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According to this article on aerospace-technology.com, many of the larger business jets seem to have a service ceiling of 51,000 feet. Other sources verify this trend.

What is the reason for this, if anything specific can be named? Some possibilities that I came up with are:

  • engineering considerations (limitations of materials, pressurization, systems capabilities etc.)
  • safety, i.e. time of usefull consciousness getting too short higher up (being less than 10 s @ 50,000ft)
  • too little to be gained going higher (no congestion anymore at that alt anyway)
  • an industry agreement (based on any of previous and/or other factors), in the same manner as european automakers' pact to limit the max speed of cars to 250km/h (excluding some model)
  • regulatory factors

Other common "steps" appear to be 45,000 ft and 41,000, with smaller models. Nice round numbers all, but surely there has to be some reason for the magical 51,000 ft figure. Interestingly, the Learjet 85 was designed to have a service ceiling of "only" 49,000 ft, and the program got cancelled at least partially due to poor sales. It seems it was simply lacking the last 2000 ft...

Examples of business jets with 51000ft service ceiling:

Cessna Citation X

Falcon 7X, 8X

Global 5000, 6000 and 7500

Gulfstream G550, G600, G650 ER

Learjet 40, 45, 60, 70/75

etc...

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  • $\begingroup$ „european automakers' pact to limit the max speed of cars to 250km/h“. The rumor says it is some sort of gentleman’s agreement, Porsche is not following it. In europe Vmax is Vmax not Vtemporary. Sustaining higher speed becomes technical challenging. You need spoilers and/or ESP (remember Audi TT). Tires with speed index V are affordable, higher speed indices like ZR or Y are expensive. I like my german Autobahns with Richtgeschwindigkeit as they are - they are not speedways! Aside physics, personal exhaustion is quickly reached, traffic (jams) and environmental conditions limit it. $\endgroup$
    – Peter
    Commented Feb 14, 2020 at 23:07
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    $\begingroup$ TT was kind of a special case, a beautiful and futuristic car, but the form of the back was such that it created lift at high speeds. Good for planes, bad for cars... I've driven cars with absolutely no problem reaching or maintaining the 250km/h limit. Many carmakers would be able to surpass that speed with relative ease, but it was easier, safer and more profitable just to make an agreement to keep thingd to a certain limit. The 51,000 feet ceiling could be a similar case. $\endgroup$
    – Jpe61
    Commented Feb 14, 2020 at 23:25
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    $\begingroup$ @Sean many car makers actually use abbreviation ESP for car stability control: en.m.wikipedia.org/wiki/… And it always makes me smile. $\endgroup$
    – Jpe61
    Commented Feb 15, 2020 at 16:50
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    $\begingroup$ ESP, ESC, DSC or vehicle dynamics controls. ESP is the trademark of Bosch. To be honest, I didn’t knew that their so many terms for this ;) $\endgroup$
    – Peter
    Commented Feb 15, 2020 at 16:51
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    $\begingroup$ I've had by far the most fun in cars with the aforementioned turned off... $\endgroup$
    – Jpe61
    Commented Feb 15, 2020 at 22:44

1 Answer 1

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If for no other reason, its likely because there is a practical maximum descent speed for a given airframe and aircraft must be able to descend from their maximum cruising altitude to 10,000 ft. in 10 minutes or less. Although under part 135 (how many of these planes are likely flown) the regulations have a few variations you can have less oxygen on board if you can descend in 4 minutes.

At 51,000 ft. you would need to lose 41,000 ft in 10 minutes or even 4 minutes. That is a descent rate of either 4100 ft/min or a blistering 10,250 ft/min which likely yields a speed at or near Vne for most of these airframes.

This Gulfstream pilot discusses a bit of it from a practical standpoint here.

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  • $\begingroup$ The Gulfstream pilot -link is excellent!! It shows (among other things) through several examples how complex this issue is from regulatory point of view. $\endgroup$
    – Jpe61
    Commented Feb 15, 2020 at 9:24
  • $\begingroup$ Plus the thing that did not come to my mind (the graph + table earlier in the article), above ~45000 feet TUC stays pretty much the same: all the oxygen you have is in your blood, you can't breath in any extra from atmospheric conditions, but TUC does not diminish that fast either. $\endgroup$
    – Jpe61
    Commented Feb 15, 2020 at 9:31
  • $\begingroup$ You could also point out that at 63000 feet blood boils at normal body temperature, which means if you go above the low 50s without a pressure suit you are getting dangerously close to instant death if you get a decompression. $\endgroup$
    – John K
    Commented Feb 15, 2020 at 15:13
  • $\begingroup$ So Concorde could do it because it could descend at high speed. Makes sense. I wonder if Concorde ever had such an event. $\endgroup$
    – TomMcW
    Commented Feb 15, 2020 at 15:38
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    $\begingroup$ @JohnK if you are referring to the Armstrong limit, no, blood would not boil because of blood pressure, not even close. Saliva, sweat and moisture in the lungs would. Fun fact: If you were flying at or above the Armstrong limit without pressurisation, and (as your last deed) decided to take a leak, you would piss steam. $\endgroup$
    – Jpe61
    Commented Feb 15, 2020 at 22:57

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