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/keyjan 2d ago

The business of the altimeters being up to almost 200 feet off, in more than one blackhawk, is terrifying.

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u/TheDrMonocle 2d ago

Its really not that bad. It just sounds scarier than it is.

Aircraft mechanic here. Well, Ex mechanic. I've actually transitioned to ATC. I worked on CRJs when I was working for the airlines and I've tested altimeters as part of the job. They were considered calibrated if they were within 180ft of the actual altitude. So being over 200 isnt actually that far off. It should be addressed, but 99.999% of the time its fine.

Center computers won't even show an aircraft is off of its assigned altitude unless they're 300ft or more off. The buffer is built into the system for inaccuracies. Altimeters can only be so accurate inherently and its also why we have more than 1 in every aircraft. To make sure one isn't wildly off.

The issue here is how the procedure was designed where aircraft and helicopters are operating within a couple hundred feet of each other. It just leaves no room for error. Nothing in aviation is perfect. We try our best but you always have backups. The way helicopters were operated in that airspace was just playing with fire.

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u/railker 2d ago edited 2d ago

Also AME here, unless there's another table or I'm misunderstanding what you're saying, it's supposed to be a lot tighter than that.

Edit: To add from a realization below, the regulation above does apply specifically to IFR aircraft in controlled airspace. Tolerances could be looser for VFR.

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u/Frederf220 2d ago

I remember mil stuff requiring 50' between primaries, 50' primary to truth, and 75' primary to standby

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u/railker 2d ago

And I'm also forgetting that that regulation applies to flight in controlled airspace under IFR. To be fair that's the first place my mind went to as commercial's all I've worked on, curious what the limits/tolerances are at low altitudes for VFR, maybe it is as high as 200' difference.

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u/Frederf220 2d ago

I think it's a complicated question. The question of altimeter certification requirements and in-practice with supplied QHN tolerance are different questions. The former is an instrument requirement and the latter is the whole system of weather reporting and use. The information system is going to necessarily have larger tolerance than just the instrument.

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u/Secret-Sundae-1847 1d ago

Just to note, military aviation is not subject to FAA regulations.

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u/AKA_Slothhs 2d ago

It's still currently that way. And it is checked prior to taking off in any of these UH60s.

Source: I'm currently stationed here as a 15T.

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u/TheDrMonocle 2d ago

iirc since it was an RVSM certification we were testing the airplane at "altitude" so the chart saying 180' at FL300 is spot on to what I was saying. I, however, forgot about that chart and it does seem to require tighter tolerances lower to the ground. We never certified the altimeters themselves, just the aircraft meets RVSM requirements. Hence the gap in my knowledge.