QFF=QNH at MSL, question

[OneIn60rule]

Can anyone give me a good EXPLANATION.


The QNH is 1022, the temp. is unknown, what is the QFF?

1022.


Is the reason because if you have higher or lower temp at this level that the pressure won't change by much more than 0.09?


Sorry if this sounds stupid.

[Xacto]

Hi there,

The correction to be applied to station barometric pressure to reduce it to sea level is given by the formula

M = P multiplied by ((10 to the power m) -1)

Where M is the correction in hectoPascals

P is the measured barometric pressure

And m = h divided by (18429.1 + 67.53T + 0.003h)

Where h = the height of the station above sea level in metres

And T = temperature at the station in degrees Centigrade.

If you need to calculate QFF from station pressure use the actual temperature measured at the station. If you need QNH use the standard temperature corresponding to the measured pressure.

Looking at a set of tables derived from the above formulae for a barometric pressure of 1000 hPa at 5 metres above sea level gives a range of corrections from only 0.7 hPa to 0.5 hPa for the temperature range of –15C to +40C. So looks like your assumption was correct.

For a station 250 metres above sea level the values of M range from 33.6 hPa to 27.6 hPa for the same temperature range. Using the standard temperature of 15.1 C for 1000hPa at 250 metres amsl gives a QNH of 1030 (M = 30), whereas the QFF could vary from 1033.6 to 1027.6 for the temperature range –15 C to +40 C.

Hope this helps.

Xacto

[ft]

Or skip the math and look at what the altimeter settings really are.

For QNH you set the altimeter to give you the correct altitude above sea level when at the reference point (threshold). For QFE you set the altimeter to read zero when at the reference point (threshold).

If the threshold is at sea level, the altimeter should read zero in both cases. Ergo the altimeter setting must be the same. If it wasn’t, the readout would be different and either the altitude QNH or the altitude QFE would be off.

Edit: Note to self: Get off high horses and soapboxes and learn to read! QFF, not QFE.

The point is still valid though, no math required. If you reduce the pressure through an enormous altitude of zero meters, it doesn’t matter a whole lot whether you use ISA or the current temperatures.

Cheers,
Fred

[Old Smokey]

OneIn60rule,

Quote - "Sorry if this sounds stupid." There are no stupid questions, only stupid answers, I hope that this is not one of them.

2 good answers so far but methinks they've drifted away from the original caveat to the question, i.e. AT SEA LEVEL.

At Sea Level, QNH=QFF irrespective of temperature.

Now, it's my turn to drift away from the original, Altimeters are subject to 2 atmospheric errors. One is variation in MSL pressure, we correct this by setting the correct QNH on the altimeter sub-scale. The other is temperature error, variations above/below ISA temperature in the column of air between yourself and Sea Level causes altimeter innacuracies, and, at ground level for a station above Sea Level, we correct this error using the same barometric sub-scale, because there is no other facility to do so - In short we fool the altimeter by setting a 'phoney' QNH on the sub-scale - QFF. On the ground at the above Sea Level airfield, QFF corrects for the temperature variation from ISA in the column of air between Sea Level and the airfield. As the aircraft climbs away from this airport, temperature error re-emerges for the column of air between the aircraft and the airfield.

Old Smokey

[ft]

Hm, I’m not sure I agree. Let’s try to synchronize our takes on the issue? With QNH set, the altimeter reads the altitude of the reference point. The altimeter assumes existing in the ISA in order to convert QFE to a fictional sea level pressure. The ISA is the only atmosphere the altimeter “knows” through it’s calibration.

The QFF is an attempt to guess a more true to life fictional sea level pressure, as it would be at sea level if the ocean was to be found nearby. It does this by using the current temperature rather than the ISA as the basis for the correction.

Thus, we fool the altimeter into reading the AD elevation by setting the actual QNH. It is not what the MSL pressure would be off a nearby drop down to the ocean, but there’s nothing phoney about it as it lives up to the definition – the reference pressure which gives the height above MSL. Nor is it QFF, as setting QFF would give a reading that is off from the actual height as soon as the temperature deviates from ISA. Yes, QFF would likely be closer to the theoretical true MSL pressure, but it would not be the MSL pressure which gives the correct altitude readout, as the altimeter is calibrated assuming the ISA.

And to address the initial question… in yet another way: At MSL, QNH and QFF both equal QFE by definition. The correction to achieve either from QFE is based on the weight of the atmosphere between the current position and MSL.

The mess when calculating it stems from the fact that the weight of that layer of atmosphere depends on the temperature in said layer. You either assume ISA temperatures and get a QNH or you base your estimate of the weight of that layer on the current temperature and get QFF.

When the current position is at MSL, there is no atmosphere between the two which means there’s no correction.


Old Smokey,
have we just invented altimeter (topic) drift to go with compass drift? :p

[Old Smokey]

ft,

Actually, I think we agree but are using different words, I'll have to read your post a bit more carefully after a sleep, just back from a 13 hour sector.

Actually, this is a continuation of a recently running thread, forget the title, it's there somewhere.

Best regards,

Old Smokey

[redblue]

Hello,

Um... reading this kind of post one has to be careful. Here are my interpretation:
QNH = actual measured pressure at SL
QFE = actual measured pressure at aerodrome (so QFE = QNH if aerodrome is at SL)
QFF = estimated pressure at SL calculated with a QFE measured at an aerodrome above SL and measured temperature

Now, Ft:
The mess when calculating it stems from the fact that the weight of that layer of atmosphere depends on the temperature in said layer. You either assume ISA temperatures and get a QNH or you base your estimate of the weight of that layer on the current temperature and get QFF.
Should we say "You either assume ISA temperatures and get a QNH'..."? Isn't QNH = actual measured pressure so if it is an estimated QNH it should be called QNH'?

Now Xacto, where can I read more about the derivation and the application of the formula you posted? Was it from a book?

[Dick Whittingham]

Redblue,

Try UK HMSO "Handbook of Aviation Meteorology"

ISBN 0 11 400365 3

Dick W