Originally Posted by
am111
Correct me if I'm wrong, but I thought the transponder always reported altitude with 1013 as its datum, regardless of what QNH has been set on the altimeters?
Edit: Just re-reading @parishiltons post, and realised that you were perhaps referring to levels/altitudes reported by the pilots rather than the transponder, in which case your comment makes perfect sense, and I was being a numpty.
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.