QFE or QNH

[seifly]

Ready for take off at the end of the runway,which is more accurate QFE or QNH and why?

[I-2021]

Depends on what you wish to know... Height above aerodrome or altitude above mean sea level. The accuracy depends on your altimeter, these settings just give different informations.

[Spitoon]

Also depends on the prevailing atmospheric conditions.

[Feather #3]

OK, I'll bite, why?:confused:

G'day ;)

[compressor stall]

What's the runway slope?

[welliewanger]

OK, I'll make a stab at it...
They are both absolutely accurate when reading zero.

As soon as the reading on the altimeter is anything other than zero a number of errors are introduced (temperature error - caused by a non-standard lapse rate and instrument error - the instrument isn't made perfectly) Neither of these errors are significant except when the temperature is very different from standard (4% error for every 10 degrees variation from "standard" temperature)

These errors are exacerbated the further you are away from the altimeter reading zero. Quid pro quo, at sea level QNH and QFE are equally accurate. At a high elevation airport or if the temperature is very different from standard, QFE is more accurate.

The Canadian AIPs have more info on temperature correction:
RAC - 9.0 INSTRUMENT ARRIVAL FLIGHT RULES (IFR) – ARRIVAL PROCEDURES - Transport Canada (http://www.tc.gc.ca/eng/civilaviation/publications/tp14371-rac-9-0-2604.htm#9-17)
Basically, it doesn't make much difference until the air temperature on the ground gets below about -15C. Lower temperatures make the altimeter OVER read. So it says you're higher than you are (high temps make it under read)

[Old Fella]

If you are going to remain within the confines of the circuit area and want to know your a/c height above the runway, say for an aerobatic sortie, use QFE if the airfield is significantly above sea level. If you are departing enroute to another location use area QNH, as everyone else should be, whilst below the transition altitude.

[Upper Air]

. . on whether your altimeter is more accurate than the Stevensons screen of the weather man. . .also, the QNH is the pressure setting of mean sea level (as you know) for a specific period and is an average of that specific area be it the Chatham ASR or the EGLL QNH . . . whereas, your reading of Zero feet on the ground is real time.