[sandywaffles]

> [Clear Air Turbulence] is particularly common around the tropopause, the boundary layer between the troposphere and the stratosphere, at altitudes between 7,000 and 12,000 meters (23,000 to 39,000 feet) .

Oh, excellent the altitudes that 99% of aircraft fly at, unaffecting the ultra rich who fly private jets at 40,000k-50,000k+.

[hiatus]

> Oh, excellent the altitudes that 99% of aircraft fly at, unaffecting the ultra rich who fly private jets at 40,000k-50,000k+.

I didn't realize private jets fly so high. What's the reason for the difference in elevations?

[pc86]

Private jets don't really fly at 50k that often but I'm sure there are some that can. 40-42k is pretty common though. There are a handful of reasons. In no particular order: 1) additional separation between recreational private flights and scheduled commercial flights 2) higher performance in small private jets with typically less than 1k lbs of people and cargo 3) winds are generally faster the higher you go so you'll usually go about at high as you're able, at least in one direction.

Most private jets aren't someone flying a billionaire around in a $60M Gulfstream, they're $3M toys being flown around by the owner to go to their ski trip.

[nickjj]

> 3) winds are generally faster the higher you go so you'll usually go about at high as you're able, at least in one direction.

I recently flew from NYC to Lisbon, Portugal and it was 6.5 hours there (flying east) and 7.5 hours back (flying west) because you go with and against the jet stream.

I wonder if there's a noticeable difference if you fly at a lower altitude against the wind. It didn't seem like the plane adjusted for that, it cruised at the same altitude both ways from what I remember. Both flights used the same exact plane type (A330neo).

[pc86]

Airlines have dispatch folks that spend a lot of time deciding what altitude and route to file. Yes there are defaults so it's certainly possible they were busy and just filed the same but especially days apart it's likely that was the optimal altitude.

They're also looking at fuel economy so even if it takes longer if the air is thinner and they burn less fuel they may still save money. I imagine there's a certain distance where those two lines cross that is probably baked into the dispatch software for each plane model.

[FabHK]

Note also that at higher altitudes the air is less dense, but the plane should fly at the same indicated/calibrated airspeed to generate the same lift, which means that it flies faster (at higher true airspeed).

Basically, with aircraft mass and angle of attack unchanged, \rho v^2 must be constant, so smaller density \rho -> higher air speed.

[ddoolin]

Traffic separation. Since they can fly at those altitudes, it makes sense to put them there, away from other commercial airliners.

[vessenes]

You got some good answers below, but in addition to those, also thinner air = less felt turbulence, so it’s more comfortable (provided your cabin pressurization is good).

[zamadatix]

You can fly over the tropopause depending how high it is at the specific location (can be lower or higher than the numbers listed) but, by definition, it'll only be "common" to observe turbulence in the range 99% of planes actually fly. I wouldn't read too much into that. As some others pointed out the height difference probably more due tot he space being faster and unused since commercial flights stay to where is more efficient.

[FabHK]

The A320 has a ceiling of 39,100–41,000 ft, the 737 of 37,000 or 41,000 ft, the 747-8 and A350-900 and A380 of 43,100 ft.

[cinntaile]

Why do they fly higher? I expected them to fly lower.

[pc86]

Ah yes those pesky billionaires putting the tropopause right below where all their private jets fly.

[joohwan]

Why is this comment being downvoted? Is it not true?