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The ICAO document Mode S Downlink Aircraft Parameters Implementation and Operations Guidance Document mentions the DAP Selected Altitude.

Is the unit of this DAP always in flight level (i.e. without QNH correction), in feet (i.e. including QNH correction) or is it whatever number the pilot entered in the MCP or FCU? If the latter, how can I (or an automated system processing the DAP) tell if the value transmitted by the transponder is with or without QNH correction?

The use of altitude seems to imply that it's always the QNH corrected value, but I cannot find a definitive reference that clearly indicates the unit of the transmitted value, neither in ICAO nor in Eurocontrol documents.

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It's the altitude in feet or meters, depending on the display setting. It is the value displayed in the MCP - either entered by the pilot or provided from the FMS depending on which is the active nav source.

Whether it's QNH or QNE or QFE depends on the airspace the aircraft is (or will be) in when it reaches the set altitude. The autopilot will guide the aircraft so that the altimeter matches the setting. Which altitude that is is determined by the setting of the altimeter reference ("STD" for QNE or the local altimeter setting for QNH or the setting for QFE.)

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  • $\begingroup$ That it's either feet or meters is really surprising to me. The mentioned ICAO document states in A3.2 (table 2.2) that the selected altitude in BDS 4,0 is 12 bit with an LSB of 16 ft. $\endgroup$
    – Markus
    Commented May 9 at 10:33
  • $\begingroup$ Otherwise, in the end it seems that the selected altitude must always be interpreted together with the barometric pressure setting (BPS) in the same BDS register. So to get the value as a "real" altitude, the value needs to be reverse QNH corrected with the BPS given in the same register. $\endgroup$
    – Markus
    Commented May 9 at 10:43
  • $\begingroup$ The pilot can select to display altitude in meters when flying in locations where that is the standard. That doesn't prevent the system from converting it to feet in the Mode-S downlink. Depending on the system, it's entirely possible that the system is using feet internally all the time, just converting the display output to meters when selected. $\endgroup$
    – Gerry
    Commented May 9 at 18:27
  • $\begingroup$ You are correct that to be accurate, you need to account for the BPS. But typically ATC wouldn't care. Their only concern would be that it matches the altitude assignment. They would expect the pilot has the correct BPS. $\endgroup$
    – Gerry
    Commented May 9 at 18:31

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