ORIGINAL: rgunder
There should be a distinction made here about what the plane "sees" and what the pilot sees.
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I alluded to this in an earlier post. I quote myself:
"It would be useless to try to rely on true airspeed alone to determine the above speeds in all but standard atmospheric conditions. In fact, it's theoretically possible that the airspeed indicator could have been marked in pressure units instead of translated to speed units the way it currently is, and with proper training and annotation of performance data, it would work just as well as the current system. You could say, "My airplane indicates 200,000 Pascal at 55% power at sea level" (I just pulled that number out of the air, so to speak, since it's greater than 1 atm.)."