Originally Posted by
parishiltons
To clarify
ATC systems correct reported level for QNH below the transition layer. So if two aircraft are operating below the transition layer at the same vertical distance above ground, one has 1013.2 set and the other QNH, then they will display different altitudes to ATC. So for example if the transition layer is A100/FL110, then FL070 will display as something other than 7000 to ATC, the size of the difference depending on the pressure difference between QNH and 1013.2.
For this reason other AIPs at ENR 7.1 (not just in Argentina) require QNH to be set when operating below the transition layer.
This is incorrect. The altitude is always broadcast independent of aircraft altimeter setting.
Why would you design the system to potentially transmit a bogus altitude because the pilots set the altimeter wrong?
Wikipedia can be a bad source so google "mode c pressure altitude" because that is what is transmitted, the pressure altitude.
And the pressure altitude is independent of the local pressure. Then the ATC system corrects for local pressure and voila you get a correct readout.
Now the number on the planes altitude indicator may be different from the one displayed on the ATC system because the pilot already set standard pressure. But that's not a problem is it?