Yesterday I was sitting at the left side window seat of this Cathay Airways plane enjoying my wine at cruise altitude, when the plane suddenly banked to the right. Lo and behold 10-15 seconds later I see a plane passing us at the same altitude at probably no more than 200 meters away. I wish I had my camera nearby so I could take a picture out the window and later see how close it was.

How common is it to actually encounter a plane at the same flight level within collision warning range?

Because I figured the turn was because they got an alert (espesially since the plane turned back to the left once the other had passed).

Was nothing exciting, the turn wasn't a violent or sharp. But enough that I noticed and wondered. Usually when you are at flight level you stay on the same course.

It's very rare.

Are you sure it was at the same altitude? It can often be deceiving.

It's rare but it can happen.

Most planes today have a TCAS installed that gives them a warning and instructions on what to do
in order to avoid possible collisions.

http://en.wikipedia.org/wiki/Traffic_Collision_Avoidance_System

if the turn wasn't violent then the plane was probably not even close to collision range

on popular airways aircraft often come extremely close to eachother from a laymens perspective but in actuality there is usually both significant vertical and lateral separation. 1000 or 2000 feet of vertical separation is very difficult to see for most people especially when the aircraft is traveling toward or away from their own aircraft at a diagonal.

Unless heading in the same direction there is commonly a distinct difference in flight-levels (odd 20s one way, even 20s the other)...
With adequate vertical separation, it is feasible to stack many aircraft - and this is commonly done in airport holding areas.

Normally the minimum separation is 3 miles for radar controlled airspace, with aircraft at the same levels, and if your flight broke these limits there should be an airprox notification which could be searched using date and flight number/airline/type.

It is hard to judge distance to a large (especially predominantly white) aircraft when there is nothing else in close proximity - I find that I grossly underestimate distance under circumstances of an isolated object with no local reference. A distance of more than 3 miles and 2000ft is entirely consistent with a turn away to maintain separations. Ultimately it is only critically important that separation was maintained at greater than the combined semi-span of the two aircraft, but that is considerably closer than

It was close enough that I at a glance read the airline logo, and see the windows clearly. As he passed us he was going upwards, and engine contrails started just as he went past. He was also going parallel with our course.

I've seen planes pass at different levels before, this was not like that.

Were you in the bank when you saw him? 1000' of vertical separation is not very much at all (and we do it all day). If you were in a slight bank, then when you look out the window the other airplane actually appears on the same vertical plane as you. If you felt no lightness in your seat or pressing down in your seat (as the plane reacted to a resolution advisory) (also..cockpit/avionics collision alarms, and their corresponding actions are always climbs or descents, not turns)..then it was probably just a normal deal. Contrails can "dit-dot-dit" too.. They can start and stop very quickly...

I'm not saying you didn't have a near-miss (near hit)..but it is highly unlikely..

Thousand foot separation more than likely. Banking was probably to take advantage of a photo shoot op.

Ah, Cathay Pacific, my favorite airline for going to the Philippines, Indonesia, Thailand, etc...
Real good service and damn cute air hostess!

One time I looked out the window to see a large UPS 747 headed towards us, or at least nearby and instantly my thought was "whoa, he's close", but upon thinking about it, figured he was much further away than I'd initially thought...747's are big machines, and it probably was 1000' below us. That's when it dawned on me that it was all perfectly safe, no close calls, even thought I could clearly see it was a -400 Freighter.

On a flight from Frankfurt to Moscow, nearly two decades ago,
we passed about four jets, at various times, that I saw out the
north windows, at a distance I would judge as about one mile. Some
were closer, some farther, some at the same height some a bit
up or down. I tried to take photos of the later ones, but
they were more distant than the one which first caught my
attention, and between the crummy resolution of the disposable
film camera (I wasn't going to risk a quality camera in that
barely-this-side-of-chaos country), the poor quality of the window,
and the poor contrast between the planes and the horizon, the
shots didn't show anything. I haven't thought about it in a long
time. I'll have to dig out the negatives and scan them, see if I
can pull out detail digitally...

TCAS only gives you vertical avoidance commands - climb or descent. No turns.

In 2010 I was out in the North Sea off Amsterdam on my way home in a Columbia 400, when a Delta A330 was asked to stop climbing due to us being at FL120, with another plane above us at FL130. The Delta plane levelled off at FL110, and I could see them both above and below me. They do seem to be pretty close, but like others have said, 1000ft of separation isn't that much, and with so big planes it appears a lot closer than it is.

I was just reading this and was going to post about TCAS issuing directions in the vertical and saw Semmern's post. This is my understanding of it also. It's unlikey the turn was for avoidance.

Interesting app is Flightradar24.com
With it you get live data from all planes sporting an ASDB transponder (or something like that)) .
Its amazing to see. You get type, alt, speed, route, squawk, and many other things.

Isn't it possible the "banking turn" was simply a waypoint course change?