Posts about: "Visual Separation" [Posts: 250 Page: 6 of 13]¶

I think the issue is there are no visual separation standards, only IFR ones. Conflict Alerting (ATC) and TCAS (aircraft) have yet another set of parameters they use in different ways. The most common reason visual separation is requested by either party is to reduce separation below 1,000\x92/500\x92, 1.5, 2, 3, 5 miles or whatever is appropriate to the categories of airspace, aircraft and flights.

This means the automated tools (which don\x92t know the aircraft are using visual means to deconflict) will go off based on a predicted or actual loss of the separation criteria that they\x92ve been programmed with. If the helicopter in this instance had passed 1/4m behind and below the CRJ, a CA may still have been generated although the conflict had been resolved visually. The controller actually picks up on the apparent proximity and queries the heli that they are still visual, to which they reply in the affirmative - there is no minimum separation for visual avoidance, just sometimes it\x92s too dang close. Which is an Airprox.

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.

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.

If I understand the sequence of events, PAT25 requested visual separation before they got to the report/hold point near Hains Point. IDK what they refer to this point as, but if the controller had denied the request and instructed them to hold at Hains Point, what would everyone have done? Is that a normal procedure? We're told that we have knowledgable senior people here so I'd like to know if that's plausible or doesn't work for some reason.

The CRJ CVR transcript does show the "Traffic! Traffic!" callout, but since this was a visual approach (non-precision in VMC even though it was at the end of an IFR flight) and visual separation was in use, why would both parties not be explicitly informed by the controller?

"5342, helo traffic on your right 1/4 mile at 300 feet, has you in sight". The CRJ FO might just have taken a closer look out the side window with that. Or, like you, they might have opted to go around.

Proper aviation safety is based on having multiple layers, in this case it was reduced to one (the helicopter pilot maintaining visual separation), which clearly failed. At least there should be active vectoring by ATC AND (visual acknowledgment of BOTH cockpits OR at least 500 ft vertical OR 1.5nm of horizontal separation). And even this is pushing it.

Additionally, any helicopters flying in such congested airspace should have a display showing other traffic so they know where to look outside.

Maybe we should check under which conditions this helicopter route with its restrictions was designed and implemented and how it is used today.

Imho the altitude sectored restrictions were never suitable and safe to deconflict helicopter traffic from traffic to finals 01 and 33, but were meant to deconflict takeoffs from 15 and 19. Could it be that someday some clever soul thought to solve increasing traffic demands by using 33 and 01 for landing despite traffic in the helicopter routes under visual separation rules, ignoring thereby that now all layers of safety had been removed bare the eyes of an helicopter crew?

I disagree. Visual separation had little if anything to do with this. The single layer was the utterly bizarre airspace design which mandates a helo route to cross a final approach track at effectively the same height which is simply insane. Once again, at Heathrow, one of the world's busiest airports, crossing helo traffic is held at clearly defined holding points if necessary by the Tower controller on the same frequency as all other Tower traffic and, (another critical difference) having positively identified the horizontally conflicting traffic by using proper definitive r/t as opposed to the lethally sloppy 'do you see the CRJ' when there are many aircraft to see it is cleared to cross behind, which is visual separation horizontally. But with 1000ft vertical too.
You cannot actively vector a helo at 200ft over the black hole of a river in the middle of an urban environment or anywhere else. You'd kill helos every month doing that. If the helo were at 100ft or so there would be no need for vectors (you never get them in London, just holding, ie orbits if necessary).
Visual acknowlewgement from the aeroplane is totally unnecessary as he is on finals to land which give total priority over all other traffic. It is up to the give-way traffic to identify and acknowlege.
500ft vertical I agree with, in which case 1,5 miles is totally unnecessary and ridiculously excessive.

This discussion is being considerably bogged down by a really surprising (to me) absence of understanding of helos and helo ops by people who clearly only fly f/w IFR and seem to have no concept of how the rest of the aviation world works. Strangely, there are other ways of aviating safely without staring exclusively at an instrument panel, following a magenta line and doing only what someone miles away in an office tells you. You simply cannot try to apply IFR airline type procedures and mindset to low level VFR traffic. It's like a train driver trying to dictate railway rules to a motorcyclist and being unaware that motorcycles just don't operate like trains... imaginingthat helos can or would come to a free air hover for separation is another example of unrealistic imagnation over reality.

Please, if you don't know anything about helicopter ops, please don't try to apply procedural IFR or Hollywood mindsets/misapprehensions to them as if there is no other waay of flying.

Confusion between 'VFR" and "visual separation" seems to be quite widespread. However the terms are not interchangeable. I don't think anyone thinks it's remotely likely that VFR would be prohibited (or in other words, that Class A airspace would be established - even the comparatively restrictive UK CAA reclassified Heathrow's zone away from Class A to bring an end to the fudge of using Special VFR clearances for helo ops). Imposing separation criteria other than "visual" does not imply that flights must suddenly switch to IFR. It would remain quite possible to apply procedural, geographical or surveillance based separation to VFR aircraft in Class B. Whether and how such procedures should account for lesser standards of altimetry, height keeping, etc in non-IFR certified ops would be a point of interest (Special VFR deals with that by requiring IFR certification).

Perhaps you mean that DoD would not be too happy with not being able to take visual separation, at night, using NVG? I think they might have to suck that up - especially the second and third aspects.

Disagree, as relying on only visual separation from the helicopter had everything to do with this. As for the rest, I think we mostly agree.

I think I indicated either having 500 ft vertical OR 1.5 nm horizontal, not both (which would indeed be excessive). Visual acknowledgment is only required within that hockey puck like area and I doubt whether it is acceptable to allow that option at night.

I do think the landing aircraft should be made aware of other traffic if it gets within the safe area, either by having that traffic on the same frequency, or by actively informing them. Situational awareness is everything and in any case this prepares them for a probable 'traffic' warning.

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.

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.

Tower pointed out the CRJ twice and PAT25 affirmed they had traffic in sight and twice requested visual separation, which was approved. Controller did everything right.

Question: In the last seconds:

Controller instructed very firmly: "PAT25, pass behind the CRJ"

There was no such readback, instead:
PAT25: "PAT25 has CRJ in sight, request visual separation"
Controller: "vis sep approved"

Does the "request visual separation" undo the "pass behind"?
(just trying to refresh my phraseology understanding, its long time passed, my PPL is not current a long time since)

That could have been better, he didn't give a traffic distance/bearing and I don't recall him (I would be happy to be corrected) Mentioning that the CRJ was transitioning from RWY 01 to RWY 33

Tower: "PAT25, traffic just south of the Woodrow Bridge, a CRJ, it's 1200 feet setting up for runway 33"
PAT25: "PAT25 has the traffic in sight, request visual separation"
Tower: "Visual separation approved"

0:26 here:

I have struggled to understand why PAT requested visual separation the 2nd time given that it had been approved in the first instance.

Perhaps, and this is big perhaps, it's a pavlovian response to whenever PAT is advised of other traffic. I listened to the TCAS RA missed approach from the previous day, and once again the response from PAT is "request visual separation". It's highly likely that the pilot requests for visual separation is the only way that this Class B airspace can operate with the mix of IFR vs VFR, and aerodrome traffic vs transits.

I fail to understand why PAT is using UHF, surely this is another slice of cheese.

The use of RWY 33 for arrival makes it easier for the ATC and the aircrew with one less runway crossing after they have landed. To emphasis the point, the following PSA actually requests RWY 33.

This discussion about the visual separation is a moot point.

Using visual separation as the only line of defence at night (and even during daylight) is utter madness. As is having aircraft circle to multiple runways, mixing high density military and civilian traffic, and a multitude of other issues.

Had full IFR separation been applied this accident wouldn\x92t have happened.

Not quite. They were asked if they were visual with a CRJ. How would they know, at that point still a couple of miles away, which aircraft lights all around them in the dark were "the CRJ" ? In fact there was more than one of this type around.

The accident aircraft was making a sidestep curving manoeuvre, a late change from a straight in to 01. The only message passed about this was it was landing on 33. No comment that it was going to break off the 01 approach. No questioning that the heli crew even understood how an aircraft now approached 33, making this unusual and last-minute change, nor that it would compromise them routing along the river.

.

Circle to 33 is a very common and understood maneuver at DCA. The clearance was issued to CRJ and visual approved to PAT 25 while the CRJ was outside the Wilson bridge, about 7 miles from the airport. Both crews were familiar with the procedure. It bit of mystery why the Army was so quick to request visual separation, but I\x92d guess it\x92s \x93Pavlovian\x94