Hacker News Reader

Comment by washywashy | Hacker News Reader

Once on a flight headed west I happened to look out the window and just then another plane flew beneath the plane I was on headed north-ish and quickly off into the distance. I remember thinking two things: 1. It was far away from us so quickly it kind of shocked me, although it makes perfect sense of course. 2. I felt like i got a really good view of it. It didn’t look far away at all altitude-wise as it passed beneath the plane I was on.

I’ve always wondered how normal this is to cross paths with other planes that closely. I’ve seen other planes way off in the distance before but never that close.

View on HN

It's very common - they space the 'flight levels' by a vertical separation of 1000 feet, and often alternate the different directions. So you might have several planes following the same path but in different directions, the planes going one way at 36,000 feet and the ones going the other way at 35,000 feet. They might also cross at other angles with the same kind of vertical separation, especially at a waypoint.

Planes on intersecting paths will be placed in separate flight levels. The standard is 35,000ft or “FL35”, and usually East bound traffic gets odd FL and West bound gets even. It’s likely the plane you saw would have collided with yours had it been on the same FL, but it was likely off by 1,000ft. The relative addition of two cruising speeds make it more dramatic too.

Small correction: 35,000 ft in flight level is FL350. (But the east/west distinction still applies to even and odd thousands of feet, or tens of flight level.)

And sometimes they do collide: https://en.wikipedia.org/wiki/Gol_Transportes_Aéreos_Flight_...

I’ve seen this before and was shocked as well. Really gave me some perspective on how much trust we put in pilots and airlines to keep us safe while hurtling through the air at hundreds of miles per hour.

Pretty common. You can check out Skyvector "World Hi" [1] to get a sense of intersecting flight routes.

[1] https://skyvector.com/

My favorite is when you pass another plane in the opposite direction along the same path but different altitude. The closing speed of two airliners at cruising speed is quite the thing to witness

Generally, required separation is 5 (nautical) miles or 1000 feet in altitude. There are some exceptions like taking off or landing on parallel runways. ATC will ensure the separation, and on large airliners, the TCAS[1] system will alert pilots if something goes horribly wrong and aircraft get too close together.

[1] https://en.wikipedia.org/wiki/Traffic_collision_avoidance_sy...

I think if you're close to any airport, it is very normal. If you're, for example, landing in Chicago, I'd be shocked if you can't see a bunch of other planes out of the window. I also once had some delay when having to fly over Dubai, where we had to fly in circles before landing, and I saw a large amount of airplanes circling around the same area above and below us. But I'm no expert in aviation by any means, so I can't really comment on the appropriateness.

Reply on news.ycombinator.com