Originally Posted by
jmmoric
Normally in older aircraft (I strongly suspect it's the same in newer aircraft), the altitude sent by the transponder is fixed at a standard QNH setting of 1013.25 hPa... So there is no connection between the altitude sent by the transponder and the shown value on the altimeter (where it can be adjusted by the pilots).
The secondary radar systems will the make the conversion to the altitude based on the local QNH that is inserted into the radar system (automatically or manually).
As long as the transponders all just transmit using the standard QNH setting, everything is fine... cause that also ensures the safety nets in for example ACAS, and radar systems....
The altimeter setting by the pilots versus the transponder altitude is what we confirm every time you show up on radar. Sometimes we catch a wrong QNH setting by the pilots, sometime we catch a malfunctioning transponder...
How flightradar24 handles it, I have really no clue... but I suspect it uses the transponder altitude as well, whether it calculates according to local QNH or standard pressure?
EDIT: A response to the FR24 users, not this case in particular.
See posts #7, #8, #11, #20 and #21.