With the risk of looking stupid, I give it a try.
ISA at 10k should be -5°C. So it is a ISA - 15 situation.
What is the danger? Because of the colder air mass, different presure levels are "squeezed" closer to the ground than would normally be expected. Therefore, although your altimeter displays pressure level (that's what altimeters do) normally assiociated with alttitude of 10 000 ft, you are in fact lower than indicated.
If 10000 is any sort of minimum altitude, this must not happen. For cold weather operations, it is an every day practice to compensate for negative ISA deviations based on actual ISA dev and height above pressure reporting station.
I think that the following question is equivalent:
Pilot at height of 7000 ft above station applies temperature correction for ISA -15°C. Then he comes to a consclusion, that in order to maintain the prescribed minimum safe altitude, he needs to fly not below 10 000 ft as indicated on his altimeter. What is the minimum altitude he is trying to observe:
a)... b)... c)...
Of course normal use is reversed, i.e. MNM Alt is known, so is your height and ISA dev, you look into a compensation chart and find out the minimum indicated altimeter value you need to keep.
Does it make any sense?
FD.
Last edited by FlightDetent; 21st Sep 2007 at 14:22.
Reason: 7500 changed to 7000 to be more exact and a second set of typos