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Cron
6th Apr 2004, 11:43
I would appreciate an explanation of QFF - my book explanations seem little different from QFE.

keithl
6th Apr 2004, 12:20
Yeah, its not obvious is it?
But QFF is more like QNH than QFE. QFE gives zero on the ground, whereas QNH/QFF both give an amsl elevation reading.

It's a bit subtle, but as I understand it you take your pressure at your location. For QNH you then increase it by assuming ICAO standard atmosphere conditions between you and sea level (imagine standing on a clifftop). For QFF you do the same, but apply the actual temperature (as opposed to ICAO) to the imaginary column of air beneath you.

Hmm... hard to express. How did I do?

FlyingForFun
6th Apr 2004, 12:36
I agree with Keithl's reply... but I also agree it's difficult to explain. I'll have a go at explaining it differently, hopefully one or the other reply will make some sense.

- Pressure increases as altitude decreases

- The rate at which pressure decreases is dependant on the temperature

- At an airfield, it's not possible to measure the sea-level pressure, because sea-level is (usually) underground somewhere. But you can measure QFE very easilly.

- Having measured QFE, you then need to convert that to sea-level pressure.

Following me so far? Right, now it gets fun.

Meteoroligists needs to know what the actual sea-level pressure is. In other words, if you drilled a hole down to sea level, what would the pressure be at the bottom of the hole? To do that, they look at the air temperature, and they work out how much the pressure would change by as you go down to sea level, based on the current temperature. (They make some assumptions about how the temperature will change as you go down towards sea level, too.) The number they come up with is QFF - and this is the number you will see on surface pressure charts.

Now, assume you set QFF on your altimeter, and you're on the ground at an airfield with an elevation of 1000'. The temperature is ISA-10 degrees. Rule of thumb says that for every 10 degrees from ISA, your height above datum will be out by 4%. The datum, in this case, is sea level, so your altimeter will indicate 1040', not 1000' as expected.

That's why pilots prefer QNH. This time, instead of looking at the actual outside temperature, you pretend that the temperature is exactly as per ISA. Because the altimeter makes the same assumption about temperature being ISA, when you set QNH on your altimeter it will correctly indicate the airfield elevation. (But if you drilled a hole down to sea level and lowered the altimeter down it, it would read incorrectly.) The datum, in this case, is the airfield elevalation.

Is that any clearer than Keithl's answer? Or have I confused you even more???

FFF
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Cron
6th Apr 2004, 12:45
It is getting clearer... additional Q:

At MSL we take 27' to represent 1 mb yes? Something tells me that temperature effects this figure - I guess there is a formula or is it whizz wheel time?

FlyingForFun
6th Apr 2004, 12:50
Yes, it will depend on temperature.

The rule of thumb I gave above is a good guideline: 4% for every 10 degrees. Since 4% of 27' happens to be nearly exactly 1', that means it will vary by about 1' per 10 degrees.

That's only a guideline, though. I'm sure I've seen someone post a link to a website that had a formula, and I think the formula took several pages of A4 - I wouldn't ask too many question because you might not like the answers!!!

FFF
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[Edit: Mods, why did this get moved to Private Flying? If it should have been moved from Questions, I'd have thought Tech Log would be the best forum for it???]

keithl
6th Apr 2004, 13:53
Yeah, Mods - what are we doing here?

Cron
6th Apr 2004, 13:55
Thanks for the help chaps, I'm taking it home now to spend an evening getting me 'ead round it.

BRL
6th Apr 2004, 15:37
Errrrrrrm, :confused: I have just got to it and will send it to Tech log......... :)

7th Apr 2004, 15:15
If memory serves there is a handy formula for working out feet per millibar:

Assume Temp. 30C, Pressure 1020

Formula:

96 X temp. in Kelvin i.e 30 + 273 = 303
------------------------
Press. in Mb

Therefore

96 X 303
---------- = 28.51....... Feet/Mb
1020

Looks nice anyway. Should work for all combinations of temp./Press.

"and I think the formula took several pages of A4". Fear not and enjoy.