Originally Posted by
Propjet88
Mode C altitude transmissions are independent of the barometric altimeter sub-scale setting. The transponder can get its information from one of two sources: an encoding altimeter, which transmits a pressure altitude reading to the transponder or (more commonly) a blind encoder permanently set to 29.92 (pressure altitude). In either case, the altimeter setting does not affect the information sent which is always based on 29.92. ATC’s computers apply the current altimeter setting converting it to altitude (which will only match your indicated altitude if you have set the correct QNH).
FlySafe
PJ88
Yes that is what an encoder does, however a pilot flies via the information displayed on the Altimeter, so if that is in error so will his maintained height. However if the aircraft is flying level this will show as an erroneous altitude being flown to ATC out by the amount the altimeter is in error by. The passing altitudes on first contact on climb will only highlight a gross error say more than 300 ft from what the screen shows and the pilots reports. Considering an Cat 1 minima is 200ft then 300ft the wrong way could put you in the ground. Hence why lower approaches have altimetry check points. Cat 2/3 rely more on radio altimetry so its not as critical.