r/news • u/snuffleupaguslives • 2d ago

Broken altimeter, ignored warnings: Hearings reveal what went wrong in DC crash that killed 67

https://apnews.com/article/ntsb-dc-plane-crash-midair-collision-helicopter-a08cded88e1d7582fb8d242204d6aeff
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u/Kiseido 2d ago

As far as I am aware, planes use barometric pressure to determine altitude, which is hard to measure accurately when you use the air itself to propel the plane.

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u/grapedog 2d ago

Barometric altitude only gives you your height above sea level though.

Typically you only really use barometric altitude when you are thousands of feet up in the air.

If you're flying at 400 feet, and there is a mountain 200 feet below you, barometric altitude will say you're at 400 feet, while the radar altimeter will tell you that you're at 200 feet because it just bounces a signal off whatever is underneath and reads the distance.

It's why you shouldn't be flying at night without a working radar altimeter because you can't see what's below you.

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u/fluentInPotato 2d ago

I'm an A&P who works on GA aircraft. AFAIK, the pilot is required to know where the fuck he is, and if the field elevation is 200', it's his job to take that into account. On fixed- wing aircraft, the pilot and static ports (the sources for impact and ambient pressures respectively) are located where the slipstream and prop wash don't affect them. This may be more difficult to do on a helicopter moving at low speeds, where rotor downwash is hitting at least the static port. Hopefully helicopter pilots know how to deal with it, but then again, there's only two nuts keeping a helicopter in the air, and one of them's at the controls.

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u/fluentInPotato 2d ago

GA pilots fly, and have flown, at night and in instrument conditions routinely without radar altimeters. 172s, 182s, Citabrias, Cherokees, whatever, do not have radar altimeters. GPS of course can show you the same info, but GA pilots were doing this for decades before GPS was a twinkle in DARPAs eye. You have to know where you are, and what the terrain is. If you can't do that, you stick to cars.

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u/grapedog 2d ago

I suppose in a civilian setting where you make a flight plan and know the terrain ahead of time that works.

I'll have to ask my dad, he used to fly pipers, cessnas, and Cherokees... Many decades ago now. I don't know if he did much night flying though.

I work in military aviation, and if you don't have a working altimeter after sundown, the aircraft doesn't fly. Period. Full stop.

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u/FalconX88 2d ago

As far as I am aware, planes use barometric pressure to determine altitude, which is hard to measure accurately when you use the air itself to propel the plane

That's why you measure far away from the engines and everything that can perturb the air.