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Comment by jackweirdy | Hacker News Reader

Bear in mind there are airports in the world at less than 0ft sea level. Very complicated to validate what a valid input actually is

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While true, my understanding is that the altimeters automatically calibrate themselves to air pressure as well as use auxiliary readings from the radio altimeter to provide an altitude system where 0ft is sea level, always. However given that the pilots seemingly didn't follow the pre-flight checklist it's possible they did not calibrate the altimeter either.

Altimeters are only pilot calibrated to ensure planes travelling in various directions can trust their vertical separation is constant even amid rising or dropping temperatures and pressures. In that context an altimeter that calibrates itself is quite undesirable

There is the radio-altimeter which uses ground based radar but that is for spoken-aloud altitude readouts and terrain warnings and as far as I’m aware doesn’t factor in Boeing or airbus autopilot.

At least in the U.K., the lingo has us refer to this always as height (ie, above the ground) and not altitude (within the air above a hypothetical sea level)

The plane can't know by itself what altitude it is and at least provide warning to override what it thinks it is?

do they not have GPS and the ability to look up airport elevations on a map?

Amsterdam represent! :)

Amsterdam is pretty mild in comparison to Bar Yehuda, which is a good 375 meters below sea level.

https://en.wikipedia.org/wiki/Bar_Yehuda_Airfield

Cool I didn't know that, thanks!

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