Originally Posted by
Lead Balloon
Reading back a number proves only one thing: The pilot heard the number and can read it back.
Well, given pilots are highly-trained individuals (of a higher class than your average trained monkey, one would like to think) it's not unreasonable to think that they, on the whole, most of the time, might be able to set their altimeter(s) QNH to the same figure as the one they just read back to ATC, however... altitude encoder errors do happen. eg. the Mode C encoder in the flight-school aircraft I recently did my test in was out by nearly 200' and resulted in some interesting conversations between me, my examiner and ATC. Stressful, to say the least!
AIUI that's one reason ADS-B is mandated in all IFR-certified aircraft - because GPS Altitude (not Altimeter altitude) is broadcast so ATC (and anyone else nearby with ADS-B In!) can read it direct - rather than rely solely on some highly-trained pilot-monkey entering a number.
Anyway, this thread seems to have drifted slightly off-topic. Some recent thermal activity perhaps?