QFF and absolute altitude

[chicken6]

Two questions

What is QFF?

What is absolute altitude?

I've got the feeling I used to know both, but can't remember now. Also posted in Tech Log.

Safe flying

------------------
Confident, cocky, lazy, dead.

[BlueLine]

QFF is the Barometric Pressure at a stated place reduced to mean sea level. AP 3340 Handbook of Aviation Meteorology.

Absolute altitude is the precise altitude at a given location however I cannot find this in a text book.

[chicken6]

Thanks for that BlueLine

However, I am now confused about the difference between QFF and QNH. It seems to me that your definition of QFF is my definition of QNH -

"QNH is what the pressure would be if we could go vertically down from here to the level of the sea".

Ah, is yours much more location-specific somehow?

I remember the definition of absolute altitude I learnt now, "the independently measured altitude of the aeroplane AMSL with no errors, or all errors perfectly accounted for".

Sound right?

------------------
Confident, cocky, lazy, dead.

[watford]

In everyday terms:

QNH - Observed pressure reduced to mean sea level using the ICAO Standard Atmosphere.

QFF - Observed pressure reduced to mean sea level using actual conditions.

Of course the real meaning, as any good WOp or TAG will tell you is:

QNH - If you set the subscale of your altimeter to read .... millibars, the instrument would indicate its elevation if your aircraft were on the ground at my station at .... hours.

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

Now, how about QHE4 and QCF? Two plastic peanuts for the first correct answer!

[Vigilant Driver]

Well my PPL Met Instructor told me it stood for:

Q for F**king Forecasters

He wasn't wrong! :)

[chicken6]

Thanks all, questions answered in Tech Log.

QNH and QFF only differ by the lapse rate from here to the sea level. QNH uses ISA lapse rate, QFF uses actual lapse rate.

Absolute altitude is the distance from the aircraft to the ground through the local vertical. The local vertical goes through the centre of the Earth, not the centre of mass of the Earth.

That's the short answers. There's also a good link to an archive of another Q-code thread.

Safe flying

------------------
Confident, cocky, lazy, dead.

[watford]

No takers?

QHE4 - I am on final leg of approach

QCF - Delay indefinite. Expect approach clearance no later than .... hours

[BlueLine]

Watford

Any thoughts on Q.P.P.?