Spitoon... See
757 Accident Link
In this case, lack of systems knowledge by both Flt Crew (who maybe should have known?) and ATC (less likely) meant that Mode C readout (relayed by ATC) was given credibility, yet of course it was suffering the same error, and played it's part in them flying into the sea.
last time I had to deal with anything like this I was told that encoding sensors were independent of displayed data because it mitigated some failure modes
Ditto - another cause to be careful! Even if these "encoding sensors" are independant, unlikely they will have a separate static source.
In short, as per ATC practice to verify Mode C, I would suggest it is the
least reliable indicaiton of altitude.
2 examples:
1. Homebuilt I fly has 2 altimeters and a Mode C/S Xpdr. All 3 have their own static inputs, but are all untilately connected to the same static ports.
2. On an Airbus, Xpdr is driven from 1 of 2 sources - which in normal use, is the source of the altimeter the AP is using*.
The logic for this is strange - it is done to ensure that where the altimeters differ somewhat, the Mode C reports the same FL as the AP maintains, and ATC do not whinge. Should that altimeter (ADC) be in error, then both the aircraft and Mode C will drift off, and nobody is the wiser i.e. the practice is to stop inconvenient ATC reports pointing out the ADCs seem to be different. Nobody seems to mind the fact the aircraft is actually at a different FL
NoD