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With rare exceptions (like the Embraer business jet, which is simply their commercial jet with a different interior), almost all private jets fly in the more rarified air above FL 400.

But, if its more efficient, why don't airliner fly at the same sorts of altitudes on long flights?

eSurfsnake
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FL360 (plus or minus a bit) is the optimal altitude for fuel efficiency, which is the top priority for airlines. And airliners all fly at roughly the same speed (again, for fuel efficiency), so ATC can just line them up in a row and let them follow each other.

The problem is that level is severely congested with all those airliners. You can’t really get out of line or pass someone, which is another reason they’re all flying the same speed—and that has become a vicious cycle because Boeing and Airbus now design their planes to be most efficient at that speed.

Business jets passengers (and thus their operators) value speed over efficiency; if they didn’t, they’d use much cheaper airline flights. However, to avoid the airliner congestion around FL360, they have to climb to FL400 or higher, where they can fly faster and likely get a more direct route.

StephenS
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  • I would question the idea that bizjet passengers value speed, and particularly airspeed, over efficiency. It's not possible to get much of an airspeed increase without going supersonic. Most of the decrease in travel time comes from being able to fly between more convenient airports, and leave pretty much whenever you want without the hassles of ticketing & security. But I suspect the major reasons are comfort and privacy. – jamesqf Mar 10 '21 at 05:35
  • The other place to operate efficiently point-to-point is the altitude largely favored by turboprops, such as the King Air or Pilatus. They usually fly at FL 250 or so, where the only other traffic is similar planes. As a result, even when the jetways at FL 360 are very congested, ATC will often clear a turbo to a fix just before final approach - even if that fix is hundreds of miles distant. Turbos can be very competitve on time from (for example) NorCal to LA as you are under the jet traffic all the time. – eSurfsnake Apr 12 '21 at 18:43
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The most interesting (and most likely the most right) is the safety issue. An airliner with engines under the wings needs to drop to FL200 in case there is an uncontained engine failure that punctures the fuselage, requiring a descent rate of 10.000 fpm or more.

A private jet, however, has engines in the back. If an engine had an uncontained failure it would be behind the pressure bulkhead, and would not require a rapid descent due to loss of cabin pressure.

Having that - as well as turbojet engines that are low bypass and thus perform better at higher altitudes than a fanjet - makes it possible for the users to fly regularly above FL 400, where there is coincidentally less traffic as well. And, the relatively large wing aids as well.

eSurfsnake
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