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Author Topic: Question re: TCAS and 'coffin corner'  (Read 359 times)

torrence

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Question re: TCAS and 'coffin corner'
« on: Sun, 13 Jan 2019 22:43 »
I have a couple of questions about a PSX scenario that I encountered recently.  I was doing a PSX KLAX-KSEA flight, flight plan alt FL380, with VNAV CRZ setting for ICAO step climbs.  I don’t know if that automatically triggers PSX ATC to keep clearing to higher altitudes, but in any case, on this flight I quickly got cleared to FL480, which was just a few hundred feet below the maximum altitude on the VNAV page.  I knew that I should probably stay lower, but decided to see what would happen (Also, I don’t think PSX has an ‘unable’ command when using the voice ATC simulation – maybe putting 0 in step climb?  Didn’t try that.)  I climbed to FL480 and had the altitude indicator nestled just between the yellow bricks as expected.  Then came the intruders.  I usually fly with the Traffic tab set for “start a semi-random preset” just to get practice occasionally with conflict resolutions.  On this flight the result was a string of flights out of KSEA coming at me on a reciprocal heading with -020 relative altitude tags.  They were remaining level, but I guess they were within the ‘bubble’ that TCAS counts as too close and I was getting CLIMB, CLIMB resolution advisories that I couldn’t comply with without stalling. 

I’ve got a couple of questions for the real-world pilots:

1.   Would you normally accept a flight level so close to the limits?
2.   If you did end up in this type of situation, would you: a. ignore the advisory, b. respond by maneuvering laterally, and inform ATC of your action on the ‘see and be seen’ principle, or c. ask the galley for more coffee.

I really got to wondering about this just the other day when I heard that the last SPACE-X launch had completed the Iridium constellation and one result would be to allow closer packing of flights on the North Atlantic Tracks.  Assuming Hoppie can get back to work soon installing his magic boxes etc.  :)

Cheers,
Torrence
Cheers
Torrence

Hardy Heinlin

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Re: Question re: TCAS and 'coffin corner'
« Reply #1 on: Sun, 13 Jan 2019 23:55 »
Really 480? Not 380?

450 should be max.

Yes, the ATC robot looks at your FMC step climb predictions.

I think there are aircraft type specific TCAS RA limits, but just type specific, not specific to the current performance. If you have an engine failure you should select "TA only" so that the intruder is the one that has to climb in case of a conflict.


Cheers,

|-|ardy

Jeroen Hoppenbrouwers

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Re: Question re: TCAS and 'coffin corner'
« Reply #2 on: Sun, 13 Jan 2019 23:59 »
2.   If you did end up in this type of situation, would you: a. ignore the advisory, b. respond by maneuvering laterally, and inform ATC of your action on the ‘see and be seen’ principle, or c. ask the galley for more coffee.

If you are this up tight in the coffin corner, you don't have lateral (banking) maneuvering margin any more, really. The only thing you can do is stay straight & level and hope for the best. You should switch the TCAS to TA Only, so that it knows that it should not tell the other TCAS that you can and will follow RAs. It would help the two systems working out a solution.

Which, of course, means you are way above your operational ceiling, as Hardy said. Only U-2 and SR-71 aircraft venture into this one-knot corner operationally.

Hoppie

Jeroen Hoppenbrouwers

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Re: Question re: TCAS and 'coffin corner'
« Reply #3 on: Mon, 14 Jan 2019 00:16 »
I really got to wondering about this just the other day when I heard that the last SPACE-X launch had completed the Iridium constellation and one result would be to allow closer packing of flights on the North Atlantic Tracks.  Assuming Hoppie can get back to work soon installing his magic boxes etc.  :)
There are two aspects to this reduced separation project.

First, having all aircraft participating in any form of automated surveillance is key. Over deep water, the use of FANS is still the best solution today. FANS has ADS-C automatic position reporting over SATCOM which provides ATC with assurance that their plots remain current. If enough aircraft report sufficiently accurate positions sufficiently often, the system is considered safe enough to reduce lateral and in-trail separation. It easily doubles the airspace capacity. Most North Atlantic tracks are already FANS-mandatory. If you do not have it, you can still use NATS, but not the popular ones. Iridium allows cheap and simple ADS-C so equipping aircraft with this SATCOM system allows them to throw pebbles at the popular tracks. Having all-new satellites does not technically change anything (that will take years) but it definitely improves the reliability. This kind of surveillance requires performance better than "95% of all position reports delivered within 90 seconds" and this surely was not easy before we got new satellites.

Second, when Iridium designed the satellites they had some excess space and power, so they could carry an extra payload. After a lot of deliberation this became what is now Aireon, a space-based ADS-B receiving and relay service. Most if not all heavier aircraft today have ADS-B (Broadcast, not to be confused with -C Contract), providing continuous rapid position updates exceeding TCAS capabilities by far. These squitters can be received by some other aircraft (not all ADS-B equipment has both IN and OUT) and by ground stations. But on deep water there are no ground stations. Aireon succeeded in receiving ADS-B OUT from 781 km up, providing a very cheap global ADS-B IN virtual radar system.

ADS-C plus ADS-B will probably be hooked up together to provide near real-time virtual radar coverage of the globe where traditional microwave radar does not reach, and then allow ATC to tighten separation a lot.

Note that the much discussed "autonomous aircraft tracking" systems (since MH-370) is neither of the above. ADS-C and ADS-B are traditional systems that can be switched off by the pilots, as nearly everything aboard an aircraft. If you want to exclude the pilots from the loop, you need significantly different equipment that has a lot of autonomous battery power and other things, so it cannot be fooled. It has to withstand malicious intent, so it moves to security instead of safety. The industry is moving here, but it is a departure from established engineering practice, and thus slow. Iridium definitely is part of it, though.


Hoppie

torrence

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Re: Question re: TCAS and 'coffin corner'
« Reply #4 on: Mon, 14 Jan 2019 00:59 »
Hardy - you're right about 450 max, my memory was faulty.  Found my flight plan printout and ATC handwritten notes.  Original flight plan CRZ was FL360, so with the ICAO step climb (4000 ft), my notes show I was given a first climb to FL400 then soon after climb to FL440 with my max something like 443 at that point I think. It was still very tight between the bricks though  :).  I didn't think about going to TA only - forgot both systems are trying to work out a solution - that would have been best option probably.

Hoppie - very informative.  The system has really advanced a long way - the very concept of a global virtual radar via satellites  is mind-blowing.

Cheers,
Torrence


Cheers
Torrence

Britjet

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Re: Question re: TCAS and 'coffin corner'
« Reply #5 on: Mon, 14 Jan 2019 10:02 »
I can count the times I’ve climbed above 410 on the fingers of one hand. Never above 430. Why would you?
These are high bypass-ratio fan engines. There comes a point where using the fuel to get up there cancels any perceived fuel flow benefit.
As for squeezing the altitude, I can think of only two occasions where you might want to climb unnecessarily high.
One is to make a “required” MNPS clearance by a boundary, the other is to climb above weather (not a good idea generally). In either case the best Mach number to fly is always .86. That will give you the maximum altitude.
The “hockey stick” as Hoppie says, is another factor. It is entirely possible to get stick shake occurring if you try to turn when in the yellow band. Tea (without biscuits) in the Fleet Office follows..
Peter.

Markus Vitzethum

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Re: Question re: TCAS and 'coffin corner'
« Reply #6 on: Mon, 14 Jan 2019 10:53 »
I can count the times I’ve climbed above 410 on the fingers of one hand. Never above 430. Why would you?

It probably depends on the mission, I guess. I agree that with a commercial payload, FL410/430 is pretty high up for the 747-400.
On the other hand, N747NA, a NASA-modified 747SP carrying an astronomical telescope (SOFIA) routinely flies at FL430 to be above most of the atmosphere. Also, VH-OJA, the Qantas 747-400 that did fly LHR-SYD nonstop flew at FL450 for the final hours over Australia (no payload, though).

But I seem to notice that many (not all) 787s routinely fly at FL400 sometimes 430, a lot of business jets at FL410 or above. So it probably also depends on aircraft type.

Markus

Britjet

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Re: Question re: TCAS and 'coffin corner'
« Reply #7 on: Mon, 14 Jan 2019 11:11 »
Oh, I know it can be done. But assuming that you are trying to maximise profit over payload, the 747 isn’t really fit for this purpose. Not enough thrust margin, and an inefficient (by modern standards) wing.
The newer aircraft can indeed get higher - but they have bigger engines, more excess thrust (being twins, which always have this) and importantly, more efficient wings..
Peter.

Jeroen Hoppenbrouwers

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Re: Question re: TCAS and 'coffin corner'
« Reply #8 on: Mon, 14 Jan 2019 12:44 »
Silly enough, these new aircraft also run into new issues that high up. The atmosphere is so thin there that a cabin pressurization problem gets to the brain very, very quickly. Hence above 430 or so the pilots are required to be masks-on (as far as I know). This is not very comfortable to say the least. Also more and more trouble is made by regulations on flying routinely this high. It seems to influence newer aircraft designs (post 787) to actually not aim for these cruise altitudes any longer.

Just as Mach 0.88 seems to be the limit, FL410 may become another regulatory/commercial limit that tech won't readily break.


Hoppie

torrence

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Re: Question re: TCAS and 'coffin corner'
« Reply #9 on: Mon, 14 Jan 2019 20:14 »
Silly enough, these new aircraft also run into new issues that high up. The atmosphere is so thin there that a cabin pressurization problem gets to the brain very, very quickly.

To say nothing of extra cosmic ray radiation exposure that crew and frequent flyers don't really need.

Hey - that's why I did this in PSX!  I wasn't worried about fuel or cost etc. etc.  Just did it because the robot ATC told me to and wanted to see what would happen  :).  Remember that's what some of us non-pilot testers did at various stages of development - just tried stuff that no real pilot would even consider to see if we could break it - so Hardy could fix it.

Cheers,
Torrence
Cheers
Torrence

Jeroen Hoppenbrouwers

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Re: Question re: TCAS and 'coffin corner'
« Reply #10 on: Mon, 14 Jan 2019 23:49 »
I do remember that during Worldflight in Coventry (John's bedroom sim, it's no longer there though, he moved house!) we routinely drove at M0.90 because we were always a bit late due to "issues"... our resident actual 747 pilot had to come to terms with it, but then realized he also could hold a cold one, and quickly adjusted to the new SOP  :-D


Hoppie