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nano404
6th Jul 2007, 23:43
What factors contribute to determining the service ceiling? Air pressure effect on the fueselage? Engine power and operating altitudes? I would imagne a few factors contribute to this?

BelArgUSA
7th Jul 2007, 12:08
Dear Nano404 -
xxx
For transport category turbojet aircraft, the maximum certificated altitude is generally the maximum aircraft altitude at which the cabin altitude can be maintained at or below 8,000 feet (passenger physiological concern).
xxx
Max certif. alt. B-747 = 45,100 feet (cabin altitude at 8,000 feet)
Max certif. alt. B-707 = 41,000 feet (cabin altitude at 8,000 feet)
Max certif. alt. B-727 = 41,000 feet (cabin altitude at 8,000 feet)
Max certif. alt. Learjet 31A = 51,000 feet (cab. altitude at 8,000 feet)
Max certif. alt. Concorde = 61,000 feet (cab. altitude at 8,000 feet)
xxx
So I would state that the max altitude is function of the maximum cabin differential pressure...
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The other factor is also the time it takes that aircraft to descent in emergency, from its maximum certificated altitude, down to to 14,000 feet. I understand that some aircraft are limited by this concern, i.e. B-737 max altitude 35,000 feet. I think that certification requires the aircraft to descent to 14,000 feet in 4 minutes or less.
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Sorry, but I am not certain about the B-737 data, it was just mentioned to me, I hope some B-737 experts will correct me.
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As far as the 747, certificated to 45,100 feet, I discussed the actual performance "ceiling" of 747s with a friend of mine, who flies the USAF E-4B (a 747 as well), he told me that he flew one to 50,500 feet and... ran out of power and ideas. Personally, I rarely fly my 747-200s above 39,000 feet. Waste of time and little is gained above FL 370 or FL 380... In order to make it up there, you must be extremely light at the end of the cruise, with tanks near empty.
xxx
:)
Happy contrails

JW411
7th Jul 2007, 12:19
I seem to remember from my RAF days that the definition of service ceiling was that height at which the rate of climb had reduced to 200 fpm.

ChristiaanJ
7th Jul 2007, 14:48
JW411, that sounds familiar. But that wouldn't be a fixed figure, but increase as the aircraft gets lighter.

nano404, wing loading and wing design play a role.
Assuming no power limitation for an instant, your absolute ceiling is determined by the altitude at which Mach buffet speed and stall speed become the same.
So that would be one factor, with a suitable margin, determining your service ceiling.
On some aircraft you may well hit that "corner" before you run out of excess power.

On Concorde the service ceiling was 60,000 ft, again based on the time needed to descend to 14,000 ft after a cabin decompression. 69,000 ft was reached during test flights.

nano404
7th Jul 2007, 15:23
Oh I see. So, if an aircraft has a good descent rate then it would be the, uhh, "certified" service ceiling but not the 'go higher and die' altitude, that is, if there is any. so i would imagine that aerodynamics, and the engine of the aircraft are factors that would determine the descent rate? (That is, not factoring in passenger and cargo) And if i built a rocket of an aircraft that could descend from 75,000 to 14,000 in four minutes (sweet hell :}), then it would have a certified ceiling of 75,000 ft?

Also, is the air ever going to get too thin for the engines, or, to support the aircraft?

Always know BelArgUSA by those "xxx"s ;)

ChristiaanJ
7th Jul 2007, 15:42
nano404,
Sorry to have been fooled into thinking you were asking a serious question.

ChristiaanJ
7th Jul 2007, 16:39
nano404,
75,000 ft to 14,000 ft in four minutes is only 180 mph vertically, you don't need a "rocket of an aircraft" for that. So there's a gaping hole in your reasoning.

A gaping hole literally.... it's the size of the hole (nominally a window) and the capacity of the pressurisation system that determine to what level the cabin altitude will rise during an emergency descent, before you get to an altitude where the passengers can survive. Those tiny windows on Concorde were there for a very good reason.

Also, is the air ever going to get too thin for the engines, or, to support the aircraft?Well, yes. Power or thrust decrease with altitude (air density), so invariably you reach a point where even at the "best climb speed" you no longer have any excess power available to climb. For light general aviation aircraft, that's generally the condition that determines the ceiling. For airliners, as said earlier, other factors come into play that may limit the service ceiling.

alexban
7th Jul 2007, 19:18
737 classic-max FL370
737 NG-max FL410 Cabin altitude 8000

skiingman
7th Jul 2007, 21:43
I had this exact same question before and some searching illuminated the difference between the certified limits and the generally accepted meaning of service ceiling (climb 100fpm). The latter seems to only come into play on modern jets when one or more engines isn't operating.

Some reading:
http://yarchive.net/air/airliners/cruise_altitude.html
http://www.dc-8jet.com/0-dc8-sst-flight.htm (50k feet in a DC-8. Oh, and they went supersonic too. :))

ChristiaanJ
7th Jul 2007, 22:07
skiingman,
Two excellent links, much better than what I found.