true altitude if ISA conditions)

The references I have been looking at (WIKI & my old books) say that QFF is QFE reduced to MSL using station temperature used in weather reports (not METAR?), so that ISOBAR lines make sense given reporting stations different altitudes (and hence pressures). If this is so, then what is the difference between QFF and QNH?

Thanks for your expertise.

[chiglet]

QFE is "Aerodrome" pressure [At MAN/EGCC corrected for each runway threshold]
QNH is "Sea level pressure".
Manchester has a 9mb difference between QFE/QNH [Approx 270 ft]
Hope this helps
watp,iktch

[ptwaugh]

Considering it didn't answer the question of what QFF is, and what is the difference between QNH and QFF... nope :)

I understand QNH, and QFE, just not QFF. Since it is a weather term, perhaps this is the wrong place to ask this question.

[LH2]

Just giving it a wild shot, but it looks to me like the difference is in the reduction method. QNH uses ISA temp and pressure gradient, while QFF uses actual station temperature?

Or in other words, the QFF value tells you what the pressure at sea level would be (useful for plotting isobar charts), and the QNH value tells you what to dial on the altimeter so that it reads airfield elevation at the threshold when you land (useful for, obviously, landing a plane)

At least that is my understanding from reading the article on Atmospheric Pressure (http://en.wikipedia.org/wiki/Atmospheric_pressure) at the Wikipedia and this list of Q codes (http://www.kloth.net/radio/qcodes.php). Other than that, meteorology is not my strong point at all.

[ptwaugh]

looks to me like the difference is in the reduction method. QNH uses
ISA temp and pressure gradient, while QFF uses actual station temperature?

Or in other words, the QFF value tells you what the pressure at sea level would be (useful for plotting isobar charts), and the QNH value tells you what to dial on the altimeter so that it reads airfield elevation at the threshold when you land (useful for, obviously, landing a plane)


Well, given that QNH only gives you the airfield's true altitude AMSL in ISA conditions (temp.), it must use an ISA reduction to MSL.

Given that if you set zero(0) on the altimeter on the airfield you are reading absolute pressure at that elevation (hence QFE), this must use local station temperature.

So, given QFF is a reduction of QFE to MSL assuming isotheric conditions, I'm guessing that this is using station temperature as well (whereas QNH assumes ISA temp).

So, the difference appears to be the assumed temperature, with QFF using actual temperature, and QNH using ISA.

Hmmmm.... guess thinking about it more cleared it up. Thanks.

Given this, it would seem if you knew the QFF below, you could set your altimeter to it, and get true altitude (meaning adjusted for temp. vs. indicated not adjusted for temp) if the amtompheric temperature gradients were only guarenteed to be standard. =)

Interesting.

[boredcounter]

Is QFF SLP in the METARS?:ugh:

[Ian Brooks]

Some where I have a copy of all the Q codes in a book given to me by uncle from when he worked as a radio operator in air traffic back in 1940
so if you have any more queries I will try and help
Infact I have it now

Taken word by word from 1937 copy of Civil Aeronautical Radio Service
QFF. Question. What is the actual barametric pressure reduced to sea level at
???? airfield.

The book is in almost mint condition and was a price of 1 shilling

Ian

[LH2]

pt,

according to my Bristol ATPL books, #4 & #5 would be correct. Coincidentally, just happened to be having a read at the Met parts last night.

[cavortingcheetah]

QFF

[At ... (place)] what is the present atmospheric pressure converted to mean sea level in accordance with meteorological practice?

At ... (place) the atmospheric pressure converted to mean sea level in accordance with meteorological practice is (or was determined at ... hours to be) ... millibars.

And, should one wish to confusticate ATC even further; there are more priceles pearls to be found here.

http://www.kloth.net/radio/qcodes.php

Must QSY. Toodle Pip!:8

[Regular Cappuccino]

"Must QSY. Toodle Pip!"

So did you QRF whence you came......? Perhaps via QFG followed by a QGH to QAL? :ok:
(Sorry Mods - the Q code has probably been done death here previously)

Back to QFF / QNH however, at my unit, the ATC staff are also qualified Met Observers, and on the occasions that we perform manual pressure calculations, we add to the (corrected) 'as read' pressure another correction for the QNH. This correction is taken from a table supplied by the Met Office and varies according to the ambient OAT.
As stated above, I suspect that the difference between QFF & QNH is only in the methodology of the calculation, and the actual difference in the results is probably in the order of 0.1mb or so.
Yes, I realise that with rounding down, this could mean a whole 1mb difference in the reported pressure, but in 9 out of 10 cases however, the difference will probably be neglible and only of interest to a meteorological anorak (try saying that after a few pints....).
Only a thought, but could the reason we use QNH and not QFF to pass to aircraft be that QFE & QFF look and sound very similar, which might lead to the wrong pressure being set during an approach to land, whereas QFE & QNH both look and sound different (especially phonetically), thus minimising that risk?
RC

[Pitts2112]

The practical application, which I haven't seen mentioned here is that QNH gives you your altitude above sea level so you then have to account for the height of ground to figure your clearance. QFE is your height above the airfield itself. Normally only used approaching and in the pattern to take out having to do the arithmetic to calculate pattern altitude. This way, when you're on QFE and the pattern altitude is 800 feet, you can just shoot straight for 800 feet. If you're on QNH at the same airfield, and let's say it's 350 feet MSL, then you have to shoot for 1150 in the pattern.

On another note, I use QFE a lot here in the UK where there is much less variation in ground height. It's much easier to calculate my clearance when doing aerobatics in the local area.

Hope that helps.

Pitts2112

[ptwaugh]

No, the difference is very real. One is for weather, one for aviation.

QFF reduces QFE, which in the current ambient temperature gives you zero on the altimeter, to sea level assuming the station temp all the way to sea level (isothermic conditions), and is used just to normalize isobars so they make sense between sites with different altitudes.

QNH assumes non-isothermic conditions, based on ISA.

[rustyflyer]

Bit late to add to this one but I think you've all missed the point!
QFE = pressure at aerodrome used for landing & operation in "the circuit"
QFF = sea-level equivalent of above (my interpretation)
QNH = sea-level pressure for a given area or Altimeter Setting Region (ASR) eg Barnsley in NW England. this is the lowest pressure in said region. All en-route aircraft using this value will be singing from the same hymn sheet if they encounter one another thus avoiding the dangers of different altimeter settings.
None have anything to do with ISA they are current values.
QFF isn't something I've encountered in modern-day aviation. Having recently gained my PPL I have a reasonable grasp of the subject :ok:

[ptwaugh]

Thank you for wanting to help, but,

a) You missed the point. (Look at the original question again)
b) You are mistaken about ISA.
c) We have solved the original question.

ISA is not just a standard pressure, but a standard "atmoshpere", meaning it defines the pressure gradients in that atmosphere. That gradient is the "map" and not the "territory" of reality in the atmoshere on a given day.

We were understanding how they are calculated, and their physical meaning, not their use (with which we are intimately familar).

[rustyflyer]

Hi pt,
I was going at half-cock anyway.......
Of course QNH can be local or Regional!
Not sure what I was thinking (or not).
I'm now more confused & somewhat intrigued by the QFF value.
Hangs head in shame & shuffles off
:O:O:O

[ptwaugh]

Don't feel bad, it's very confusing (unless you are a meterologist I suppose).

As a pilot, you can basically ignore QFF, but know that it is what is used to create those nice ISOBAR lines on the all the weather maps you (hopefully) look during pre-flight.