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Comment by Xcelerate | Hacker News Reader

I wonder how often these close calls occur. I was on a flight a few days ago and using an app to track my flight while on the plane. We were at 32,000 ft and according to the app another aircraft was headed straight toward us at 33,000 ft. Just before the two planes overlapped on the app, I looked out my window and saw the other plane go right above us. It was absolutely insane how close it appeared and how astonishing a 1,000+ mph speed difference is. (If you're curious which flights: DL75 and TK1344 on Feb 1, 13:04:20 GMT).

My understanding is that planes passing this close to each other is relatively common, as 1,000 ft is the minimum vertical separation for RVSM airspace, but I was still surprised that the other plane was directly above us and that some sort of optimization algorithm isn't used to maintain horizontal separation as well just in case there is an error in communication on the flight levels.

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Airline pilot here: passing another aircraft opposite direction on the same lateral track with only 1000ft of vertical separation is about as normal as it gets. I’d say that it has occurred on almost all of the flights in my career so far. In certain areas of poor ATC coverage we sometimes use SLOP (strategic lateral offset procedures) to add a bit more margin for a height error.

Edit: fun fact, you can actually hear the passing jet from the flight deck just after it has passed, a slight whoosh can be heard. Depending on conditions you can also feel a brief bump when crossing their wake (since the wake descends).

That's really cool! I can only imagine from a pilot's perspective it must look like you're about to collide though; that would be a bit nerve-racking for me.

There was an accident over Brazil?

Each plane was at the exact same height, and given their digital altimeters they were at the exact same height and one planes wing clipped the one going the other way. Only the smaller plane crashed.

Commercial airliners have the Traffic Collision Avoidance System that operates independently of ground based systems or ATC clearance to avoid collisions. Adding more separation would be the opposite of an optimization algorithm in the case of transoceanic flights - it would reduce the number of planes able to fly in the most efficient parts of the jet stream at one time.

What’s interesting though is that you were not flying at 32,000 ft, but rather flight level 320, which actual altitude was determined by a barometric altimeter set to a standard pressure. It may deviate considerably form the actual altitude of 32,000 ft above the mean sea level, but the deviation is dictated by local atmospheric pressure, thus shared by all aircraft in close proximity.

So does everyone fly higher on hot days?

Not a pilot, just some jerk on the internet.

One minor potentially life saving standard protocol is that flights are supposed to stick to certain elevations depending on the direction of travel. Half remembering this, but East flights are to be at odd elevations (31,000; 33,000; etc) while West bound are on evens (32,000; 34,000; etc).

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