Posts about: "Radar" [Posts: 135 Page: 4 of 7]¶

not an American readers, but was thinking the same couple of days ago, so I found this:
https://www.faa.gov/air_traffic/publ...section_6.html

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Still, if the helo crew misidentified the A319 as the CRJ, and they assumed the A319 will start turning right soon to circle for 33, turning right as well would have increased horizontal separation. And the extended centerline for 01, which the A319 was aligning with, does actually intersect with Route 4 a bit after the Wilson Bridge. And, looking at the radar replay, the A319 did actually turn right for a bit, to align to 01. At around the same time, the helo starts turning right.


I agree that these are just guesses, but the alternative seems to be that the helo just drifted to the right randomly, for no specific reason, which seems unlikely.

Well I only replied to the CRJ not crossing "Route 4" as some people paint on screenshots...

the A319 was adjusting a few degrees left and right, but wouldn't call that 'turning' (but of course its position would make it an ideal target for misidentification)



Well, it's probably my worn out eyes, but I don't see really any reliable evidence of the helo turning right ... remember it came out of Route 1 which ends in one big right-hand turn until joining Route 4 which only after passing KDCA airfield, has a very slight course adjustment to the left.

And the "amateur MLAT" tracking of the helo, is only a rough indication of the trajectory with a wide margin of position error and should be interpreted more like the right side hereunder:

Could they have been over-correcting for a quartering tail wind from their right which was at 15mph gusting to 25mph? That's not insignificant.

The radar video from AV Herald seems to be a recording of the actual radar display, not "amateur MLAT". Of course, without an overlay with the map and the routes it's a bit hard to say how much the helo deviated from its route. I tried to create such an overlay, but I can't vouch for its accuracy:



And yeah, with the route overlayed on the radar, the deviation from the route doesn't seem to be that great, since the route itself also turns right.

The issue here, I believe, is that there's different standards for visual separation vs radar separation. A separation that results in a CA from the radar may still be entirely legal and acceptable. It's like trying to work out if two cars at an intersection will crash into each other from their GPS trackers: the data just isn't good enough and you don't know if one car will actually stop at the stop sign at the last second.

Radar orders also need to be given and actions taken sooner than if the crews are doing it of their own initiative. So a radar CA needs to be visible say 15 second pre-collision so ATC can wait for the radio to be clear then order pilots to manoeuvre. Pilots can aim to cross visually at more like 5 seconds.

I'm not saying that this is overall a good idea, but the fundamental reason you fit more planes in with visual separation is that you can put them closer together with (given good visibility) not too dissimilar safety.

[Edit: too late... Fullwings got this.]

One aircraft gets a PULL UP, the other gets a NO CLIMB. Ideally you would have each aircraft advertise how much altitude it can gain/lose in 5/10/15 seconds and make the decision based on that. It would also fix needing to turn to TA only after engine failure.

To the contrary, you're legally obliged to..


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CA will activate whenever certain separation minima are going to be lost (600\x92 and 2 miles for example).
Visual separation is a perfectly legal tool that allows separation to reduce below radar minima.
CA will (almost) always activate when applying visual separation. That doesn\x92t mean the aircraft are on a collision course or are definitely going to crash, it just means they will be in confliction and separated by less than the IFR radar minima.

Its activation in this scenario (and countless others in the days before) was totally normal. Anyone who thinks it should have been reacted to differently or that it was a last line of defence really doesn\x92t understand the role of Conflict Alert.

And it\x92s not the role of the radar centre to phone tower and warn them of a conflict alert between two aircraft they would expect to be visually separated - that\x92s a scenario that probably happens several times a day.

What is your source for the G/S? According to the surreptitiously recorded radar display, the helicopter ground speed appears to be closer to 80 kts.

apologies, I may have misheard it somewhere.

I think we can take this (radar recorded) data-source as pretty reliable, it matches (rounded) the ADS-B reported CRJ speed of 121kts at 375ft (QNE)

I think the confusion comes from the "amateur MLAT" tracking, which calculates the GS based on the multilaterated position calculations, which have a (relatively) large margin of error:


And probably PAT25 was doing initially something in the region of 100kts GS (edit: averaging all but last calculated GS, gives 105kts as average - and over more datapoints, longer trajectory, calculated average GS becomes more reliable)

but at the end it seems there might possibly have been a decreasing GS trend:

Just for illustration, this is how it's done in London (or was some time ago last time I did this sort of work). Accurate compliance with routes is strictly enforced and clearances are SVFR day and night unless this has changed. Almost invariably clearance to enter would state the route/s ie 'H4 H3 Bagshot Mast' (straight through) and no other instruction required - that takes you E -W right through the centre of London and out to the W passing 5 miles S of Heathrow. Right hand rule applies and opposing traffic on the route is always advised.
Usually, only if crossing LHR you'd be cleared to enter via requested route with limit Bedfont/Sipson, sometimes Airport Spur to hold (orbit) and change from Heathrow Special to Tower for the crossing itself. There's a further hold at Twin Taxiways between the runways. Altitudes are shown. Note there is usually unrestricted passage on routes H3 and H10 along the river directly under the approach. This system works seamlessly and with - to date - total safety.
Accepted the aairport we are discussing has more varied runway directions than Heathrow so the situation would be a bit more complex but I can't see why a similar system couldn't be devised - with defined clearance limits, sensible vertical separation and, critically, coherent and specific controller voice procedure.

There's no reason not to make landing traffic aware of helos holding close in if appropriate and indeed that happens, but no way is their visual contact required.
The entire system operates on visual 'separation'. Helos cross visually behind traffic as cleared, but with vertical separation. It's as safe as the system can be made. How else could it work? It requires no controller vectoring and the time and space margins that would be required if radar separation was used would render the slick, efficient visual system cumbrous, unacceptably high end unnecessary workload and probably unworkable.
Please, once again let's stop applying this insular f/w procedural IFR mindset to VFR helo traffic. There seems to be a procedural IFR mental blockage that can't see that 'visual separation' occurs in three dimensions, not just two. Helos are perfectly capable of ensuring visual separation as long as the traffic has been correctly identified and with vertical separation as here even if a mistake is made there is 800ft clear vertically. Also, VFR does NOT mean, as many seem to imagine, blundering about randomly at will, it is often every bit as disciplined and controlled as IFR as Shackman reiterates below, these routes are rigidly enforced to within a hundred metres or so and woe betide the transgressor.

The elephant in the room here is a combintion of a ridiculously hazardous two-dimensional crossing procedure combined with culpably sloppy & imprecise r/t which offers no second slice of cheese, not matters of visual separation. I'm well aware that our transatlantic cousins are sensitive to criticism of their relaxed, easygoing and informal ways in the air but in this case they self-evidently were the direct cause of 70 odd deaths. While they may regard European style as excessively pedantic there's no doubt whatsoever that had European standards applied here this event would have resulted in nothing more serious than a MOR and an Airmiss report.

For those unfamiliar the light grid squares are 1Km so the Sipson and Bedfont reporting/holding points is ony about 500m from the runways.

meleagertoo forgot to add - and RIGIDLY radar monitored and enforced. Get one bit wrong and you are given immediate radar controlled turn to the nearest 'edge' of the TCA and invited to telephone LHR on landing. I was a pax with our CO flying when he went about 300ft past Kew Bridge on H10 - he wasn't very happy, and to compound his error had an airmiss filed against him by an aircraft on approach to 27R.

This describes very effective procedural separation between VFR and IFR traffic.

As you've described it, the only place where the Heathrow system operates using visual separation between helicopters and airline traffic is when crossing the airport behind landing aircraft. That's a very different risk proposition to visual separation away from the airport.

Procedural separation has the same benefits in terms of reducing controller workload.

There's the rub. Easy to achieve when the traffic in question is on a runway, such as "cross 27R behind the landing A380"... not so assured otherwise. I stand to be corrected, but I would be very surprised if helicopters using routes underneath the approach had to report and maintain visual contact with each aircraft in the stream as it passed overhead. Separation is built into the procedures: not delegated to the pilots.

Agreed, but what you are describing is procedural separation with 800ft vertical separation.

Not just procedural separation: rigidly radar monitored procedural separation, no less. This is a very, very different thing to visual separation.

I agree with your underlying point that blanket application of IFR separation criteria would be inappropriate. But there are modes of separation besides the false binary of 'visual' and 'IFR' which can be applied to VFR traffic.

I don't think it make a significant turn right, and I believe that what you are seeing is positional inaccuracy due to MLAT position limitations in the ADS-B Exchange data. According to Flightradar24, " MLAT position calculations have a general accuracy of 10-100 meters and 1000 meters in the worst cases." Given that, I believe this previous post from MikeSnow AA5342 Down DCA represents our best current understanding of the actual helicopter track and its relation to Route 4 (until we get more information from the NTSB). I generated my own zoomed overlay and got essentially the same results



The position of the collision shown in the radar data overlay is consistent with the position of the RJ as shown in ADS-B Exchange at the time of the collision (approximately 01:47:59Z according to the NTSB timeline). See this link from DIBO for the RJ Track with timing AA5342 Down DCA In my experience, times in ADS-B Exchange are generally accurate to within 2 seconds. Given all that, I believe that the Black Hawk was within the horizontal bounds of Route 4 at the time of the collision and that it did not make the right turn we see in the ADS-B Exchange map.

Edit: Corrected route number and helicopter

IFR separation is not appropriate for VFR traffic. Class B airspace has specific rules for separation of VFR/VFR and VFR/IFR aircraft. The helicopter routes do not appear designed to provide the required separation for VFR helicopters, at least where Route 4 and RWY 33 final approach cross each other. Was the design at fault OR was the daily application at fault? If the design was at fault, it\x92s an FAA DOD problem. I find it hard to believe the FAA didn\x92t address the correct separation distances in the design phase\x97it\x92s pretty clear. The FAA may have planned crossing traffic wouldn\x92t happen or there\x92s a FAA LOA defining how crossing traffic would be controlled, but the FAA didn\x92t say \x93okay, let\x92s go with 150\x92 vertically\x94.

Did the FAA or the Army assume they would always be able to apply visual (NOT VFR) separation. Visual separation does not necessarily mean 500\x92 vertically and 1.5 nm or radar target separation; it means \x93I see you, I miss you\x94. Did the operating plan always direct crews to use visual separation as the default plan? I hope not, but it is only I see it being written.

Only to Rwy 33, not Rwy 1. Yuge difference.

1.5NM is obviously not applied, look at the south flow arrivals TCAS RA the day prior. It, a PAT merged previously with a SWA 737 at TCAS TA altitudes with Collision Alert to ATC radar, before causing the later RA.

The capitol cities of both Great Britain and France both have heli-route structures, and both have good radar coverage ( at least they did 20 years ago ). Depending on traffic, they’ll hold you at one of the check points. And the reason we can only have eyeball control in our capitol is…………..?
And, oh-forgot to mention: if one ignores the radar controller’s instruction to hold, said helicopter is met by the law ( least-thats what happened to a guy I was following in Paris, who blatantly ignored a hold command at Bagnolet. By the time I arrived at the Le Bourget helipad his ship was surrounded by gendarmes and a black van.

The Thames going by the Battersea Heliport is narrower than the Potomac, yet they do two way traffic. Is there an underlying reason why radar control of traffic on the DC heliroutes is a forbidden subject?

But, S-S, if the two flying machines we are discussing were under radar control, the accident would not have occurred.