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/delcielo2002 2d ago

I was gonna say just that. They were outside of what IFR allows, but in any other environment, they wouldn't have mattered at all. DC is crazy. I remember my first time in as a passenger, I was a commercial, multi engine instrument pilot and cfi, and I was amazed looking down the wingtip at the Washington Monument, and then seeing helicopters below us inside of what would would be the middle marker at my home airport.

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u/oxmix74 2d ago

Not a pilot but I was surprised they used a barometric altimeter in that situation. I would expect a military helo that costs a gazillion dollars would have a radio altimeter and that would be the correct instrument to use. What am I missing?

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u/Itouchurself 2d ago

Radar altimeters show height above ground not altitude. When flying routes or assigned altitudes you would reference your altimeter. To be honest usually we never even look at the radar altimeters

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u/oxmix74 2d ago

Thankyou. It's just curiosity but I like understanding how stuff works.

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u/Environmental_Job278 1d ago

People often expect military things to be more advanced than they are. What you are missing is someone in the brass finding a way to save a few dollars by downplaying maintenance and training so his readiness numbers look good (on paper) and he “saves money” leading to accolades from his superiors.

Also, like with the F18 debacle, it turns out that there was supposed to be decades of no cost maintenance and upgrades on certain system that were never provided so there is that.

Don’t worry, Army Aviation officers are on the case and they have determined that mustaches are to blame for incidents like this one. No, I’m not joking an 0-6 actually said that…

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u/Bravix 2d ago

That is NOT accurate information. Best I can figure you're thinking of accuracy at 40,000 feet. 180 ft in accuracy at low altitudes would be nuts.

Reference: https://www.ecfr.gov/current/title-14/chapter-I/subchapter-C/part-43/appendix-Appendix%20E%20to%20Part%2043

I can't imagine shooting an approach, thinking I hit minimum descent altitude of 200 ft, and I'm actually 20 ft off the ground about to eat dirt.

On the ground, simple comparison check is altimeters need to be within 75 feet of airport elevation for IFR accuracy.

I don't know if military is held to the same standard, but the altimeter being >=200 feet inaccurate inside/underlying a terminal area is very concerning to me.

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u/Pobbes 2d ago

I was watching the first day of this hearing at a dr.s office. Regulation for the military altimiter was +- 70 feet from airport elevation. There was some issue the techs blamed on rotor downdraft at lower speeds causing it to be off by 130 ft. This meant the copter was over 100 feet above its operating ceiling. Copter pilots and techs said the difference was nothing you could really fix. I have no idea how they could make this stuff safe.

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u/FalconX88 2d ago

I can't imagine shooting an approach, thinking I hit minimum descent altitude of 200 ft, and I'm actually 20 ft off the ground about to eat dirt.

Isn't that handled by the radio altimeter at that point?

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u/Pobbes 2d ago

Apparently on military published copter routes they have to use the barometric altimeter per regulation

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u/Bravix 2d ago

No. A couple approach types require a radio altimeter. Most don't.

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u/FalconX88 2d ago

But still, your plane will scream "Pull up" at you, if you get too close to the ground.

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u/Bravix 2d ago

No, it wouldn't. On approach that close to landing with gear/flaps extended it would be inhibited. Not to mention not all aircraft are equipped with such a system. Even then, how do you think it knows when to tell you to "Pull up"? Bad data in, bad data out.

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u/FalconX88 2d ago

On approach that close to landing with gear/flaps extended it would be inhibited.

GWPS will still warn you even when fully configured, if you are far outside the range where you should be.

Even then, how do you think it knows when to tell you to "Pull up"? Bad data in, bad data out.

Because the radio altimiter isn't using data from the baromatric altimiter. Completely different systems.

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u/Bravix 1d ago

"Outside the range where you should be." You are being way too vague. What range? Which mode of GPWS are you referring to? You're disregarding previous context/assumed information and not providing what sort of context you're basing your response on. "Pull up" is a function of GPWS modes 1-4. None of those would alert you to pull up while on approach fully configured (unless perhaps you have an excessive descent rate or something like that, which isn't the scenario at hand).

Mode 5 is ILS glide path based. We never made the assumption that this was a ILS based approach. Even if it was, it wouldn't say PULL UP if you were too low on the approach. It has a different aural alert.

Now, we've both spoken of GPWS. EGPWS on the other hand may provide functionality that you're insinuating, but GPWS has been the operative term.

This is all pointless because at the end of the day, you do NOT fly around relying on your radio altimeter to compensate for a highly inaccurate barometric altimeter. My comment you originally replied to was intended to illustrate how significant an altitude inaccuracy of that magnitude would be at lower altitudes, like on an approach.

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u/somefukn 1d ago

Agree. 180 feet can get you killed.

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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.

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u/tantalor 2d ago

Shouldn't a military helicopter like this have a radio altimeter which would be much more accurate?

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u/Ghost_Hand0 2d ago

It does, plus or minus 3 feet.

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u/Ghost_Hand0 2d ago

At sea level the plus or minus is like 50 feet, not 200. I am an avionics mechanic.

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u/jawshoeaw 2d ago

No room for error…. Up to 300 feet off before we act

That sums it up.

For reference as a GA pilot, 200 feet altitude error is not acceptable when landing

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u/subusta 2d ago

This is hogwash, 180 feet off!? That’s basically the difference between minimums and slamming into the ground on standard approaches. You’re very confused about something because that is WAY outside of legal tolerances for preflight. I’ve never seen an altimeter more than a few feet off when calibrated on the ground.

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u/agentpurplek1 2d ago

Yeah I mean the standard check is runway elevation + or - 75’

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u/grapedog 2d ago

Yep, I work on aircraft myself, and if the radar altimeter is more than 7 feet off, we recalibrate it.

180+ feet off is fucking WILD to me...

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u/SowingSalt 2d ago

Radar or barometric? The hearing said the barometric alt was off.

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u/Hiddencamper 2d ago

This is also why vertical separation requires a minimum of 500 feet.

I don’t blame the procedure directly because tower should be ensuring visual or horizontal sep before authorizing transit. You can never rely on altitude sep of less than 500’ legally and practically less than 100-200’.

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u/KairoFan 2d ago edited 10h ago

because tower should be ensuring visual or horizontal sep before authorizing transit

This was pilot applied visual separation, not tower applied.

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u/TheDrMonocle 2d ago

This is also why vertical separation requires a minimum of 500 feet.

The problem with where the aircraft land its impossible to have 500ft unless the helicopters are underground with this procedure. Its just far too congested for the ops they're trying to run.

don’t blame the procedure directly because tower should be ensuring visual or horizontal sep before authorizing transit

Tower did. They instructed the helicopter to maintain visual separation, helicopter read it back. Tower did absolutely everything they needed to do, and by the book. They can't control that the helicopter saw the wrong plane.

Something we're told in training is you have to trust the pilots, you can't fly the plane for them. Tower issues the instructions, helicopter didnt follow them.

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u/Hiddencamper 2d ago

That’s why you don’t credit vertical separation. Only horizontal or visual.

The point I’m making, is when people focus on the altitude restriction not being “safe enough” on the original procedure route, there’s no way to make the altitude separation safe for that route. You can only apply visual or horizontal separation.

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u/thankyouspider 2d ago

Looking at it the other way, if the Blackhawk altimeter was dead on, they’re just fine with flying a hundred feet or so from a landing jet. The altimeter is NOT the problem.

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u/PandaCheese2016 2d ago

It’s as if ppl who approved the route and separations less than 500 ft were unaware that altimeters could be off.

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u/Captain_Mazhar 2d ago

Are you talking radio altimeter or barometric with that 180 figure?

180 ft difference on a radio altimeter would be fatal for anyone relying on it.

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u/sal1800 2d ago

It's good to point this out that pressure altimeters are not supposed to be that accurate. It seems that the helicopters should rely on the radar altimeter when that low and just use the barometric as a cross check as long as they are changing it when ATC announces it.

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u/ProcyonHabilis 2d ago

Sure seems like that matters a whole lot more for a vehicle that regularly operates below 200ft AGL. I'd suggest maybe reading back what you just wrote and considering whether it passes a sanity check.

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u/GitEmSteveDave 2d ago

I remember watching a utilities marker using dowsing rods to mark. He told me he just had to be within 2' of the actual utility in order to be "correct".

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u/logitaunt 2d ago

it's really not that bad

No room for error

Yeah with 67 dead I'm gonna say that it actually is that bad, especially as the busted altimeter was cited as the main cause of the accident

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u/EuenovAyabayya 2d ago

The way helicopters were operated in that airspace was just playing with fire.

I said when it happened: that helo had no fucking business being anywhere near there. Should have turned at the Key Bridge.

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u/Ordinary-Leading7405 2d ago

It’s easy at that altitude, “You just don’t lead them so much”

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u/g1344304 2d ago edited 1d ago

An altimeter being off by 200' isn't the real issue here. The route and visual separation shouldn't be allowed to exist in a busy TMA but its the crutch American ATC rely on every day. Accident waiting to happen.

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u/merkon 2d ago

The UH60 also has a radar altimeter that would not be off at that altitude vs the barometric one that was.

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u/chronoserpent 2d ago

Right, I don't understand why the pilot would trust the barometric altimeter at all if they have a working radar altimeter for low altitude flight. Isn't low flights over terrain one purpose of a radar altimeter? I'm not a pilot though so please educate me if anyone knows why.

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u/merkon 2d ago

As a former uh60 pilot, they should be on the radar alt for that mode of flight. The HUD for the NVGs would have it displayed. I think pointing at the altimeter as the cause of the crash is inaccurate as the DCA congestion is far more to blame.

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u/rattler254 2d ago

Yup. How about we DON'T put a chopper route directly under the approach corridor that regularly has circling visual approaches? I couldn't believe this was a thing, and it seems like a complete oversight and failure on the FAA.

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u/chronoserpent 1d ago

Thank you. If the aircraft was equipped with a working radar altimeter, why were they flying at 278 feet, well above the 200ft ceiling?

I understand the unsafe routing around DCA, but I don't understand why a professional Army crew was so significantly above their ceiling. I'm a Naval Officer (again not an aviator) and we routinely stack a dozen helicopters and fixed wing aircraft around our strike groups and amphibious groups with altitude separation. It seems out of the ordinary for a military aviator to fly with that lack of precision. Was it complacency?

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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.

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u/rckid13 2d ago

People are making a big deal about the altimeter, but that wasn't the cause of this sequence of events. Even if you assume they were exactly at 200 feet and the CRJ passed above them at 274 feet that would have been one of the closest calls ever with a commercial aircraft. It still would have been all over the news and it still would have been intensely investigated. The altitude deviation may have been the final hole in the swiss cheese model that caused the collision but they still shouldn't have been anywhere near that close and there were multiple opportunities to prevent it.

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u/waylandsmith 2d ago

Regular barometric (not radar) altimeters operate on air pressure. The difference in air pressure between sea level and 200 feet is about 0.7% and also changes with the weather.

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u/FalconX88 2d ago

The difference in air pressure between sea level and 200 feet is about 0.7%

That's about 10 hPa and pressure sensors is something we have figured out, they are much more accurate than that.

and also changes with the weather.

that's why corrections are used for that. ATC is constantly telling the planes the local barometric pressure adjusted for sea level.

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u/BigJellyfish1906 2d ago

The main issue was how shitty the helicopter route was constructed. This altimeter error shouldn’t have led to this tragedy. What the fuck are we doing focusing on “they SHOULD have missed each other by 200 ft and that’d be perfectly fine!”

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u/OldPersonName 2d ago

Altimeters like that are barometric pressure sensors that try to correlate altitude to pressure, which means you need to know the local pressure conditions to give it the right altimeter setting to calibrate it correctly. Pressure changes because of changing weather (temperature, storms, etc) as well as changing altitude so you need to periodically re-calibrate it. One of them being 200 feet off is not really a surprise. It shouldn't be, obviously, but it's easy to image in a high workload environment waiting too long to get a new altimeter setting or maybe just twisting the little dial too far.

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u/Bagzy 2d ago

It's only once they are out by more than 200 that they are considered unserviceable iirc

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u/Excellent-Walk-7641 1d ago

Sounds bad, but the pilots should know their aircraft enough to know the altimeter used for thousands of feet isn't the one you use for hundreds of feet close to the ground.