c5load
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Higher Ceiling Limitations?

Thu Nov 19, 2009 6:34 pm

I know since jet airliners have surfaced that the service ceiling has been capped around 40-43K. Since engines have now become so advanced, why haven't the ceilings become higher? I think it'd be kinda cool to cruise at 50-55K!
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bri2k1
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RE: Higher Ceiling Limitations?

Thu Nov 19, 2009 6:40 pm

A number of reasons. Among them are the extra time it would take to descend to breathable altitude in the event of a loss of pressurization, and the cost/weight of building a structure capable of the higher differential pressure.
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KELPkid
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RE: Higher Ceiling Limitations?

Thu Nov 19, 2009 8:20 pm

When you get into the really high altitudes (above FL410), in the US at least, you bump into the requirement that one flight crew member must always be wearing an oxygen mask. This is because your useful time of consciousness at those altitudes, if a decompression were to happen, is only a matter of seconds.

A bit of an "inconvenient" regulation...  Wink This is also why you don't see 747's (certified with a service ceiling of 45,100') up above FL410 very often...well that, and the 747 is definitely approaching the "coffin corner" of the flight envelope up near its service ceiling.
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BMI727
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RE: Higher Ceiling Limitations?

Thu Nov 19, 2009 8:42 pm



Quoting KELPkid (Reply 2):
This is also why you don't see 747's (certified with a service ceiling of 45,100') up above FL410 very often...

Didn't 747SPs regularly fly up in the mid-40s? And the 787 should be able to make FL430, but how often that will be used is yet to be seen.
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Fly2HMO
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RE: Higher Ceiling Limitations?

Thu Nov 19, 2009 9:17 pm

Every once in a while you'll find high performance bizjets cruising up that high (ie. Citation X)
 
njxc500
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RE: Higher Ceiling Limitations?

Thu Nov 19, 2009 10:45 pm

It appears that "if" the pilots have quick donning masks available, neither has to wear one.

Just for reference business aircraft like Globals climb direct to 430. That's really the lowest they fly regularly. Also usually don't pass 470 unless they are on a really long ride.


http://www.flycrj.com/2009/06/09/whe...n-required-in-part-121-operations/

For Pressurized Turbine Aircraft Operating under Part 91:

Above FL250, there must also be at least a 10-minute supply of supplemental oxygen available for each occupant
Above FL350, one pilot must always wear an oxygen mask that is supplying oxygen or can automatically supply oxygen if the cabin pressure altitude exceeds 14,000 feet MSL (or Mean Sea Level); however, if the aircraft is below FL410 and there are two pilots at the controls and each pilot has a quick-donning mask (i.e., they can place it on their face within 5 seconds) available (which we do), then neither has to wear a mask while operating the airplane unless one of the pilots leaves their station. (Reference: FAR 91.211)
 
vikkyvik
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RE: Higher Ceiling Limitations?

Thu Nov 19, 2009 11:32 pm



Quoting C5LOAD (Thread starter):
I know since jet airliners have surfaced that the service ceiling has been capped around 40-43K. Since engines have now become so advanced, why haven't the ceilings become higher? I think it'd be kinda cool to cruise at 50-55K!

Obviously this is an artificial limitation, but I believe I remember that the FAA only allows commercial airliners to fly up to FL450 (is that for Part 121? I never remember). I believe it's driven by, as Bri2k1 stated, the need to descend to breathable altitude in a certain amount of time.

I'd imagine it would take a significant amount of time and money to get them to increase that max altitude, and I'd assume it's simply not worth it currently.
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David L
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RE: Higher Ceiling Limitations?

Fri Nov 20, 2009 7:10 pm



Quoting Vikkyvik (Reply 6):
I believe I remember that the FAA only allows commercial airliners to fly up to FL450 (is that for Part 121? I never remember)

How did Concorde operations fit in with that or was it never above FL450 in US airspace?
 
KELPkid
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RE: Higher Ceiling Limitations?

Fri Nov 20, 2009 7:47 pm



Quoting David L (Reply 7):
How did Concorde operations fit in with that or was it never above FL450 in US airspace?

Good question. Braniff used to "rent" the Concorde from BA and AF between turns for a short while (1979-1980 IIRC). They flew it between DFW and IAD and JFK, if memory serves correctly...so they had to deal with many inane FAR restrictions, like you can't go above Mach 1 (that would be like trying to drive a Porsche on the autobahn at 55 MPH).

I've often wondered how Concorde crews and operators dealt with potential situations like, say, the potential for a decompression at FL600 (if it happened, and someone wasn't wearing the proper gear, like a positive-pressure oxygen mask, the aircraft and all aboard would have been lost, unless the autopilot was set up to initiate an immediate descent to 10,000 feet in that event...). SR-71 and U-2 pilots wear full pressure suits because, at the altitudes they operate at (even higher than Concorde, admittedly), a decompression would be deadly without it.
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vikkyvik
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RE: Higher Ceiling Limitations?

Fri Nov 20, 2009 8:02 pm



Quoting David L (Reply 7):
How did Concorde operations fit in with that or was it never above FL450 in US airspace?

I have no idea.

Tried searching around a bit...found this:

As the maximum operating altitude of modern jet transports increases, so does the physiological risk associated with cabin depressurization. Some existing large commercial transport category airplanes type certificated prior to Amendment 25-87 are approved to operate up to 45,000 feet altitude (See Attachment 4). Special conditions were issued for operation up to 51,000 feet for several executive business jets and the Concorde (60,000 feet) to address cabin depressurization concerns.

Both the business jets and Concorde shared a common performance characteristic; specifically the ability to conduct a rapid descent following a sudden loss of cabin pressure. Also, business jets typically feature rear fuselage-mounted engines which incorporate an aft pressurized bulkhead located forward of the rotor burst zone which decreases the likelihood of experiencing a rapid cabin decompression following an engine failure. Amendment 25-87 incorporated criteria similar to the provisions of the special conditions into part 25 to ensure occupant safety following any failure scenario including uncontained engine failure.


From here:

http://rgl.faa.gov/Regulatory_and_Gu...901D98625713F0056B1B8?OpenDocument
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bri2k1
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RE: Higher Ceiling Limitations?

Fri Nov 20, 2009 8:27 pm

I remember reading that was part of the reason for the small windows, too. I guess if you make the fuselage robust enough, there won't be many things that cause rapid decompression that you could survive anyway?
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BMI727
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RE: Higher Ceiling Limitations?

Fri Nov 20, 2009 8:53 pm



Quoting Bri2k1 (Reply 10):
I remember reading that was part of the reason for the small windows, too.

That may have had more to do with thermal issues, since high altitude business jets do not have small windows. The Gulfstreams have huge windows.
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KELPkid
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RE: Higher Ceiling Limitations?

Fri Nov 20, 2009 8:57 pm



Quoting Bri2k1 (Reply 10):



I remember reading that was part of the reason for the small windows, too. I guess if you make the fuselage robust enough, there won't be many things that cause rapid decompression that you could survive anyway?



Quoting BMI727 (Reply 11):
That may have had more to do with thermal issues, since high altitude business jets do not have small windows. The Gulfstreams have huge windows.

I wonder if the Brits (1/2 of Concorde's engineering team) were a little gun shy after the Comet I? After all, the metal fatigue that lead to the explosive decompressions was originating at the corner of a rectangular window frame...
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David L
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RE: Higher Ceiling Limitations?

Fri Nov 20, 2009 9:38 pm



Quoting KELPkid (Reply 8):
I've often wondered how Concorde crews and operators dealt with potential situations like, say, the potential for a decompression at FL600

As far as I know, it was certified for (at least FL600) in the same way that other types were certified for their operational ceilings.

Quoting Bri2k1 (Reply 10):
I remember reading that was part of the reason for the small windows, too

It was. With the obligatory "as far as I know"... Concorde could descend to a "safe" altitude within the regulation time with two (I think) windows out.

Quoting BMI727 (Reply 11):
That may have had more to do with thermal issues

They certainly got warm but I think it was more to do with potential decompression issues.

Quoting KELPkid (Reply 12):
I wonder if the Brits (1/2 of Concorde's engineering team) were a little gun shy after the Comet I? After all, the metal fatigue that lead to the explosive decompressions was originating at the corner of a rectangular window frame...

I don't think that had a lot to do with the window size. We built quite a few pressurised airliners with bigger windows between the Comet I and Concorde, after all.  Smile

Quoting Vikkyvik (Reply 9):
Special conditions were issued for operation up to 51,000 feet for several executive business jets and the Concorde (60,000 feet) to address cabin depressurization concerns.

Ah, I suspect that's the key issue.  thumbsup 

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Starlionblue
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RE: Higher Ceiling Limitations?

Sat Nov 21, 2009 12:59 am



Quoting David L (Reply 13):
With the obligatory "as far as I know"... Concorde could descend to a "safe" altitude within the regulation time with two (I think) windows out.

Indeed. As I recall pretty much any modern airliner can hold pressure with one window gone anyway. There'll be a stiff breeze but no decompression.
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DashTrash
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RE: Higher Ceiling Limitations?

Sat Nov 21, 2009 3:45 am

Most airplanes take an incredible performance hit at high altitudes. With the X, when you get above 470, whatever mach number you have at level off is where the airplane will stay. It just will not accelerate anymore.

 
David L
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RE: Higher Ceiling Limitations?

Sat Nov 21, 2009 11:03 am



Quoting David L (Reply 13):
Concorde could descend to a "safe" altitude within the regulation time with two (I think) windows out.

I could have worded that better... Concorde's descent to a safe altitude from typical cruise FL was deemed as safe as in other types and it could sustain an appropriate cabin altitude with one (possibly two) windows out because they were small.
 
Bellerophon
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RE: Higher Ceiling Limitations?

Sat Nov 21, 2009 1:07 pm

C5LOAD


... I know since jet airliners have surfaced that the service ceiling has been capped around 40-43K....

Like this one for example:  Wink

http://i303.photobucket.com/albums/nn142/Bellerophon_photos/P8160022-1.jpg



KELPkid


...if it happened, and someone wasn't wearing the proper gear, like a positive-pressure oxygen mask, the aircraft and all aboard would have been lost...

As you suggest, a rapid-donning, positive-pressure, O2 mask was indeed part of the flight crew emergency equipment, and the crew tested annually in its correct use, something that was not as easy as it may sound!


... I've often wondered how Concorde crews...dealt with potential situations like, say, the potential for a decompression at FL600...

In common with all other jet passenger transport aircraft, Concorde had an emergency descent procedure which met all the targets and fulfilled all the requirements imposed by the various regulatory authorities in whose airspace she flew.

Starting from FL600, the initial rate of descent would have been around 12,000 - 15,000 fpm, reducing on passing through FL500 and increasing again on passing through FL400.

On Concorde, once below FL500 an emergency descent also became a deceleration manoeuvre, which brought with it the necessity to move fuel forward rapidly to keep the CG within limits as the aircraft Mach number decreased.

Various emergency descent profiles were tried during test flying. The one that was finally adopted for line operations gave an average rate of descent of around 7,000 fpm, and kept the CG within limits throughout.

We need to keep a sense of proportion about the risks involved in operating aircraft at these altitudes. It certainly isn't something to be treated lightly, but as someone who was trained for and operated at those sorts of altitudes for years, I do not believe the problems are as dangerous as you suggest.

My search function doesn’t seem to be working, so I can’t find and post a link, so allow me to re-post something I wrote on Airliners.Net a few years ago, with apologies for the thread creep!

Quote:


In the event of a total and near-instantaneous decompression to 60,000 ft, then the ramifications would have been very serious. Passengers exposed to atmospheric pressure at FL600 for any appreciable length of time would have had only a few seconds of consciousness followed by a merciful lapse into unconsciousness.

In addition, the sort of damage or failures necessary to have caused this would have brought with it a whole host of other problems, not the least of which may have been that the aircraft had ceased to be a viable flying machine - the early Comet accidents being a case in point.

However, in the overwhelming majority of decompressions, experienced over many years on all aircraft types, the aircraft did not explosively depressurise to ambient atmospheric pressure, even if it may have felt like it to the occupants.

Whether due to pressurisation system failure, discharge valve failure, a small hull breach, a door or window blow-out, or just plain human error, the cabin took time to decompress, often a considerable amount of time.

It is this time, the time the cabin takes to climb which provides the flight crew with a safety margin, precious seconds in which to act to protect passengers and crew from extreme cabin altitudes.

On Concorde, this involved the crew in protecting themselves (pressurised O2 masks) analysing the situation (what warnings?, what cabin rate-of-climb?) rectifying if possible (re-instating packs, selecting alternate systems, closing errant valves manually) or, if control of the cabin had been irretrievably lost, initiating an emergency descent.

The cabin altitude on Concorde was typically around 5,000 ft in the cruise, and in common with most commercial aircraft, various flight deck warnings would occur as the cabin altitude rose through 10,000 ft, and again passing through 14,000 ft, to alert the crew to any problem, assuming their own eyes, ears, sinuses and lower intestines had not already done so!

There were also many protection devices fitted to Concorde to ensure that the cabin altitude never exceeded 14,000 ft, however, even had they all failed and the cabin had been climbing at 5,000 fpm, it would still have taken 36 seconds before the cabin altitude exceeded 8,000 ft.

It would have taken 108 seconds before it exceeded 14,000 ft and around 3 minutes for the cabin to exceed 20,000 ft, by which time the aircraft would have been well on its way down to safety in an emergency descent.

In most cases, the cabin altitude would never have got above 20,000 ft, and the overwhelming majority of these incidents, though alarming, would have been highly survivable for all occupants. The chances of passengers ever being exposed to atmospheric pressure at FL600 was an extremely remote possibility.

One small point I would like to emphasise is, that when people talk about Time of Useful Consciousness, it is the TUC at the cabin altitude by which the O2 mask is likely to be have been donned, not the TUC at the aircraft’s cruising altitude when the decompression commences, that we should refer to.


...unless the autopilot was set up to initiate an immediate descent to 10,000 feet in that event...

No such autopilot mode existed on Concorde, nor was any other mode on the autopilot used, as any emergency descent was required to be flown manually.


Best Regards to all

Bellerophon
 
A342
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RE: Higher Ceiling Limitations?

Sat Nov 21, 2009 1:25 pm



Quoting DashTrash (Reply 15):
With the X, when you get above 470, whatever mach number you have at level off is where the airplane will stay. It just will not accelerate anymore.

That's interesting. Could it at least maintain M0.92 if you accelerated to that speed before the climb?
Exceptions confirm the rule.
 
DashTrash
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RE: Higher Ceiling Limitations?

Sun Nov 22, 2009 3:47 am



Quoting A342 (Reply 18):
That's interesting. Could it at least maintain M0.92 if you accelerated to that speed before the climb?

It won't be able to maintain .92 in the climb up to 510. The power setting for high speed cruise is usually at the top of the cruise detent using all available power. Even going to takeoff power doesn't give much more push.

It's definitely possible to have a higher mach number up there than we had on that flight, but I think the most you could ever possibly see is in the .84 range. The airplane is certified to go that high, but there isn't much advantage to doing it. It really peters out above 450, but does exceptionally well in the 410-450 range.
 
peterpuck
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RE: Higher Ceiling Limitations?

Tue Nov 24, 2009 8:06 pm

Have a look into cosmic radiation and decide if we really want to be flying any higher!
 
rwessel
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RE: Higher Ceiling Limitations?

Wed Nov 25, 2009 4:56 am



Quoting Peterpuck (Reply 20):
Have a look into cosmic radiation and decide if we really want to be flying any higher!

Cosmic radiation exposure is a somewhat complex subject, since much of the damaging ionizing radiation is a secondary effect (the high energy proton hits the atmosphere resulting in a gamma, plus other stuff). In fact, past about 50kft, the amount of ionizing secondary radiation from cosmic rays starts (slowly) tapering off, since there isn't enough atmosphere for the protons to hit (on the flip side, you and your aircraft start getting hit more directly by the protons).

In any event, there's not too much difference once you get past 40kft - you've already lost the vast majority of the atmosphere’s screening effect, but are still well within the shielding from the earth's magnetic field.
 
KELPkid
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RE: Higher Ceiling Limitations?

Wed Nov 25, 2009 5:14 am



Quoting Bellerophon (Reply 17):

Thanks for that, Bellerophon. Always nice to hear from someone who's been there, done that. Glad to see that you are still an active member!  Smile
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