I was on a recent transcon staring out the window and listening to Ch9 when another UA jet came flying by in the opposite direction and a little below us. It looked really close. There was no mention of it on Ch 9, no evasive action taken by my plane, but man, the other plane looked really close (enough that I could tell it was a 757 and not a 767) If it wasn't for the fact that it went zooming by and that I didn't expect it, I probably could have read off its aircraft identification number (you know, NUA9AF or whatever)

Of course, I am no expert on this, but maybe someone can tell me ways of knowing when close is too close? Meaning, I know the requirement is 1000ft or something like that, but what does that look like when you're staring out the window?

Well the wing span of the 75 is ~125 feet, i.e. 1000 feet in my opinion is enough to justify extra scotch guard for the window seats.

I've had it happen more times then not on my way to/from JFK. Route is always populated and see three to five planes in a 10-20 min period. I think 1000 feet is more then enough distance, the pilots can see them coming and know when/if they are too close. but im no pilot.

I've read a number of references to this sort of thing indicating that there is sort of optical illusion that makes other aircraft seem much closer than they actually are. I know I've seen some that seemed remarkably close, it can be a little unsettling.

sometimes 1000 ft seems pretty close....

I've seen stuff that's really seemed close, but I doubt actually was. I think when you're above the clouds, it probably distorts your sense of distance...or something!

Drink more!

Almost all commercial planes have Terminal Alert and Collision Avoidance System (TCAS) onboard. This system uses radios to query other aircraft in the vicinity and calculates if they are on a potential collision course. If they are, the system directs the pilots to take evasive action.

Try Googling TCAS, or see the Wikipedia entry for it. Lots of info.

IIRC, the two planes which collided a few weeks ago in Brazil both had TACS. Was it ever determined what when wrong (besides the surviving pilots being at the wrong altitude)?

I had thought the private jet's transponder was turned off--I could be wrong but I think I remember hearing that.

A good arguement for keeping your seatbelt fastened, regardless of whether or not the sign is lit! I was on a flight several years ago, approaching our destination so everyone was already buckled in with 'tray tables up'. The plane suddenly went nose down, banked hard left & dropped a couple thousand feet. Yes, my hearing has recovered. No thanks to the lady sitting beside me who decided to scream her head off, convinced that we were all going to die. It was not typical turbulence. Nor was it even close to a typical landing descent. I later learned that a private plane crossed too close & the TCAS kicked-in. Unfortunately, I never saw the other plane, so I can't say how close it looked.

While the transponder was off on the Legacy meaning TCAS was not working either, tapes indicated ATC had both planes cleared at the same altitude, last I read. The investigation continues and the US-based pilots are still unable to leave Brazil.

Too close is when you hear it.

If you got to less than 5 miles separation or 1000 feet, you would likely feel some effort (climb, turn, descent) to get separation. But yes, 1000 feet looks close. Head on the closure rate is 16 miles a minute or a mile every 3 seconds. By the time you process it, the other plane's about two miles away.

Too close is something like this, the deadliest mid-air collision on record, that happened 10 years ago nearly to date:

http://en.wikipedia.org/wiki/Saudia_Flight_763

May they too RIP.

That's why I book aisle seats and don't look!