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Most private jets have a service ceiling of 51,000 ft. A service ceiling is the altitude at which a plane can climb no more than 500 ft/min, right? However, on flightradar24 I don't remember having ever seen a jet flying that high, nor at 49,000 ft. The highest-flying jets on flightradar24 are at 47,000 ft but that's very rare, usually 45,000 ft are the highest cruise of jets; usually these are Gulfstreams 650/700, Cessna Citation jets and Global 6000s.

If these private jets have service ceilings of 51,000 ft, why don't they fly that high on flightradar24 and at what occasion would a jet do so (very little remaining fuel e.g.?)?

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  • $\begingroup$ @Bianfable Thanks for the edit but I think the flightradar24 tag should be added, for the answer may be something like flightradar is less able to show planes that high above the planet, or that pilots at these altitudes make themselves invisible on flightradar for some reason. So perhaps planes do fly there but are invisible on fr24. I don't know. $\endgroup$
    – Giovanni
    Nov 12, 2021 at 9:29
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    $\begingroup$ I don't think it has anything to do with FR24, but if you feel strongly about it, you can edit it back in. $\endgroup$
    – Bianfable
    Nov 12, 2021 at 9:35
  • $\begingroup$ @Bianfable I don't think so either, but it's mentioned in my question often so I'll reedit it in. $\endgroup$
    – Giovanni
    Nov 12, 2021 at 10:05

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Generally when you run right at service ceiling you are near the edge of the operating envelope with little energy margin if things go off the rails, so you are in a relatively dangerous place (also, aerodynamic forces, and damping, are drastically reduced in the thin air, but the inertial mass is the same, which tends to exaggerate overcontrolling/overshoot tendencies).

Less so if you are very light, but business aircraft are usually operating "heavy", in the first part of the trip especially, because the interior weighs so much (empty weight is permanently high) and passenger load is relatively small.

So if you are going to cruise at ceiling you have to be extra careful monitoring speed/mach to make sure you don't run out of energy margin. If you run through some smooth mountain down wave air, that starts you gently descending, to which the autopilot reacts by pitching up to keep the flight level, and you aren't paying careful attention, you might find yourself with speed decaying toward shaker in very thin air.

You have little thrust margin, and at this point, if you are sliding onto the back side of the power curve, you are in big trouble and need to descend immediately to regain energy.

So if 45000 ft is high enough to clear 99% of the weather, there is little need to go all the way to FL510 when you are much safer, from a general operational perspective, at FL450. The only motivation to go that high would to be to get clearance over the top of a cell in the tropics (mostly to avoid the hail that can come shooting out the top) without having to divert upwind of it (once you're down near the equator, even 51k may not be high enough and you still have to go around them).

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  • $\begingroup$ Concorde flew higher because if was a way to limit drag (fuel consumption) at M2, drag being proportional to the square of the speed. Bizjets don't have this problem. $\endgroup$
    – mins
    Nov 12, 2021 at 16:42
  • $\begingroup$ And the bizjet operator may see some theoretical cost saving going there, but it's not worth it, when the boss is sitting in a $100,000 dollar chair in the back and isn't going to care anyway. $\endgroup$
    – John K
    Nov 12, 2021 at 16:58
  • $\begingroup$ As you climb to the tropopause it stops getting colder, so thermodynamic efficiency of the engine does not improve any more, and since you are Mach-limited, you are not flying faster, so propulsive efficiency does not improve either (jet engines get more efficient with TAS). Tropopause is on average at FL360, and rarely above maybe 450, so you shouldn't see much cost saving above that. $\endgroup$
    – Jan Hudec
    Nov 12, 2021 at 19:12
  • $\begingroup$ @JanHudec At the equator, the tropopause can be as high as FL600 but that's unachievable by current bizjets anyway. The following link may be interesting: airspacemag.com/daily-planet/… The link put one record wrong however: the Proteus reached a 19,015 m (62,385 ft) altitude record for horizontal flight while the value in the link, 19,277 m (63,245 ft), is in fact Proteus' absolute altitude record (for a parabola, obviously). $\endgroup$
    – Giovanni
    Nov 13, 2021 at 6:40
  • $\begingroup$ At the other extreme, in the high the arctic in January, it can come down into the high teens. The U2 cruises at 70000. But its allowable speed range is about 10kt. $\endgroup$
    – John K
    Nov 13, 2021 at 13:36

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