Radio Altimeter Page: 1 of 2

PPRuNeUser134364
January 31, 2025, 09:48:00 GMT
permalink Post: 11818051

Quote:

Originally Posted by clearedtocross

Sorry fdr, I humbly disagree. While it is near impossible to stop a light heli manually like a Robinson R22 without proper ground reference, those big junks used for all-weather rescue operations all have hover-capable autopilots. Press the button and the thing holds position even in strong winds. I am sure a Blackhawk has this feature too. And hover og at sea level is not an issue here. And I am sure you should not be allowed to fly a heli at night if you cannot perform a reasonable 360 flown shy above transition speed. Another question is if you should be allowed to fly at 200 feet at night over a built up area. But that's another story. There are so many risks staring at you with these procedures it's a wonder an accident did not happen before.

Do you have a lot of time on a 'big junk'? Randomly coming to the hover at night is not like stopping your car at a red light. And not all 'big junks' have auto-hover, particularly free-air hover capability. Even if the Blackhawk does have auto-hover, it may well have limitations and it's not always just a simple case of pressing a button. e.g. is the surface (water or land) suitable for rad alt hold? What are the rad alt height limitations? Is the auto-hover based on doppler? If so, is there a max altitude for its use? The 'big junk' I am familiar with even has a min speed on instruments (not that this was instrument conditions, but it was night which has some similar characteristics).

Despite what some people appear to be suggesting here, in my experience it's not normal just to stop a helicopter to wait for a passing aircraft to fly by.

And maybe even more importantly, why would you stop if you have no reason to suspect that the flightpath isn't clear?

Subjects Blackhawk (H-60) Hover Radio Altimeter

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meleagertoo
February 02, 2025, 10:36:00 GMT
permalink Post: 11819671

Quote:

Originally Posted by CaptainDrCook

What should have been the vertical separation? I'm just a lowly PPL holder, but I imagine if the CRJ was at 325 feet, even a ceiling of 200 feet is too high for the helicopter.

Not just from a collision perspective, but a wake turbulance issue.

And maybe more importantly, what should have been the horizontal separation? Surely it should have been at least 500 feet after the passing CRJ (not based on air law, just common sense). Clearly there was no horizontal or vertical separation in the end, but just how far off minimums was the helicopter? Seems nearly impossible to be that far off the expected flight path.

Quote:

Dozens of posts back, I asked the question that many recent posts have been keying on... if everyone was where they were supposed to be, they would have passed one over the other with 150 feet of separation! In what world is that OK?

Heavens above!
Can all these non-aviation pundits here please get it into their heads that just because the helilane has a cieling of 200ft and the glideslope is 325 or whatever it does not imply that helos can, would or might EVER be allowed to pass 125 ft under an aircraft on finals nor would any sane helo pilot (there are some!) do so. That would be insane, as surely this common sense you speak of should tell you? What's a lateral 500ft got to do with air law or anything else, ever? You're muddling completely unconnected and irrelevant matters. Have you not read/heard the ATC transcripts? Helos are not given clearance to and cannot cross until landing traffic is clear (as this helo one was told) - ie until it has passed unless the incoming is sufficiently far away for there to be no possible confliction.
How far off minimums (actually a maximum)? - you've already answered that question yourself. 125ft.
The insanity of this routing procedure is that in the event of an accidental horizontal incursion into the track of an inboud as happened here there is in theory only 125 ft of vertical clearance to prevent a disaster which is nowhere near enough of a safety margin. That route should have been, imho, at least 5-800ft or more above two dots up on the glideslope.

Once again, helicopters never, never ever come to a free air hover for separation purposes - this is a ridiculous concept for numerous reasons that are too long to go into here, and would be downright dangerous at night over a black hole at 200ft. They slow and orbit if they have to, maybe slow right down if wind direction and speed allows, but never hover.

I know not everyone here is experienced on helos but if so could they please refrain from speculating on operating procedures? All this guff about altimeter accuracy is completely irrelevant and has created a huge amount of unnecessaty noise. The aircraft was flying a visual sight-picture approach where an altimeter barely features at all and helos at low level, especially at night and over water do not use the baro altimeter. They exclusively refer to rad-alt.

Finally, all those who think a visual self-positioning clearance as employed in this case behind crossing traffic is somehow hazardous are completely incorrect. Once again, at Heathrow the helilane crosses 27 L and R thresholds at (iirc)1000ft. The only clearances given as you approach the boundary is to the effect of 'cross NOW (directly) over the threshold', 'hold (at a VRP clear N/S of the threshold)' or, having confirmed and read back landing traffic visual and identified the formula is repeated, 'after the landing traffic 2 miles cross behind'. It's perfectly safe as as it isn't done at the same height as the airliner but with a large vertcal clearance too.

btw, does Marine One fly this route?

Last edited by meleagertoo; 2nd February 2025 at 11:41 .

Subjects ATC Altimeter (All) CRJ Hover Radio Altimeter Separation (ALL) Vertical Separation

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PPRuNeUser134364
February 02, 2025, 16:35:00 GMT
permalink Post: 11819905

I know I'm fighting a losing battle but here goes.....

Quote:

Originally Posted by DespairingTraveller

I'm struggling to make sense of the numbers being bandied about here. Apparently the NTSB has said that the CRJ was at 375 feet,

But from the beginning, various plots have shown the collision occurring in mid-river. A little work with Google Earth will show that the extended 33 centre line reaches mid-river about 3,300 feet from the aimpoint of runway 33. A 3 degree glideslope and some trig will put it at 175 feet at that range, with obvious implications. 375 feet would be a big error with not much more than half a mile left to run.

What am I doing wrong?

Also, helicopter route 4 hugs the eastern bank of the Potomac until Goose Island and doesn't cross to the western bank until Wilson Bridge. So why did the Black Hawk perform a 45 degree right turn while still abeam DCA? (See, e.g., post #25) Was it intending to route direct to Fort Belvoir, ignoring the heli routes? (I think I've seen it was operating out of Joint Base Anacostia en route to Fort Belvoir, so the fact it was still abeam DCA must have been more than obvious.)

Puzzled.

What you are doing wrong is making guesses based on incomplete/inaccurate data that is in the public domain.

Assuming the NTSB figure of 375ft is correct, you still need more info for it to mean anything. Was that the AGL, AMSL or SPS height/altitude?
How accurate are the 'various plots' that indicate the collision occurred mid-river? They may be right; they may not be.
Was the CRJ on final or was it still positioning to final? If it wasn't on final then the trigonometric calculations of what height it should be at might not be correct.
What official evidence is there that the Blackhawk made a 45 degree turn, or could that be a data error in the publicly available information?

In summary what I am saying is that, despite all of the internet sleuths plotting tracking data, none of it is official and it is all subject to various errors. Whilst interesting to form an understanding of the circumstances, it can't be assumed to be accurate to within a few feet (vertically or laterally).

Quote:

Originally Posted by JohnDixson

Regarding UH-60L altimeters:
All UH-60 A and L Army aircraft incorporated the APN 209 radar altimeter. Our Sikorsky tech fellow for Avioics/electronis reports the accuracy in this area is 1-2 feet. The radar altimeter position in the instrument panel is just to the right of the attitude indicator and its top matches the top of the attitude indicator. The barometric altimeter is immediately below it.
Both pilots have the same setup.

That 1-2 feet error would be a theoretical accuracy. The readout in the cockpit would be subject to significant errors during turns/pitch changes etc; without knowing the exact installation on a Blackhawk, it might also unlock based on the surface conditions. More importantly, the ATC instructions would probably be based on an altimeter setting and not Rad Alt.

Quote:

Originally Posted by uncle_maxwell

Instead of no RA below 500ft (or whatever the floor is), how about telling one conflict to climb and the other one to \x91not climb\x92? \x91Not climb\x92 could then be understood (and trained) to mean \x91descend a little, terrain/aircraft/wx permitting or fly level\x92. Lots of ifs and buts, spurious warnings, limitations for when 3 or more conflicts, TCAS vs. GPWS considerations etc. but perhaps worth a thought.

How would that have worked in this scenario? Who do you think should have been directed by TCAS to do what? Does the Blackhawk even have TCAS?

Subjects ATC Altimeter (All) Barometric Altimeter Blackhawk (H-60) CRJ DCA NTSB Radar Radio Altimeter Route 4 TCAS (All)

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uncle_maxwell
February 02, 2025, 16:54:00 GMT
permalink Post: 11819920

Quote:

Originally Posted by SAR Bloke

I know I'm fighting a losing battle but here goes.....

What you are doing wrong is making guesses based on incomplete/inaccurate data that is in the public domain.

Assuming the NTSB figure of 375ft is correct, you still need more info for it to mean anything. Was that the AGL, AMSL or SPS height/altitude?
How accurate are the 'various plots' that indicate the collision occurred mid-river? They may be right; they may not be.
Was the CRJ on final or was it still positioning to final? If it wasn't on final then the trigonometric calculations of what height it should be at might not be correct.
What official evidence is there that the Blackhawk made a 45 degree turn, or could that be a data error in the publicly available information?

In summary what I am saying is that, despite all of the internet sleuths plotting tracking data, none of it is official and it is all subject to various errors. Whilst interesting to form an understanding of the circumstances, it can't be assumed to be accurate to within a few feet (vertically or laterally).

That 1-2 feet error would be a theoretical accuracy. The readout in the cockpit would be subject to significant errors during turns/pitch changes etc; without knowing the exact installation on a Blackhawk, it might also unlock based on the surface conditions. More importantly, the ATC instructions would probably be based on an altimeter setting and not Rad Alt.

How would that have worked in this scenario? Who do you think should have been directed by TCAS to do what? Does the Blackhawk even have TCAS?

It could have instructed the CRJ to climb (meaning initiate go-around immediately) and the heli to \x91not climb\x92 (meaning descend if practicable). Or it could have instructed heli to climb and CRJ to not climb (meaning continue descent or level and look out). I am saying that tech is there in principle and the 500ft RA floor was decided on 20-30y ago, probably to limit complexity and risk of dangerous RAs, but that this limitation could be revisited in future, especially with lots more data and modelling capability to assess.

Subjects ATC Altimeter (All) Blackhawk (H-60) CRJ NTSB Radio Altimeter TCAS (All) TCAS RA

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Sven Sixtoo
February 02, 2025, 17:44:00 GMT
permalink Post: 11819956

Quote:

Originally Posted by mahogany bob

I assume that the RJ and the helo both definitely had the same pressure setting ( airfield QNH ) set on their altimeters.

Baesd on my experience, the helo crew would most likely have been operating on radalt with QNH on a baro alt as a secondary. However, when we were doing coupled low-level flight overwater, we (RAF SAR) used to set the baralt to match the radalt at 200 ft before descending.

Subjects QNH Radio Altimeter

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Lonewolf_50
February 05, 2025, 14:41:00 GMT
permalink Post: 11822257

Quote:

Originally Posted by MechEngr

They didn't even have to hold, just slow to 50 knots would have been more than enough.

60 knots, but slowing down would not be a bad idea. The SH-60 and UH-60 fly smoother at 60 than 50 IME, due in part to how and when the horizontal stab changes pitch based on the FBW set up for that flight control surface.

Quote:

Originally Posted by Hadley Rille

How does a Blackhawk pilot bust airspace by about 125ft? Wouldn't they have their arse handed to them?

Their penalty was a bit more severe than that: they died, and sadly took a bunch of other people with them. I also am puzzled at the delta between their altitude requirement on that route and the altitude at impact. As John Dixson had noted, UH-60's have a rad alt.

Quote:

Originally Posted by JohnDixson

Dibo/fdr: keep wondering why the Hawk crew made that last correction to the right.

So it wasn't just me.

Last edited by Lonewolf_50; 6th February 2025 at 13:20 .

Subjects Blackhawk (H-60) Radio Altimeter Route Altitude

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RSJ245
February 05, 2025, 19:26:00 GMT
permalink Post: 11822473

The last hole in the Swiss cheese was a vertical separation of aprox. 125ft by design. But the worst-case altitude tolerance stack-up results in about 50ft of separation. H60 tub to top of TR disc = 16', H60 @ 200' +/- 25' = 225' as measured from tub (rad alt location) to the water (I am using the NTSB +/- 25 number). So, the highest point of the H60 is 241' unless the MR cone height exceeds the top/height of the TR disc. CRJ was at 325' +/- 25 so it goes to 300' for this worst-case analysis and it's got landing gear hanging off the bottom of the A/C by 5 or 6'. A bad design was compromised by a minor non-conformance. I am not a pilot, spent 45 years as a QA engineer in that beautiful H60 factory.

Subjects CRJ NTSB Radio Altimeter Separation (ALL) Vertical Separation

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meleagertoo
February 07, 2025, 08:11:00 GMT
permalink Post: 11823433

Quote:

Originally Posted by fdr

It is highly probable that the 60 was on altitude on their instruments

On the cobntrary, it is vanishingly unlikely that the 60 crew even glanced at their baro alt. They were flying HEIGHT - that is AGL, on radalt and radalt alone. No helo ever flies at that sort of height by reference, even fleetingly, to bar-alt. That instrument is totally redundant in such a case (except for mode C reporting)

Subjects Radio Altimeter

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fdr
February 07, 2025, 15:40:00 GMT
permalink Post: 11823717

Quote:

Originally Posted by meleagertoo

Point taken, if the crew were flying a RADALT rather than a BARO. That would give a correction to their altitude of.... 10 ft, at the maximum, that being the elevation of the RWY33 approach end. The Potomac has a fall gradient which indicates the end of the runway is around 4 ft above MSL, which would give the error in RADALT to BARO. If the end of the runway is at water level, then it's 10 ft.

OTOH, the CRJ is not flying a BARO ALT, it is descending on a visual glide slope that would approximate something near 3 degree, 5.2%, from whatever aiming point they had chosen, +/- the vertical error from that ideal glide slope. They stuck each other with the UH60 striking from below. John D and LW50 can suggest the static system error that is in the -1 for the UH60L, I don't have the FM for that type. The static pressure ports are on the 2 pitot static heads that are above the cockpit area, just behind the rear edge on the pilots doors. For the UH60A,

Quote:

the static sources for the two systems are interconnected and provide static pressure to both pilot's airspeed indicators, altimeters and, vertical velocity indicators. In addition to standard cockpit instrumentation, ram and static pressures are converted into electronic airspeed signals by an airspeed transducer and an air data transducer to be utilized by the Automatic Flight Control System (AFCS) and Command Instrument System (CIS) USAAEFA PROJECT NO. 77-17 AIRWORTHINESS CHARACTERISTICS UH-60A (BLACK AND FLIGHT EVALUATION HAWK) HELICOPTER SEPTEMBER 1981 FINAL REPORT

.

...so for the A model, the drivers get raw static for their ALT displays. Later models with EFIS systems would take the same data and process that from analog to digital, and that would normally be done by an ADC system, which can remove the errors that arise from direct static sources with some rat cunning. For our jets, the ADC data does not correct all static errors, that is why we generally see a negative transient of altitude and VS rate at rotate, the flow conditions around the static ports are changing. The helicopter has the static ports in the wake of the rotor, which alters with CT, and with J so pretty much is a mess for getting nice n' tidy accurate pressure altitude displayed. The RADALT is better, it is subject to errors as well due to attitude changes but they are generally tolerable by the choice of the fan shape of the transmitter. Bottom line is, assuming that the aircraft should have missed by a hair vertically given the wide range of errors that would apply to the helicopter instruments is immaterial to the fact that they were otherwise going to be in a grossly unacceptable vertical separation in any circumstance. That they arrived at the same place in space and time is a consequence of a very straight forward error of identification of a single target when confronted with multiple targets, which we have known to be an issue for about a century. Hard to blame the PF in getting caught out doing a practice that is known to be hazardous but which is institutionally tolerated as "business as usual".

Last edited by fdr; 7th February 2025 at 15:55 .

Subjects CRJ Radio Altimeter Separation (ALL) Vertical Separation

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MarcK
February 07, 2025, 19:16:00 GMT
permalink Post: 11823873

Quote:

Originally Posted by meleagertoo

More likely, they were flying visual reference to the PAPI.

Subjects Radio Altimeter

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Wide Mouth Frog
February 14, 2025, 21:30:00 GMT
permalink Post: 11828220

Quote:

Originally Posted by parabatix

deltafox44
Not at all. The briefing indicated there may be a possibility that the altimeter in the BlackHawk displayed an inaccurate altitude reading and that the discrepency was in the order of approx 100' given the height at which the collision is known to have occurred.
Briefing the RT comms, NTSB stated that a portion of the ATC instruction to the BlackHawk to 'pass behind the CRJ' was received in the Blackhawk (according to the CVR), truncated due to the BlackHawk keying the mic at the same time. Apparently, the words 'pass behind the' were missing from the BlackHawk CVR.

I'm not sure they did say that about the altimeter, they said that there was bad pressure altitude data recorded on the FDR. That's not the same as saying the altimeter display was wrong, although it does mean that it's going to be hard to infer what was actually displayed on the Baro Alts. I would have thought a military crew would be pretty solid on altimeter cross checks though so I think that's all a bit of a red herring. The Potomac is (give or take the tide) at sea level so I think we can be pretty confident that the RadAlt figure of 278ft is good for an altitude too.

Subjects ATC Altimeter (All) Blackhawk (H-60) CRJ CVR NTSB Pass Behind Pass Behind (All) Radio Altimeter

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WillowRun 6-3
February 14, 2025, 21:38:00 GMT
permalink Post: 11828226

Quote:

Originally Posted by Wide Mouth Frog

From the transcript,
8:43:48, pilot stated the Black Hawk was at 300 feet, the instructor pilot stated 400 feet, there is no discussion of the discrepancy; the investigation will probe this.

True, this is some minutes before the collision. But it is a discrepancy of 100 feet as referred to in the prior post.

Subjects Altimeter (All) Blackhawk (H-60) Radio Altimeter

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Lonewolf_50
February 14, 2025, 22:00:00 GMT
permalink Post: 11828241

Quote:

Originally Posted by Wide Mouth Frog

When I checked the field elevation at National/Reagan/DCA, I see 14'.

Subjects Altimeter (All) Radio Altimeter

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DIBO
February 14, 2025, 23:10:00 GMT
permalink Post: 11828287

Quote:

Originally Posted by meleagertoo

so I'm not sure what to think of this statement from the NTSB brief:

Quote:

Originally Posted by NTSB autogenerated transcript

14:40 the first term is Radio altitude
...
15:04 parameter is not the primary means the
15:07 pilots would have used to determine
15:08 their height during flight the pilots
15:10 are not typically navigating using radio
15:13 altitude it is often different from what
15:15 they see on their primary Al altimeters
15:18 the next term is barometric altitude
15:21 this is typically the altitude the
15:23 pilots would use while they were flying

As they also corrected some previous released information (like PAT25 was on V HF - I know, not really relevant, but it was incorrect when they stated UHF previously), I wouldn't be surprised if they have to come back on this " not using radalt " statement

Subjects NTSB PAT25 Radio Altimeter

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MechEngr
February 14, 2025, 23:39:00 GMT
permalink Post: 11828297

Barometric altitude is the only reading that all participants can share. Trying to avoid terrain? Radalt makes sense. Trying to comply with a corridor, barometric altitude. If there is a problem that the radalt is way too low for the barometric, that should be a call to the ATC to find out what the reading is at the airport.

Subjects ATC Radio Altimeter

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DIBO
February 15, 2025, 00:52:00 GMT
permalink Post: 11828325

Quote:

Originally Posted by MechEngr

but for this accident, this was all close to irrelevant, as QNH was very close to std.pressure (18ft. diff), Potomac AGL was almost MSL (3ft diff), even airfield/TDZ elevation is only 14ft / 13ft. So baro alt., encoding & radalt all should have been very close. Equipment rounding was even larger (or less small), with the UH60's mode C apparently even reporting rounded to the nearest 100ft (and TWR's BRITE displaying in 'hundreds' only).

Subjects ATC QNH Radio Altimeter

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Stagformation
February 16, 2025, 23:07:00 GMT
permalink Post: 11829527

Accepting that the altimetry may be a side issue in this accident, but one explanation for the apparent altimeter discrepancy of around 100ft could be if pilots adjusted baro-alt to read same as radalt while over the water. Is this SOP in helicopter ops? Or were they given an altimeter setting to apply when they checked in on freq? The Potomac is tidal in that area with a tidal range 3ft or so, so radalt is near equivalent to amsl.

So maybe the handling pilot misread 300ft radio and set an indicated 210ft on her baro-altimeter, while the pilot monitoring correctly set 300ft on his baro-altimeter, but without a crosscheck being performed. Subsequently if they adjusted flightpath to fly 200ft on the handling pilots baro alt then that would explain quite a lot\x85.

eg The helicopter flying at 290ft radio (actually 278ft at impact); the pilot monitoring\x92s nudge to the pilot handling to descend a bit; the transponder mode C reporting 300ft from the pressure altitude capsule in the handling pilots baro altimeter (which we\x92ve seen on the various ATC traffic videos).

Subjects ATC Altimeter (All) Radio Altimeter

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deltafox44
February 16, 2025, 23:34:00 GMT
permalink Post: 11829539

Quote:

Originally Posted by Stagformation

Who knows. If the 2 aircraft collided only for 2 ft, had the tide be low they would not have collided... Thus the tide would be to blame

Subjects Altimeter (All) Radio Altimeter

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Lonewolf_50
February 17, 2025, 01:08:00 GMT
permalink Post: 11829568

Quote:

Originally Posted by Wide Mouth Frog

So this just cracks me up. He's in the middle of the river where the route says it's up the East bank, and that's OK because the routes are not defined with no procedural separation from landing traffic. He's instructed to pass behind the CRJ, but that would involve him either holding short or deviating over the city at 200ft at night, but instead he chooses to plow right on. The helicopter is out of his standard altitude, and the jet is way above the glideslope, and ATC encourages them to sort it out themselves. And the helicopter crew are wearing NVGs. What could possibly go wrong.

Not quite funny once the body bags fill up.
Your litany of how the holes in the cheese lined up might be missing a detail or two, but any of those holes not lining up might have avoided this tragedy.

Quote:

Originally Posted by Chiefttp

The debate about how the altimeters could have been calibrated wrong seems like they are looking for an excuse that most pilots won’t believe.

I think I agree with you.
The rad alt is right there.
At night over water at low level, the pilots I flew with did not ignore their rad alt.
It was a part of one's scan.
If I know that field elevation is 14', and my rad alt isn't at 200' or less on a route where max altitude is 200', a correction is needed now, before the error gets larger. (The separate issue of going behind, and that tower guidance apparently being stepped on, is another pair of holes in the cheese).
I am at a loss to understand the apparent magnitude of the altitude error (they were still too close laterally, yes), but as I've been out of the cockpit for a few years I am not aware of what's being taught these days.

Quote:

Originally Posted by Wide Mouth Frog

The next time I hear someone in authority say that safety is our number one concern, I think I'll probably choke on my own vomit.

On most airlines, they do have a barf bag, still, in the seat pocket in front of you. Suggest you vomit into that and avoid the choke hazard.

Subjects ATC CRJ Night Vision Goggles (NVG) Pass Behind Pass Behind (All) Radio Altimeter Separation (ALL)

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meleagertoo
February 20, 2025, 08:54:00 GMT
permalink Post: 11832026

For Heaven's sake! Altitude, altitude, altitude QNH QNH QNH!

A published glideslope (usually nowadays defined by ILS or GPS) is predicated on HEIGHT above the touchdown point , ie it is a physically fixed plane referenced to the ground. How the altimeter is set is completely irrelevant to an aircraft on the glideslope from that point of view. But this aeroplane was on a visual aproach so altimetry is also irrelevant as it is not constrained by a rigidly defined instrumented glideslope.
While the helilane limit is, technically, predicated on an altitude that's really just semantics in this case as it is over water that is at sea level and as any PPL should know that gives height above sea level with QNH set. Thus to all practical intents and purposes the heli routes are flown at heights, and as the rad alt is an order of magnitide more accutate than a bar-alt pilots will likely set its bug at 200ft and and such a low level as this where bar-alt inaccuracy is very significant will (or should) fly that, despite having set QNH because that's the requirement. That way both aircraft are on the same plane of reference, ie vs. the ground = height.

The graphic above is surprising illustration of how a lateral error of just 200m puts a helo firmly into the acceptable glideslope parameters and surely demonstrates more clearly than anything else we've seen the insanity, even irresponsibility of the design of this piece of airspace. Mind, get 200m off track in the vicinity of Heathrow, let alone crossing it and you'll be ordered out of the system the way you came in with a telephone number to call...

Still no word on why this route couldn't have been designed with a sensible vertical separation above the f/w flightpath, someone must know.

Subjects Altimeter (All) QNH Radio Altimeter Separation (ALL) Vertical Separation

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