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I heard in a TV show that private jets such as Gulfstream can fly at about 50,000 ft, higher than other civil jets.

Is it an aerodynamic reason (lighter aircraft to be sustained in a less dense air) or does it depend on the fact that a smaller fuselage is more resistant to lower outside pressure at a higher altitude?

I think that – if possible – flying higher, in order to fly faster and with lower fuel flow, is always better. Am I correct?

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    $\begingroup$ Jet engine efficiency does not increase above the tropopause, at around 11km, due to the way temperature behaves beyond it, see here: aviation.stackexchange.com/questions/12541/… $\endgroup$ Aug 6 '19 at 8:48
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    $\begingroup$ Re, "higher is always better:" If that were a simple truth, then long-range flights all would be in space planes. $\endgroup$ Aug 6 '19 at 22:15
  • $\begingroup$ @SolomonSlow Technically, the longest long range flights are: Any flight around the earth one or more times are in some kind of space plane. $\endgroup$ Aug 8 '19 at 20:45
  • $\begingroup$ @VolkerSiegel It seems odd to call a plane with an service ceiling of 51,000 feet and rather limited maneuverability in a vacuum a space plane. $\endgroup$
    – 8bittree
    Aug 8 '19 at 21:11
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There are several reasons and none of them really have to do with a private jet being more aerodynamic than a commercial jet.

Private jets have better power to weight ratio than commercial jets so it is easier to ascend to greater heights. Commercial jets also carry more fuel which needs to be burned off to lighten the plane to go higher.

It’s easier to pressurize smaller cabins than larger ones (this is a significant factor for commercial jets).

Flying higher is usually better for avoiding weather that can even affect commercial airliners in the 30ks.

There is much less traffic up above 40k and it allows the private jets to fly more direct routes at a faster speed and more safely as well.

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    $\begingroup$ You forgot the bragging rights :-) $\endgroup$
    – Antzi
    Aug 6 '19 at 9:10
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    $\begingroup$ They are designed for higher cruise speed, which explains the higher power/weight ratio. Add to that a lower wing loading for operating out of regional airports. The higher cruise altitude is just a consequence of the desire to fly faster and from more airports. $\endgroup$ Aug 7 '19 at 7:47
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There are three main factors that let corporate jets with 50000+ ceilings get up that high.

First and most importantly, they have very large wings. Partly this is because of the need for fuel volume for range requirements, but it's mainly that you need the extra wing area to be able to fly that high with a reasonably efficient indicated airspeed at 50000 ft (500 kt TAS is only 250 kt indicated at FL500) at reasonable weights.

Next they use turbofan engines like the BR700 with lower bypass ratios than airliners because a lower bypass ratio is somewhat more efficient in very thin air.

Lastly, they operate with higher cabin pressure differentials, 9-10 psi rather than around 8 psi to get an 8000 ft cabin at 50+ thousand feet.

The structure has be a bit heavier to handle the pressure differential, but not that much because they are designed for way less cycles than an airliner, 3-500 cycles per year vs 2-3000 cycles for a regional airliner. If you took a corporate jet as-is and put in a regular interior and started running it 4 or 5 flights per day flogging it like a mule, it'd start breaking down in short order. Conversely, corporate aircraft created as conversions of regional airliners designed for 3000 cycles per year will run pretty much forever in the light duty corporate role.

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    $\begingroup$ The structure does not necessarily need to be heavier. Since the diameter of the cabin is smaller, the square-cube-law helps you out, i.e. you can have higher pressure differential using the same materials and wall thickness. $\endgroup$
    – Bianfable
    Aug 6 '19 at 13:37
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    $\begingroup$ Not necessarily; Global's tube is the same diameter as the RJ's. $\endgroup$
    – John K
    Aug 6 '19 at 14:03
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    $\begingroup$ Agreed, but that's a large long-range business jet compared to a small regional airliner. $\endgroup$
    – Bianfable
    Aug 6 '19 at 14:07
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I think the other important factor that the others haven't addressed is Mach number.

The speed of sound is doesn't change much with altitude (it actually drops slightly due to reducing temperature) and if a plane gets too close to it, shockwaves start to form in areas where the airflow is accelerated (such as on the top of the wing) causing a lot of excess drag.

By flying higher, a plane gets a higher ground speed and true air speed for the same indicated air speed (IAS). However the limit is when the true air speed (TAS) results in a Mach number that starts to cause shock waves. To go higher, a plane has to reduce its indicated air speed to maintain the same Mach number. Therefore the limiting height is when IAS approaches stall speed at the same time as the Mach number limit is reached. (see Coffin Corner)

Now private jets are usually designed to use smaller runways than big airliners, so they have a slower stall speed (due to relatively large wings for their weight). Their maximum Mach number is similar (an easy way to tell is that the angle of wing sweep is similar) so the height at which minimum IAS = maximum TAS is higher.

EDIT: True air speed (TAS) is the actual speed of the air, similar to the aircraft's ground speed (after allowing for wind). Indicated air speed (IAS) is the effective aerodynamic air speed, or "how much force you'd feel if you stuck your hand out the window". IAS falls relative to TAS as the air gets thinner.

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  • $\begingroup$ How much faster would a Gulfstream at 50,000ft be able to fly a route compared to a commercial flight at 35,000? (say SFO-JFK?) $\endgroup$
    – Aaron Ash
    Aug 8 '19 at 12:58
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    $\begingroup$ @Aaron, no faster. It's flying at the same Mach number and thus approximately the same true air speed and ground speed. The thinner air means that the indicated air speed is lower, so there's less drag and the engines don't need as much thrust. $\endgroup$ Aug 8 '19 at 13:09
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I am quite surprised that no-one mentioned safety as a reason in the previous answer. In 1996, the FAA adopted Amendment 25–87, which imposed additional high-altitude cabin-pressure specifications, for new designs of aircraft types.

The standards adopted as Amendment 25-87 pertain to operation of subsonic airplanes to a maximum altitude of 51,000 feet, although many of the requirements addressed therein relate to operations at lower altitudes (below 41,000 feet) as well.

Typical systems to be considered are engine bleed air systems, air conditioning systems, power sources, outflow valves and their associated control systems, etc. Failures or a combination of failures which expose the occupants to: (1) cabin altitudes in excess of either 25,000 feet for more than 2 minutes, or (2) cabin altitudes that exceed 40,000 feet for any duration, shall be shown to be extremely improbable (see Glossary).

Airliners altitude are thus limited by safety regulations in case of rapid pressure loss. On modern airliner a major engine failure might lead to a quick depressurization and even passenger sucked out of the window. In this case you have to lower the altitude below 25'000ft in less than 2 minutes which means a decent rate of approximately 10'000ft/min. To avoid any structural overstress, you can't have much higher decent rate than that thus limiting the maximal altitude you can fly. The 40'000ft absolute limit is not unbreakable as the A380 was certified to fly at 43'000ft.

On the other end private jet have the engines mounted on the aft fuselage, and behind the pressurization bulkhead. Which means that fast depressurization in case of an engine failure is totally unlikely and they can therefore bypass this rule. Furthermore they operate at higher cabin pressure which means in case of an air pressurization system failure, they will have a bit more time to descend to lower altitudes. Therefore they can use higher altitude up to 51'000ft to avoid traffic and increase fuel efficiency.

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  • $\begingroup$ wait so why does engine location affect depressurisation? I thought it was simply a matter of using bleed air for the cabin $\endgroup$
    – Abdullah
    Oct 5 '20 at 17:05
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    $\begingroup$ In case of engine uncontained engine failure some part might puncture the airframe. The private jet pressurized cabin ends before the engine location thus mitigating the risk of depressurization. $\endgroup$
    – MaximEck
    Oct 5 '20 at 17:16

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