Very bad plan, Ian W:
Selecting a different pressure setting produces an almost linear shift of the indicated altitudes (30ft/hpa).
On the other hand the cold temperature effect is increasing with elevation above the source - e.g. at -20°C at the airport the altimeter at 200ft shows 30ft less, at 2000ft the error is 280ft.
Folowing your suggestion: If you correct for 280ft (about QNH minus 10hpa) then you will be clear of your obstacle at 2000ft, but you will end up about 270ft above a charted 200ft-minimum . Not desired.
When you correct for the 30ft error at the 200ft minumum, you will strike the obstacle at 2000ft. Also not desired.
Changing the pressure setting during the descend? Unthinkable!
Your proposal sounds like an instruction for CFIT...
If you are not happy with a cleared altitude you get from ATC (due to temperature) just tell the controller! In affected areas ATC are usually already adding some hundred feet to keep you away from terrain.
Last edited by FA10; 11th Aug 2011 at 12:49.