Originally Posted by
DaveReidUK
AFAIK, the altitude that ATC see on Mode C/Mode S isn't affected by your altimeter setting - it's always based on a 1013 hPa datum.
Mode C is not, however a strange quirk of Airbus avionics suite sends the last used before setting the STD through the downlink S mode.
The extended squitter seems to have a data filed for selected Baro ref, however the last value is retained for transmission instead of STD when the crew changes to that.
Which brings us to another unexpected conclusion. Mode S must have been transmitting Q1011 after this almost doomed crew accepted the wrong instruction.
Hmm, some opportunities there!