What drives the altitude on the Passenger inflight display?

[paull]

Landed at Nice with Swiss and the inflight display showed altitude of minus 25m just before we landed, followed by a firm landing and then an altitude of (+)8m.

Is this driven off the same source as cockpit instruments? and if so does it indicate an altimeter fault, or does the software look at the RoD and extrapolate the height at the next refresh?

Apologies if question is not 'tech' enough.

[DaveReidUK]

Is this driven off the same source as cockpit instruments? and if so does it indicate an altimeter fault, or does the software look at the RoD and extrapolate the height at the next refresh?Sounds like pressure altitude before correction for QNH, i.e. what the transponder sends rather than what the crew sees.

[asc12]

Because it changed after landing, it sounds more like a GPS-derived altitude to me, but I've wondered the same thing in the past.

[jimjim1]

I would think that connecting it to the flight instruments would be very expensive (much testing and qualification) so something independent is my bet.

Speed and position are also often displayed and you want it to work over the ocean too so GPS seems favourite. Just needs an external antenna.

[bvcu]

Fed from FMS on a lot of modern widebodies

[Skyjob]

Older technology used to present flight levels as altitude derived though FMS or Transponder.
Newer technology can use GPS of aircraft or an internal one.

Neither technology makes correction for QNH like an altimeter.
Hence when flying level below transition you can see all wonderful manoeuvring altitudes displayed. But as aircraft is then generally climbing or descending it doesn't really matter to passengers, plus the system always lags a little as well.

Maybe you can remember on your recent flight, that the passenger display is always in even feet or equivalent meters in cruise, but depicting the flight levels flown as a constant not as an actual corrected altitude above ground, but advertises it to passengers as "altitude".
So the display in the cabin is always annunciating the "flight level" as an "altitude".

Think the initial system design is not taking into account people reading too much into the system discrepancies with adequate background knowledge about pressure changes etc. Made for Mr & Mrs Smith to tell them how high they flew in cruise to XXX.

[paull]

Thanks Skyjob, as you say, if it is transponder based then I guess what I saw on landing was probably within spec (+/- 125feet) , just didn't expect to see such a wobble.

[DaveReidUK]

The reason for the change to +8m reading on landing is probably because of dodgy conversion logic between feet and meters.

Although the aircraft's systems know it's on the ground (via the squat switches), I suspect that the implied 0ft AGL altitude is being interpreted wrongly by the passenger display system as zero to the nearest 25ft (25ft being the resolution of the altitude encoding on most modern aircraft).

And 25ft happens to be 8m when rounded to the nearest meter.