Aircraft separation over the Atlantic

[DaiB]

Was just wondering what methods are used to keep aircraft separate over large bodies of water such as the Atlantic, and indeed what the legal limits are for separation.
I was flying to Boston from the UK last year and glancing out of my window saw another aircraft at the same altitude at some distance. As I watched we got closer and closer together until it felt like we were more or less flying side by side. I had a real sense of the speed we were travelling by watching the contrails fly off the back of the other plane, and could almost make out the livery on the aircraft, despite the bright sunlight. With the very real sense of speed it felt as though a fairly minor change of direction by either aircraft could spell rapid disaster...
I came very close to alerting a member of cabin crew, as it seemed we were just destined to get ever closer! Does this sound unusual? Considering the levels of traffic accross the Atlantic is there a risk involved? I would assume that there are proximity alarms, and that one of the planes would notice the other before it was too late, but on this occasion it felt incredibly close!

[11Fan]

DaiB,

First off, welcome to PPRuNe.

There are a number of methods used to maintain separation. As the other aircraft was - as you say - "some distance", there is a possibility that there may already have been a vertical separation of 1000 feet. In addition, I would also suspect that both aircraft would have had Traffic and Collision Avoidance Systems (TCAS) active and very likely were aware of each others presence.

Quite frankly, and not intending to alarm you, it's usually the ones you don't see…... but then again, that's what TCAS is for.

I'm fairly certain you were fine. Add to that, relative distance is somewhat distorted at speed and elevation.

Happy flying, and again, welcome to PPRuNe. :ok:

Best regards,

11Fan.

[Sobelena]

DaiB, rest assured that you were legally seperated. The other a/c was probably several miles away and at that distance a vertical seperation of a 1000ft will look as if at same level. Although there is no radar coverage there exists a very strict and long established procedure which you can read all about here: http://avsim.com/geoffschool/airlinecourse/atlantictracks.htm

[DaiB]

Thanks for the responses! The linked article above was particularly interesting as although logically some sort of system must operate, I wasn't aware of the details. It's all very reassuring!
I must say though, it did feel like something was a little awry in this case, and I'm no 100% sure that what I saw fits into the scenarios in that link. The aircraft was certainly not several miles away - 2 or 3 at the absolute most. It is concievable I suppose that it could have been at a different height, but it was absolutely at my eye level and given the proximity I think 1000 feet difference in altitude would have been noticeable - and as it drew closer there was no percievable change in our vertical seperation.
Anyway, I'll take your words for it that it was a normal occurence that just looked odd to someone who doesn't fly accross the Atlantic very often!!

[Rainboe]

Look mate, you don't have to be suspicious- you had no drama. I have been flying across the Atlantic for 35 years, and control is top notch. Briefly, Gander Oceanic control west of 30W, and Shanwick Oceanic control east of 30W. Pilots navigate and report every 10 degrees of longitude. Height separation on each route is 1000'. Routes at the moment are 1 degree, or 60 miles, apart. Some pilots apply a track offset of 1 or miles right of track as Nav systems are so accurate that in the remote event of an error, you won't be bang on course against another aeroplane coming the other way. You were looking at another flight 1000' searated, and maybe 1 or 2 miles away. Do you not think if you saw the other flight, one or the other's pilots could also? And they appear to have been not worried by it? When you get an explanation here from people that do know, you can rely on it, especially as your knowledge base is zero on these matters- I know what you thought you may have seen, what actually happened was different. It is common for people in holding patterns when turning to think the aeroplane above or below is at exactly the same altitude- another scenario when people don't believe the explanations!

[DaiB]

A very thorough explanation, for which I thank you.

In my own defence, the first response contained the phrase 'fairly certain' - which allows for error, and the second linked to a highly informative article which nevertheless did not make clear to me the concept that two aircraft could occupy the same track at a horizontal as well as vertical distance, although with 60 miles between each one I suppose it's inevitable that such freedom is available.

I've merely taken the opportunity to compare what I saw with the offered explanations of how things work, maybe discuss it a little. I certainly don't mean to question anyones knowledge or advice; my last sentence said virtually the same thing your last sentence did - that I do not fully understand, but will nevertheless accept that the occurrence was probably normal.

As it happens the explanation you have given me is a very enlightening and pertinent one, and answers my questions thoroughly. The tone of it though would make me think twice before asking another one.

[Rainboe]

DaiB mate, your post #4 more or less discounts, or casts doubt, on the explanation given to you. I have as positively and concisely as I can given you a further exlanation and reassurance about it, plus advised you of the reason you saw what you saw- the track offset procedure being applied by some flights. I didn't sign off with 'love' or 'best regards', but you asked, and you were told. You can ask what questions you like, but if you throw doubt on the answers you receive, then you must accept the answers you get! In this age of GPS, the sheer accuracy of computer navigation now leads to flights happily flying along close to each other mid-Atlantic, but always separated vertically. What you saw is actually very common.

[TopBunk]

DAIB

Unless your surname is Yeager, I can advise you that the most you may see of an aircraft at 60nm distance is the contrail, or possibly if the sun is in the right position, you may see a glint off the aircraft. In reality, you need to be at less than 30nm to discern the shape. Often, even with TCAS, if approaching head on, you won't see the target until inside 20nm, quite often 10nm.

Rest assured that this aircraft would have been at a different altitude. Even as an experienced airline pilot, I find it difficult at distance to judge relative altitudes - often something I think is above turns out to be below. From the cabin through a side window you haven't got a cat in hells chance of quantifying anything.

About 4 years ago I had a note passed forward from a PPL holder sat down the back claiming that we had had a near miss (hate that term!) over northern Europe and that he had witnessed it. He gave me his mobiloe phone number, and I seriously believed that he was likely to contact the press, so I phoned our Media Comms group to forewarn them. Nothing resulted, not was there any abnormal incident during the flight.

[treadigraph]

I was in the back of an A319 flying up to Edinburgh last week. In the cruise a 737 slid over the top of us, maybe 20 degrees heading difference. My instant reaction was "bloody hell, he's close", but with a second look it was obvious that he was certainly 2000' higher and very likely more.

I suspect that the 737's vapour trail gave an illusion of nearness - I don't believe I've ever seen another trail quite as close as that before (I don't fly as often as I would wish!), therefore I am rather more used to seeing them from the ground than regularly from the air.

Incidentally, I can't think of any mid-air collisions out over the North Atlantic in recent memory. I seem to recall something like a DC-6 and a Connie collided off the west coast of Ireland in the 1950s?

My only worry mid-Atlantic is whether there will be any turbulence severe enough to stop lunch/dinner being served! :}

[DaiB]

Rainboe

I appreciate the point you're making; I think it was probably my blood alcohol level at the time which helped interepret your slight exasperation as open hostility...

I wouldn't consider myself a nervous flyer, so its interesting how a lack of knowledge can still turn an everyday occurence into an alarming experience!

[Rainboe]

Treadigraph, contrails actually descend fairly rapidly- I seem to recall they can descend at 600fpm. I have been following higher aircraft by anything up to 20 miles and have found us going in an out of his vortex- quite irritating as it can still give an unnatural bumpiness after that time. The high speeds involved with high altitude flight also give an impression of being closer than you are actually.

[treadigraph]

Funnily enough Rainboe, you've reminded me that on that trip last week I was watching another aircraft on a parallel course ten miles or so west of us - the trail (quite a short one, perhaps a mile or two long) appeared distinctly "nose up"

Cheers

Treadders

[forget]

This probably a load of bolleaux but - years ago Pilot License No 1 explained to me that, if a distant aircraft appeared above the horizon, then he was above you. If it appeared below the horizon, then he was below you. :confused: Does it work? I suppose it does - when you think about it. Only downside - you need to see the horizon.

[Rainboe]

Not quite right! It would work if you were at ground level- any plane above the horizon would be above you, and any plane below the horizon would be in deep doodoo (or probably Indonesian), but at 37,000', the horizon is significantly below you, and it is very hard to judge. You often see planes coming towards you that appear to be heading over you, and suddenly they seem to duck and shoot below you. It's almost impossible to judge sometimes. The only reliable way I found was to suspend the pilot clipboard like a spirit level through the hanging hole and look along the top edge at your target- that shows you better than anything else.

[Capt Pit Bull]

Take a look at this expanation (http://www.pprune.org/forums/showthread.php?t=58559).

pb

[Rainboe]

Dat's very useful PB! I could have worked it out for myself.....if my geometry wasn't 45 years old!

[Bagheera]

One of the problems for passengers, or indeed for any of us, I believe is that the standard seperation of 1000ft vertically seems like it should be a huge margin. The scale of the number and the fact that little else in our lives is measured in feet anymore gives us little perception of what 1000ft actually is like. The one place we do come across it frequently, if you would like to use your imaginations, is at a motorway junction. The countdown markers prior to the junction are 300/200/100 metres. 300 metres is approximately 1000ft. So now imagine 1 Boeing 747 at the 300 metre board and a second one at the actual turn off and you will see that it is not as large a seperation as we may have thought.

[forget]

A picture's worth a thousand words ----

(Now all to scale.)

http://i21.photobucket.com/albums/b270/cumpas/1KFT2.jpg

[Rainboe]

Yer top Jumbos littler than the bottom one! It's an inch shorter (none of that Euro-crap metrication- give 'em good old Imperial!)

[forget]

Quite right Rainboe - quite right. A Friday afternoon oversight on my part. Just pretend the top one is the ref, the 1,000 feet is based on that. And then imagine you're closer to the bottom one. That works! ..... and easier than fixing it.:) On second thoughts - I'd better fix it. Stand by.

PS. Fixed - all to scale now.

[Rainboe]

Now...are they 100 series or 200 series? (pilots are a bit like that- pedantic to the extreme!)
Observe, everyone, how they both look at the same level! So what hope has the eye of judging it in the air?

[Avman]

I'd say a 200. I'm probably wrong but didn't the 100s only have 3 windows on the upper deck?

[forget]

It's taken from an old Jane's - 747-200B.

[forget]

Here's a real photograph of 1,000 feet separation. Cracking picture but one helluva visual case for NAV off-sets. This guy (SIA Capt) has a Gold in air to air photographs. Look at http://www.airliners.net/search and do a Keyword search on Bailey. Brilliant!

EVA Air B747-400 passing 1000 feet below a China Airlines A340 in RVSM Airspace.


http://i21.photobucket.com/albums/b270/cumpas/0637285.jpg

[Avman]

Just goes to show how difficult it is to judge height differences. If I would've had to guess I would have said there's 2000ft between them. The long lens probably makes it deceptive though.

[BOAC]

DaiB - do not be put off - many grown PPruners fear the bark of the awoken Rainboe:D

Folks- this is going a bit 'photoshoppish' and I think the original query has been covered, so please continue photosnaps on the appropriate thread.