411a, I must disagree with your reasoning about corrected station QNH.
As we all know the altimeter works by sensing the pressure of the atmosphere around it, the instrument is sensitive to changes in this pressure. For it to provide any useful information a datum must be established and this is often QNH but could be airfield elevation (not quite the same thing as QNH), QFE (zero feet indicated at the airfield elevation) or standard setting of 1013.25mb/29.92 inches. Any movement of the instrument up or down away from this datum of course causes the instrument to sense a different pressure. The instrument is calibrated and marked such that a movement upwards of say 1000 real feet produces a similar reading on the instrument face. But lets be clear that all the instrument is sensing is the change in atmospheric pressure over that 1000ft column of air.
The crux of the matter is that the rate the pressure changes over altitude varies depending on the air temperature. The international standard atmosphere model (ISA) gives a standard rate for this change and the altimeter is calibrated so that on an ISA day when it is 1000 actual feet above the datum in use the instrument reads 1000ft. If the temperature is colder than ISA (air is more dense) then the instrument will read 1000ft before it has travelled a 1000ft upwards, ie. it will be reading low. The higher you are above the datum the worse the error becomes. These errors are normally small at the MDA because you are not far above the source datum but even so at -20 degrees C you need to add a 100ft to a 700ft MDA. At 6-7000 feet above an airfield you may have to add 800ft to a published check height to achieve required terrain clearance.
The QNH reading at the airfield may very well be corrected for temperature but that only makes the elevation reading correct on the airfield, if the atmosphere is colder than ISA then corrections must be applied to published altitudes. It's something the Canadians have been doing for years and there is some good stuff on the web about it, have a look at:
http://www.fmcguide.com/media/calgary.pdf
http://www.bluecoat.org/reports/Long_98_Cold.pdf
http://www.atlasaviation.com/feature...ltimeter_t.htm
This is something that we have been slow to pick up on in our company, and perhaps in the UK in general but we are now encouraged to think about it in sim. details that include low temps. and I think we are all much more aware of it than we were a few years ago.
Sorry for such a long post but I couldn't seem to explain myself any quicker than that.
Cheers.
PS. I see that several others have posted whilst I was slaving away on this but havn't got time to go through it again to take out any duplication.