On general aviation planes there are usually two altimeters that are independent. The instrument a pilot uses to fly with (round dial gauge or digital version in glass cockpit) and one inside the transponder itself. If you lose your altimeter you actually can call up atc if you have a mode-c transponder and have them read it to you. It’s separate.
No idea how it works on a passenger jet, but I would be shocked if it was different.
On the aircraft in question (the 757-200) there are 3 altimeters (captain, FO, standby/backup) fed by two different colocated static ports (that were both taped over). The transponder sends the altitude reported by the captain's altimeter.
This is why the crew trusted the captain's altimeter over either of the other two because it precisely lined up with what ATC was telling them. Neither set of parties knew that they were the same incorrect figure derived from the same source. The captain correctly diagnosed that the entire pitot-static system had gone to shit, but still trusted ATC's figure, right up until they started hitting the ocean, even with the GPWS alarm from the (correctly functioning, entirely separate) radar altimeter blaring in the cockpit for over a minute.
aaronmdjones|1 year ago
This is why the crew trusted the captain's altimeter over either of the other two because it precisely lined up with what ATC was telling them. Neither set of parties knew that they were the same incorrect figure derived from the same source. The captain correctly diagnosed that the entire pitot-static system had gone to shit, but still trusted ATC's figure, right up until they started hitting the ocean, even with the GPWS alarm from the (correctly functioning, entirely separate) radar altimeter blaring in the cockpit for over a minute.