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I've noticed some pilots preselect their destination QNH on their side of the FCU. In order to preselect the QNH, you need to momentarily switch out of STD which made me wonder:

Assuming your side is switched on for altitude reporting (on the ATC/TCAS panel):

  1. Will ATC receive a different altitude while the pilot is preselecting their QNH?
  2. Does TCAS use this barometric altitude during interrogations? So could this trigger a TCAS if the QNH difference to STD is drastic enough?

I know that it usually on takes a few seconds to do, but could be longer if the QNH difference is big enough or if the pilot is distracted.

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2 Answers 2

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Mode-C transponder reporting is done using QNE barometer settings, regardless of the QNH setting by the pilot. This precisely avoids the problem that you mention. The goal of the transponder is to avoid other airplanes, so a common setting like QNE is the logical choice. Maintaining enough height above the ground is the pilots' responsibility, so QNH is only for the pilots display.

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    $\begingroup$ Note that the barometric pressure setting does not affect the altitude transmitted by the transponder (which is indeed QNE based),but a Mode-S transponder can provide the barometric pressure setting if the radar is asking for it (It is in BDS register 4,0). The altimeter setting is also part of the ADS-B transmissions. $\endgroup$
    – DeltaLima
    Apr 17 at 17:45
  • $\begingroup$ @DeltaLima Neither is used for separation purposes though but is of course relevant for terrain warning. The real odd one out is FLARM which AFAIK uses GPS altitude. $\endgroup$
    – Sanchises
    Apr 17 at 19:28
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    $\begingroup$ FLARM uses GPS height indeed, but that is understandable. That way it can be a stand alone instrument that doesn't need static air pressure, which makes it easier to install. Barometric pressure setting is not use for separation purposes directly, but can cause alerts in ATC systems if it is not set to the correct value for the airspace the aircraft is flying in. $\endgroup$
    – DeltaLima
    Apr 17 at 19:41
  • $\begingroup$ @DeltaLima Right! My club flies in a TMZ so I've never understood why FLARM doesn't just use pressure altitude since "obviously" everyone has a transponder too. $\endgroup$
    – Sanchises
    Apr 18 at 6:01
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ATC will not see a different altitude. The radar ATC is known as secondary surveillance radar or SSR, where transponders return a pilot set 'squawk' code and the pressure altitude. These transponders have their own altimeters and report the pressure altitude, which is not adjustable to the pilot.

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  • $\begingroup$ Do you mean QNE instead of QFE? $\endgroup$
    – user38116
    Apr 17 at 17:05
  • $\begingroup$ That was in error @user38116, transponders send the airplane's pressure altitude, not QNH, QNE or QFE. $\endgroup$
    – GdD
    Apr 17 at 17:24
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    $\begingroup$ Note that the barometric pressure setting does not affect the altitude transmitted by the transponder (which is indeed QNE based),but a Mode-S transponder can provide the barometric pressure setting if the radar is asking for it (It is in BDS register 4,0). The altimeter setting is also part of the ADS-B transmissions. $\endgroup$
    – DeltaLima
    Apr 17 at 17:46
  • $\begingroup$ If it is in fact part of the ADS-B transmission, would ATC not be alerted by a deviation in altitude? $\endgroup$
    – user38116
    Apr 17 at 18:50

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