I agree that this subject is going around in circles. I do not claim that a correction is unnecessary to an altimeter for low temperatures, and all the great stuff included here should be considered as valid. My point was solely that so far as I am aware, and I base it on the reference I gave before,as well as practical experience in real cold and real hot conditions, that the ADIRU as fitted to the Boeing airplanes I am familiar with, have temperature compensation. I do not know what that really means, since I cannot get more information from the manufacturers, and it could well be that it is a partial compensation. It seems reasonable to me that the engineers would have looked at things like Approach requirements and set it up to fully compensate at normal landing altitudes, and to have uncompensated readings at cruise. it is a computer after all.
Why this should cause such angst among you I do not have any idea, since I have repeatedly said you should follow your own opinions.
With regard to the altimeter correction, the FRM is the Fault Reporting Manual and it gives the correct tolerances for writing up the instruments. It is not 70, or 75 feet, as most will claim, since that is for pilot reference to establish that the baro setting is right. If the check fails, the reason is probably that the baro setting or the parking position height reference is wrong.
Yes, the flight I commented on was out of tolerance, but I thought it reasonable to do a couple of flights and observe it before writing it up, and since it was ok on the next leg, I did not pursue it further. It would have been released under the MEL anyway. So sue me.
And the point of the altimeter correction for temperature is to find the True Altitude. For that you need to know the ambient temperature for the little bit of air you are currently in. The temp on the ground, or even a few hundred feet below you, is irrelevant. Using the ground (airfield) temp to correct the altimeter is a second-best way to do it, accurate only if the lapse rate is standard. But if that is all you have, go ahead.
These are opinions, not facts.