How does ATC get a plane's Altitude , because ATC appears to think the plane is at a different Altitude to where it really is.
This is NOT an issue at STD Pressure. (Clear skys)
- Plane Altimeter indicates correct altitude, set to Std Pressure.
- AP holds the plane at the desired Altitude.
- GPS indicates same correct altitude.
ATC does NOT complain about an incorrect altitude
But if weather is not Clear Sky, and altimeter setting is not STD.
After setting altimeter to correct Value
- Plane Altimeter indicates correct altitude, and current correct Altimeter setting
- AP Holds the plane at the desired altitude.
- GPS indicates that same correct Altitude.
But ATC complains that plane is at Incorrect Altitude
Its almost as if ATC is reading the Transponder which is always calibrated to Std Pressure, and is not taking into account that that altitude needs to be corrected by them to show the True Altitude of the Plane.
This may be associated with the use of the B key to set Altimeters, because currently, the B key ALSO sets the Pressure sensor in the Transponder, so it showns ALTITUDE, and not Flight Level