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| JRBarrett
February 02, 2025, 03:43:00 GMT permalink Post: 11819495 |
That's assuming perfect equipment accuracy. My understanding is that the tolerance of e.g. an IFR altimeter in the USA is 75'. If that's correct, one IF aircraft with a 'legal' altimeter indicating 325' could in fact be at 250' and another IFR aircraft with a 'legal' altimeter indicating 175' could in fact be at ... 250'. I'm hoping that the avionics in the aircraft involved in this terrible tragedy were more accurate than that, but I always exercise caution in taking numbers out of avionics and ATC systems as 'gospel truth' to the foot. The altitudes on RADAR displays don't increase and decrease in 1 foot increments; nor do the outputs of aircraft transponders. RADALT is different.
Subjects
ATC
Altimeter (All)
Barometric Altimeter
Blackhawk (H-60)
CRJ
IFR
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| Lead Balloon
February 02, 2025, 04:12:00 GMT permalink Post: 11819502 |
The maximum allowable altimeter error between sea level and 1000 feet is +/- 20 feet, and even a simple stand-alone barometric altimeter has to be able to meet that requirement. The RVSM-certified Air Data Computers on something like a CRJ are typically much more precise than that - more like +/- 5 feet at almost all altitudes. I would assume the air data system on a Blackhawk would be equally precise at low levels.
Subjects
Altimeter (All)
Barometric Altimeter
Blackhawk (H-60)
CRJ
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| JohnDixson
February 02, 2025, 15:22:00 GMT permalink Post: 11819856 |
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. Subjects
Altimeter (All)
Barometric Altimeter
Radar
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| magyar_flyer
February 02, 2025, 15:44:00 GMT permalink Post: 11819866 |
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. So unless a very unlikely failures they knew their altitude within a few ft. Subjects
Altimeter (All)
Barometric Altimeter
Radar
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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.....
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. 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).
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.
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.
Subjects
ATC
Altimeter (All)
Barometric Altimeter
Blackhawk (H-60)
CRJ
DCA
NTSB
Radar
Radio Altimeter
Route 4
TCAS (All)
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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, 20:09:00 GMT permalink Post: 11932030 |
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. 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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| DaveReidUK
January 27, 2026, 23:09:00 GMT permalink Post: 12028272 |
Probable Cause Statement:
The NTSB determines that the probable cause of this accident was the FAA's placement of a helicopter route in close proximity to a runway approach path.
Their failure to regularly review and evaluate helicopter routes and available data, and their failure to act on recommendations to mitigate the risk of a mid-air collision near Ronald Reagan Washington National Airport, as well as the air traffic system's overreliance on visual separation. In order to promote efficient traffic flow without consideration for the limitations of the see and avoid concept. Also causal was the lack of effective pilot applied visual separation by the helicopter crew, which resulted in a mid-air collision. Additional causal factors were were the tower team's loss of situational awareness and degraded performance due to a high workload of the combined helicopter and local control positions, and the absence of a risk assessment process to identify and mitigate real time operational risk factors, which resulted in miss prioritization of duties, inadequate traffic advisory advisories, and the lack of safety alerts to both flight crews. Also causal was the Army's failure to ensure pilots were aware of the effects of air tolerances on barometric altimeter in their helicopters, which resulted in the crew flying above the maximum published helicopter route altitude. Contributing factors include the limitations of the traffic awareness and collision alerting systems on both aircraft, which precluded effective alerting of the impending collision to the flight crew's. An unsustainable airport arrival rate, increasing traffic volume with a changing fleet mix and airline scheduling practices at DCA, which regularly strain the DCA Atct workforce and degraded safety over time. The Army's lack of a fully implemented safety management system, which should have identified and addressed hazards associated with altitude exceedances on the Washington, D.C. Helicopter routes. The FAA's failure across multiple organizations to implement previous NTSB recommendations, including Ads-b in and to follow and fully integrate its established safety management system, which should have led to several organizational and operational changes based on previously identified risk that were known to management and the absence of effective data sharing and analysis among the FAA aircraft operators and other relevant organizations. Subjects
Altimeter (All)
Barometric Altimeter
DCA
FAA
NTSB
Probable Cause
Route Altitude
See and Avoid
Separation (ALL)
Situational Awareness
Visual Separation
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| Undertow
January 27, 2026, 23:41:00 GMT permalink Post: 12028291 |
Also causal was the Army's failure to ensure pilots were aware of the effects of air tolerances on barometric altimeter in their helicopters, which resulted in the crew flying above the maximum published helicopter route altitude.
Subjects
Altimeter (All)
Barometric Altimeter
Blackhawk (H-60)
Route Altitude
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| paulross
January 29, 2026, 12:41:00 GMT permalink Post: 12029104 |
This Thread Re-Mixed
As new information has emerged I have just rebuilt the website that re-organises this long thread by subject.
You can find it here: https://paulross.github.io/pprune-th...DCA/index.html All 1,829 posts are organised into 68 subjects. Changes:
The project is here: https://github.com/paulross/pprune-threads . Issues can be raised here: https://github.com/paulross/pprune-threads/issues or PM me with ideas. Subjects
Altimeter (All)
Barometric Altimeter
Final Report
Findings
Helicopter Working Group
Hot Spots
NTSB
NTSB Docket
Probable Cause
Route Altitude
Safety Recommendations
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| Musician
January 29, 2026, 13:55:00 GMT permalink Post: 12029155 |
weird that they don’t even mention the Blackhawk PF’s straying from altitude constraints, the IP repeatedly tells her about her deviations multiple times as per the transcript, baro altitude limitations or not they were both aware she wasn’t meeting the limits of the corridor (that the margins are so fine in that airspace is absurd of course)
https://www.ntsb.gov/investigations/...CA25MA108.aspx
31. Due to additive allowable tolerances of the helicopter’s pitot-static/altimeter system, it is likely that the crew of PAT25 observed a barometric altimeter altitude about 100 ft lower than the helicopter’s true altitude, resulting in the crew erroneously believing that they were under the published maximum altitude for Route 4.
.
27. The PAT25 instructor pilot did not positively identify flight 5342 at the time of the initial traffic advisory despite his statement that he had the traffic in sight and his request for visual separation.
Subjects
Altimeter (All)
Barometric Altimeter
Blackhawk (H-60)
CRJ
Findings
NTSB
PAT25
Route 4
Separation (ALL)
Traffic in Sight
Visual Separation
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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 |
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. 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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| Musician
February 18, 2026, 01:30:00 GMT permalink Post: 12038924 |
Page 242 ff. in the final report pretty much exonorates the PF in the helicopter, in my opinion.
.
Altimeter testing showed that the 100-ft pressure altitude discrepancy seen in the FDR data for the accident flight was observed on three other UH-60L helicopters operated by the 12th Aviation Battalion. These altimeter testing results also showed that the pressure altitude data recorded by the helicopters\x92 FDRs, when corrected for local conditions, was representative of what was indicated on the right side altimeter. Therefore, the FDR pressure altitude data for the accident helicopter, when corrected for local conditions, was likely representative of what was indicated on the IP\x92s barometric altimeter during the accident flight.
[...]. The NTSB concludes that, due to additive allowable tolerances of the helicopter\x92s pitot-static/altimeter system, it is likely that the crew of PAT25 observed a barometric altimeter altitude about 100 ft lower than the helicopter\x92s true altitude, resulting in the crew erroneously believing that they were under the published maximum altitude for Route 4. For the northern segment of Route 4, which included the area of the collision, of the 523 flights analyzed, 260 flights (49%) were identified as exceeding route altitude limitations at some point during the flight. Had the error tolerances of barometric altimeters been considered during design of the helicopter route maximum altitudes, the incompatibility of a 200-ft ceiling and barometric altimeter errors may have been identified. Subjects
Altimeter (All)
Barometric Altimeter
Final Report
NTSB
PAT25
Route 4
Route Altitude
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| Ver5pen
February 19, 2026, 14:18:00 GMT permalink Post: 12039724 |
Page 242 ff. in the final report pretty much exonorates the PF in the helicopter, in my opinion.
.
Altimeter testing showed that the 100-ft pressure altitude discrepancy seen in the FDR data for the accident flight was observed on three other UH-60L helicopters operated by the 12th Aviation Battalion. These altimeter testing results also showed that the pressure altitude data recorded by the helicopters\x92 FDRs, when corrected for local conditions, was representative of what was indicated on the right side altimeter. Therefore, the FDR pressure altitude data for the accident helicopter, when corrected for local conditions, was likely representative of what was indicated on the IP\x92s barometric altimeter during the accident flight.
[...]. The NTSB concludes that, due to additive allowable tolerances of the helicopter\x92s pitot-static/altimeter system, it is likely that the crew of PAT25 observed a barometric altimeter altitude about 100 ft lower than the helicopter\x92s true altitude, resulting in the crew erroneously believing that they were under the published maximum altitude for Route 4. For the northern segment of Route 4, which included the area of the collision, of the 523 flights analyzed, 260 flights (49%) were identified as exceeding route altitude limitations at some point during the flight. Had the error tolerances of barometric altimeters been considered during design of the helicopter route maximum altitudes, the incompatibility of a 200-ft ceiling and barometric altimeter errors may have been identified. the PF had also shown their handling skills were not to standard earlier in the check ride (I\x92ve seen it mentioned their abandoning a manoeuvre earlier would\x92ve been a fail normally) hence monitoring their trainees parameters would\x92ve been even more taxing for the instructor I don\x92t know how anyone can pretend these things didn\x92t at least play a part in the Swiss cheese. if the PF had been as equally capable as the instructor and performing their scan (the CGI reenactment shows that much of the CRJ\x92s flatboats occurred within the PF\x92s side of the scan) would the outcome have been different? Possibly. the conduct of that flight was the final hole in the Swiss cheese arguably Subjects
Altimeter (All)
Barometric Altimeter
Blackhawk (H-60)
CVR
Final Report
NTSB
PAT25
Route 4
Route Altitude
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| punkalouver
February 19, 2026, 17:02:00 GMT permalink Post: 12039783 |
I have not read this report, as I am deep into many other ones these days but I did take a quick glance on page 242 that was referenced and it talks about altimeter additive errors. The report states: "The allowable tolerances are additive, with the total error having the potential of exceeding 100 ft.". The report also states: "The NTSB concludes that, due to additive allowable tolerances of the helicopter\x92s pitot-static/altimeter system, it is likely that the crew of PAT25 observed a barometric altimeter altitude about 100 ft lower than the helicopter\x92s true altitude, resulting in the crew erroneously believing that they were under the published maximum altitude for Route 4". My question to other people on this thread is: Did the NTSB do some sort of evaluation of this particular helicopter in order to come to a reasonable conclusion that all errors were in such a way that they were all in the direction of resulting in the helicopter being higher than indicated as opposed to errors potentially cancelling each other out(or partially so)? Subjects
Altimeter (All)
Barometric Altimeter
NTSB
PAT25
Route 4
Separation (ALL)
VFR
Visual Separation
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