Posts about: "Radio Altimeter" [Posts: 40 Page: 2 of 2]

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Lascaille
February 26, 2025, 12:31:00 GMT
permalink Post: 11836357
Originally Posted by Cobraguy
to ask if there ever was a quantitative analysis ... the possible errors in the helicopter-borne equipment, the Static ports could be subject to some biases ... the approach without benefit of a Glide Slope, the VASI or PAPI is visual and thus "probably" more challenging ... be better than "10 to the minus nine". Need to have data from both low-hour and high-hour pilots on a non-coupled approach.

Next quasi- related thought::: when the CVR recorded a verbal disparity of 100 feet between pilot and examiner, shouldn't that have raised questions of "Why"- especially when at low altitudes MSL? As I understand it, there would be 3 or 4 places where Baro Alt was displayed; the two mechanical bar alt indicators, AND the altitude display(s) on the pilot(s) NVG HUDS. If the pilot under evaluation was fully on the ANVIS HUD, and if that pilot failed to set the Bar Alt "correction" in terms go In-Hg, then the pilot could readily be seeing inaccurate Bar Alt digits on the HUD.
I like healthy food but this much word salad would choke a horse.

Helo is going to be using radalt. Everything is radalt when the heights are below ~1000ft because the alternative is often fatal. This has been covered extensively. As to the rest... Wat?

Subjects CVR  HUD  Night Vision Goggles (NVG)  Radio Altimeter

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galaxy flyer
February 26, 2025, 14:05:00 GMT
permalink Post: 11836401
Originally Posted by Lascaille
I like healthy food but this much word salad would choke a horse.

Helo is going to be using radalt. Everything is radalt when the heights are below ~1000ft because the alternative is often fatal. This has been covered extensively. As to the rest... Wat?
well, the Army pilot in the Mover and Gonky video referenced here said, they\x92d be on Baro altimeters because RADALT would be bouncing around too much to be useable, over water, over land, over bridges, etc. Second, he stated that 200\x92 was the procedure altitude\x97no higher but no lower, either. Top for traffic, bottom for noise, I\x92d guess. He also stated the holding at Hains Point was common a few years ago.

Subjects Radio Altimeter

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Lonewolf_50
July 31, 2025, 19:00:00 GMT
permalink Post: 11931499
Originally Posted by ATC Watcher
-flying at 300ft targeting 200ft is "acceptable" by the Army
-200ft restriction on the chart is a only a "recommended target" in VFR not a hard restriction i unless instructed by ATC ..
Something smells wrong about some of this.
(I need to see a bit more of the documentation on the difference between the hard altitude (200') that I was under the impression was on that route, as opposed to the "recommended altitude" statement made there...maybe it will make more sense to me then).
As to altimeter errors.
The UH-60L has a radalt.
Are you trying to tell me that the alleged acceptable error for a rad alt is 80'-130'?
I think not.
I doubt that the rules have changed that much since I was last flying a Blackhawk. (yes, it has been a while).
Will do a bit more reading, thanks.

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

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MechEngr
July 31, 2025, 19:18:00 GMT
permalink Post: 11931506
Depending on radalt for this flight mode is a problem when flying over buildings or terrain when one wants to maintain an absolute altitude relative to a common datum. If done perfectly radalt would be hammering back and forth with every chimney and tree and park and car. I'm sure they use smoothing to give something for the crew to read, but it doesn't tell how high the terrain is that the measurement is made from - it only reports clearance to the terrain when one wants clearance to other aircraft.

For TF/TA radalt is the go-to instrument, but for coordinating multiple aircraft to maintain vertical separation, barometric altitude is more suitable. The problem being that barometric altimeters are subject to a lot of measurement and reporting errors.

I am sure that GPS-RTK could be used to fix the absolute altitude with great precision, but I am also sure that depending on an easily denied measurement source on a military aircraft is not going happen.

The correct solution for operating in a civilian airspace is to use ADS-B In/Out for all manned aircraft to provide appropriate and timely situational awareness. While ADS-B is also subject to denial, it offers far greater benefit in civilian airspace over GPS-RTK in that it tells the pilots where all the other aircraft are rather than simply being more precise about where their own aircraft is.

It is clear that the helicopter crew not knowing where the passenger jet was was the primary cause of the collision.

Arguments about the error in the altimeter readings are suitable to emphasize that depending on them in a crowded airspace is a fool's choice and should have been spotted a long time ago as insufficient to provide clearance.

Subjects ADSB (All)  Altimeter (All)  Radio Altimeter  Separation (ALL)  Situational Awareness  Vertical Separation

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Lonewolf_50
July 31, 2025, 20:35:00 GMT
permalink Post: 11931531
Originally Posted by MechEngr
Depending on radalt for this flight mode is a problem when flying over buildings or terrain
They were flying over the river, not the built up areas. Not sure how much time you have flying in a Blackhawk, but if you are over the water at night at 200' your radalt is giving you better indications of how far you are above water than your bar alt. If the two disagree, which one do you think you'll be using?
(Same is true in the Seahawk).

As to your point on the cause of the accident, yes, they were not aware of the traffic coming into 33 (for reasons beaten to death already).

I'm going to offer a contributing cause that I think merits consideration: there was mention made early on of there being 1, not 2, tower controllers on duty at the time when apparently 2 is the normal number.
Had there been two, the helicopter might have been handled differently, but we'll never know.
Spoiler
 


Subjects Blackhawk (H-60)  Radio Altimeter

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BFSGrad
July 31, 2025, 22:12:00 GMT
permalink Post: 11931558
Originally Posted by Lonewolf_50
They were flying over the river, not the built up areas. Not sure how much time you have flying in a Blackhawk, but if you are over the water at night at 200' your radalt is giving you better indications of how far you are above water than your bar alt. If the two disagree, which one do you think you'll be using?
(Same is true in the Seahawk).
My recollection from the CW3 Roth (former 12th AB pilot) interview was he said the opposite; i.e., due to the frequent bridges/islands in the river, PAT25 would have been flying referenced to baro for route 1 and 4 altitude limits.

Subjects Blackhawk (H-60)  PAT25  Radio Altimeter

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dragon6172
July 31, 2025, 22:18:00 GMT
permalink Post: 11931562
Originally Posted by Lonewolf_50
They were flying over the river, not the built up areas. Not sure how much time you have flying in a Blackhawk, but if you are over the water at night at 200' your radalt is giving you better indications of how far you are above water than your bar alt. If the two disagree, which one do you think you'll be using?
(Same is true in the Seahawk).

As to your point on the cause of the accident, yes, they were not aware of the traffic coming into 33 (for reasons beaten to death already).

I'm going to offer a contributing cause that I think merits consideration: there was mention made early on of there being 1, not 2, tower controllers on duty at the time when apparently 2 is the normal number.
Had there been two, the helicopter might have been handled differently, but we'll never know.
Spoiler
 


I have flown that route a handful of times during my time in the Marine Corps (admittedly it's been more than 20 years ago). I recall being asked to do a 360 turn over the Wilson Bridge for spacing (we were northbound) for traffic landing on 33.

In regards to your other comment about them not seeing the CRJ, I have believed from the time that the radio traffic was public that the controller was not specific enough when he asked PAT 25 "do you have the CRJ in sight?" Something along the lines of "PAT 25, CRJ on final for RWY 33 is at your 10 o'clock less than a mile, pass behind that traffic" should have drawn the helo crews eyes to the left to hopefully see the traffic. Even better would have been to have the helo do a left 360 over Hains Point.

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

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BFSGrad
July 31, 2025, 23:00:00 GMT
permalink Post: 11931572
Originally Posted by ST Dog
And I forget who, but there was mention of RADALT varying with the depth of the water.
Don\x92t recall hearing that. Roth referred to the river clutter causing the radalt to bounce around.

To complicate further, the Army Day 1 witness, CW4 Lewis, indicated she would have flown route 1/4 using radalt as her altitude reference. She also offered that she had zero DC route experience.

Subjects Radio Altimeter

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MechEngr
July 31, 2025, 23:36:00 GMT
permalink Post: 11931584
No matter, radalt only gives the altitude above some actual thing, not a shared pressure altitude that all air vehicles can agree on. AA5342 was not on a radalt path. AA5342 was flying over buildings.

EDIT: I also appreciate the arriving jet was on a geometric glide slope and that radalt under the glideslope could have been sufficient, but if one is expecting that vertical separation is sufficient then there needs to be certainty to that altitude and clearly that isn't possible on barometric altimeter and should never have been accepted by anyone.
Last edited by MechEngr; 1st August 2025 at 05:39 .

Subjects AA5342  Altimeter (All)  Barometric Altimeter  Radio Altimeter  Separation (ALL)  Vertical Separation

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ST Dog
August 01, 2025, 00:29:00 GMT
permalink Post: 11931595
Originally Posted by BFSGrad
Don\x92t recall hearing that. Roth referred to the river clutter causing the radalt to bounce around.
Once the recording/transcript is available I'll try to find it. I remember it struck me as odd.

To complicate further, the Army Day 1 witness, CW4 Lewis, indicated she would have flown route 1/4 using radalt as her altitude reference. She also offered that she had zero DC route experience.
Was it her that later said she'd use baro since the route was MSL not AGL?

Again need to recheck against the transcript. my memory may be fuzzy. I was doing 3 different things at the time.

I have several things I want to revisit from the 2 days so far.

Subjects Radio Altimeter

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ST Dog
August 01, 2025, 20:09:00 GMT
permalink Post: 11932030
Originally Posted by BFSGrad
Don\x92t recall hearing that. Roth referred to the river clutter causing the radalt to bounce around.

To complicate further, the Army Day 1 witness, CW4 Lewis, indicated she would have flown route 1/4 using radalt as her altitude reference. She also offered that she had zero DC route experience.
I had time to look through the live transcript I still have open.

It was her.

Lewis: In my experience, when flying at low altitude, I would be referencing the radar altimeter.

Mueller: And when would you transition roughly between the two?

Lewis: In my experience, certainly 200 feet and below, I would be referencing the radar altimeter. However, if I was flying on a published MSL route, I would be referencing barometric altitude.

and later

(unsure): This is a scenario based, one based on what the chairman asked about flying over the Potomac River route. If you had a route ceiling of about 200 feet and you were to look at your barometric altimeter and see about 160 feet, but then it's all on your radar altimeter, about 280 feet. How would you triage that situation in your experience?

Lewis: So if the barometric altimeter is reading 160 feet, you're saying and the radar altimeter is reading significantly higher than that, I would still my primary concern on the MSL route would be staying below the 200 feet. And I would definitely, you know, take note of that and and maybe write the aircraft up for some, you know, maintenance action. But at that particular time I would as long as there was nothing wrong with the aircraft before we took off. As far as the field elevation and barometric altimeter, I would, you know, continue to assume that I was below that 200 foot MSL.

Subjects Altimeter (All)  Barometric Altimeter  Radar  Radio Altimeter

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Sailvi767
August 03, 2025, 16:13:00 GMT
permalink Post: 11932824
Originally Posted by Lonewolf_50
Something smells wrong about some of this.
(I need to see a bit more of the documentation on the difference between the hard altitude (200') that I was under the impression was on that route, as opposed to the "recommended altitude" statement made there...maybe it will make more sense to me then).
As to altimeter errors.
The UH-60L has a radalt.
Are you trying to tell me that the alleged acceptable error for a rad alt is 80'-130'?
I think not.
I doubt that the rules have changed that much since I was last flying a Blackhawk. (yes, it has been a while).
Will do a bit more reading, thanks.
My question exactly. Where is the radar altimeter in this discussion. I also can\x92t believe that when flying a helo at night over water cross checking the radar altimeter with the baro altimeter is not just good practice but required!

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

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Easy Street
October 21, 2025, 14:54:00 GMT
permalink Post: 11973706
Originally Posted by Lonewolf_50
What has that got to do with this event? They were not on an IFR flight plan.
Also, as an aside the term "altitude" is typically used in aviation. (Yes, I know that DH for a precision approach is "decision height"...and HAT is shown on approach plates (Height Above Touchdown).
Thank you, I am a professional pilot so I do understand the difference in meaning. 'Height' is the correct term here as the route was defined with reference to the surface and the pilots were using the (badly named...) radalt as their height reference.

Why do I think the IFR ACS might be relevant? Because it specifies the accuracy the FAA requires of skilled pilots when separation is to be achieved by procedural means. The fact that the "designed" separation between the 33 approach slope and the top of Route 4 was less than the allowable error for skilled pilots could be used to rebut an allegation of negligent flying as the cause of the accident. The Army no doubt has its own standards document, but I'd be surprised if it was radically different. Building the argument off the FAA's own document forces the focus onto its route design and visual separation procedures.

As to why this might be relevant to VFR flying - is it your opinion that parameters should be flown more accurately in VFR than in IFR? In any case, I have now dug further into the ACS and the same tolerance is prescribed for commercial VFR helicopter operations (see CH.VII.A.S8).
Last edited by Easy Street; 21st October 2025 at 15:16 .

Subjects FAA  IFR  Radio Altimeter  Route 4  Separation (ALL)  VFR  Visual Separation

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BFSGrad
October 21, 2025, 16:07:00 GMT
permalink Post: 11973734
Originally Posted by Easy Street
...and the pilots were using the (badly named...) radalt as their height reference.
The interview transcripts indicated that the 12th AB Blackhawk pilots used barometric altitude as the reference for flying the DC routes.

Subjects Blackhawk (H-60)  Radio Altimeter

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Easy Street
October 21, 2025, 18:22:00 GMT
permalink Post: 11973780
Originally Posted by WillowRun 6-3
(I'm imagining that military aviators may disagree insofar as it may be an article of faith as well as military regulation that the FAA is absolutely the one responsible party for civil controlled airspace, but as a legal point I think plaintiffs will attack it.)
Route 4 wasn't restricted to use by military helicopters, so it should be possible to argue the unsafe design point based purely on the FAA's own specification. As to expectations that the Army pilots should have flown the route to tighter tolerances, even a tolerance of plus zero would have been grossly unsafe on a procedural basis: altimeter errors alone would take up most of the 50-odd foot "separation", and variances in airliner approach slope angle the rest. Besides, "plus zero" is an impossible tolerance to achieve when maintaining an altitude or height. The only way of flying that route not above 200 feet on a "IFR-esque" procedural basis with an achievable tolerance would be to fly 150 feet plus or minus 50 feet, which would demand total focus on height keeping via radalt (it would be hopelessly unsafe to attempt to fly that low on barometric instruments).

Subjects Altimeter (All)  FAA  Radio Altimeter  Route 4  Separation (ALL)

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Musician
October 21, 2025, 19:09:00 GMT
permalink Post: 11973814
Originally Posted by Easy Street
Besides, "plus zero" is an impossible tolerance to achieve when maintaining an altitude or height. The only way of flying that route not above 200 feet on a "IFR-esque" procedural basis with an achievable tolerance would be to fly 150 feet plus or minus 50 feet, which would demand total focus on height keeping via radalt (it would be hopelessly unsafe to attempt to fly that low on barometric instruments).
I'vd argued before that any route that does not achieve 500ft above terrain is unsafe by the FAA's own standards, for much the same reasons.

Subjects FAA  Radio Altimeter

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JohnDixson
January 31, 2026, 18:23:00 GMT
permalink Post: 12030315
This was supposed to be a checkride including Night vision Goggle usage and a bunch of submittals regarding the use and accuracy of the bardo metric altimeter system, and not much, if anything regarding the radar altimeter system accuracy.
The UH-60 Maintenance Test Flight Manual includes a barometric altimeter accuracy check, which is accomplished by calling the tower for the local altimeter setting, set the altimeter accordingly and comparing the altimeter readout to the elevation of that heliport/airport. THE ACCEPTED MAXIMUM ERROR IS 70 FEET. So, the baro altimeter readout can be 70 ft off and you are OK to fly: VFR or IFR.

The radar altimeter ( APN-171 or later APN-209 ) accuracy is similar at +/- 3 ft then 3% of indicated altitude, so, for the H-4 Route Maximum altitude of 200 ft. The Rad Alt could be as much as 9 ft off.

The NVGs do show Rad Alt.

Haven\x92t seen any submittal indicating the Rad Alt was inoperative or unusable.

Subjects Altimeter (All)  Barometric Altimeter  IFR  Night Vision Goggles (NVG)  Radar  Radio Altimeter  VFR

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DaveReidUK
January 31, 2026, 21:00:00 GMT
permalink Post: 12030376
Originally Posted by JohnDixson
This was supposed to be a checkride including Night vision Goggle usage and a bunch of submittals regarding the use and accuracy of the bardo metric altimeter system, and not much, if anything regarding the radar altimeter system accuracy.
The UH-60 Maintenance Test Flight Manual includes a barometric altimeter accuracy check, which is accomplished by calling the tower for the local altimeter setting, set the altimeter accordingly and comparing the altimeter readout to the elevation of that heliport/airport. THE ACCEPTED MAXIMUM ERROR IS 70 FEET. So, the baro altimeter readout can be 70 ft off and you are OK to fly: VFR or IFR.

The radar altimeter ( APN-171 or later APN-209 ) accuracy is similar at +/- 3 ft then 3% of indicated altitude, so, for the H-4 Route Maximum altitude of 200 ft. The Rad Alt could be as much as 9 ft off.

The NVGs do show Rad Alt.

Haven\x92t seen any submittal indicating the Rad Alt was inoperative or unusable.
None of the references to NVGs in the hearing made mention of a HUD capability.

The Helicopter Operations presentation included a slide stating that pilots are "Trained to use barometric altimeter to navigate helicopter route ceilings".

Subjects Altimeter (All)  Barometric Altimeter  HUD  IFR  Night Vision Goggles (NVG)  Radar  Radio Altimeter  VFR

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BFSGrad
January 31, 2026, 22:24:00 GMT
permalink Post: 12030414
Originally Posted by JohnDixson
The radar altimeter ( APN-171 or later APN-209 ) accuracy is similar at +/- 3 ft then 3% of indicated altitude, so, for the H-4 Route Maximum altitude of 200 ft. The Rad Alt could be as much as 9 ft off. The NVGs do show Rad Alt. Haven\x92t seen any submittal indicating the Rad Alt was inoperative or unusable.
Extensive discussion in the interview transcripts about why all of the DC area helicopter pilots (medical, LEO, military) use baro rather than radar altitude when flying the DC helicopter routes.

Subjects Altimeter (All)  Night Vision Goggles (NVG)  Radar  Radio Altimeter

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Lonewolf_50
February 18, 2026, 17:39:00 GMT
permalink Post: 12039290
For Chiefttp:
The question of currency, proficiency, and recency fairly leap off of the page, yes.
Originally Posted by DaveReidUK
At the height the collision occurred, there would never have been a TCAS RA.
Inconvenient facts never stopped a lawyer from bringing a case to court, though.
Originally Posted by Musician
Page 242 ff. in the final report pretty much exonorates the PF in the helicopter, in my opinion.
No, it does not.
Originally Posted by Musician
Yes, but the visual separation was the responsibility of the PIC instructor, not the PF.
Wrong. Visual lookout is a responsibility for all members of the crew. That's a shared responsibility, and briefed before every flight. Note that in a Blackhawk, there are a variety of zones where the pilots are effectively blind (starting at about the four o'clock position and reaching to about 8 o'clock position) but the forward quarter isn't usually one of those. (Won't comment on the goggle issue here...)
Originally Posted by Musician
You are of the opinion they should've checked that the altimeter was working correctly? Is that a normal item on a pre-flight checklist?
Not just pre-flight checks.
I am not sure how much low level, over water, at night flying that you have done, but I have done quite a bit of that. If you are flying in such a regime, and there is a substantial mismatch between your radalt, and your baralt, and you have a hard altitude limit, you don't ignore your radalt.

Subjects Altimeter (All)  Blackhawk (H-60)  Final Report  Radio Altimeter  Separation (ALL)  TCAS (All)  TCAS RA  Visual Separation

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