QantasHeavy
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41,000 Feet -- Will There Be A Higher Standard?

Sat Dec 22, 2007 1:10 pm

I have been a frequent flyer for more than 10 years and have never flown above 41,000 feet. Been on over a hundred 747-400s, 777s etc. that are capable of flying higher, but FL410 always seems to be the top. I have had work colleagues say they have been to 43,000 feet in the 777 going to the Middle East and some claim 45,000 in the 747 near the top of descent. They seem to be credible, but I have never done it.

Is this a function of the airways/ATC, the hassle of an SOP stating one pilot must wear an O2 mask above FL410, or the load/performance of the aircraft... or all of the above?

Concorde obviously flew very high and private jets go way up there too. Any reasons why the 787 and A350 generation would not be designed to go higher -- doesn't it save more fuel? Certainly should be a smoother ride.

My first thread on TechOps. Thanks.
 
rsbj
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RE: 41,000 Feet -- Will There Be A Higher Standard?

Sat Dec 22, 2007 1:43 pm

It doesn't always save fuel climbing higher. At any weight, an aircraft has an optimum altitude and climbing above it usually carries a slightly higher fuel burn penalty than being the same amout below optimum. Our 737-700W's have an optimum altitude of FL400 at 130,000lbs. Cruising at FL390 has a fuel burn penalty of roughly 1%, where FL410 would be 1.3%.

Also, during certification, the plane must be able to descend to 14,000' in four minutes or less from it's maximum certified altitude. I'm told this is why the 737NG was not certified to FL430, it is too slick, and/or it's speed brakes are not effective enough, to make it down in time.

One more item; one pilot must put on his oxygen mask above FL410, so some pilots are discouraged from climbing above FL410.
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QantasHeavy
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RE: 41,000 Feet -- Will There Be A Higher Standard?

Sat Dec 22, 2007 2:47 pm

Thanks RSBJ.

I wonder how Concorde handled this. Assume one pilot had to be on O2 and if there had been a depressurisation of that cabin at 55,000 feet it could make the descent without overspeeding because it was a high-speed aircraft.
 
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longhauler
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RE: 41,000 Feet -- Will There Be A Higher Standard?

Sat Dec 22, 2007 2:57 pm

In Canada, when cruising above FL410, one of the pilots must be continuously on oxygen. This adds other considerations, as not only is it cumbersome to wear, but also, Oxygen capacity now becomes an issue.

Quoting QantasHeavy (Reply 2):
I wonder how Concorde handled this.

I am not sure what happens on Concorde, as I am not sure that any mask would work at such a high altitude! One would almost need a pressure suit.

This in mind, I have always found it curious that some (but not all) BAC-111s did NOT have a passenger oxygen system installed at all! I can only assume that they had a very fast descent rate to lower altitudes. I have always found it interesting that any passenger transport aircraft would be licenced in North America without oxygen masks at the passenger seats. (Some versions of the Trident also were not equipped with masks!)
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tb727
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RE: 41,000 Feet -- Will There Be A Higher Standard?

Sat Dec 22, 2007 5:14 pm



Quoting RSBJ (Reply 1):
It doesn't always save fuel climbing higher. At any weight, an aircraft has an optimum altitude and climbing above it usually carries a slightly higher fuel burn penalty than being the same amout below optimum. Our 737-700W's have an optimum altitude of FL400 at 130,000lbs. Cruising at FL390 has a fuel burn penalty of roughly 1%, where FL410 would be 1.3%.

Plus 99% of the time you get way better winds in the mid-30's, at least over the US. I'd rather be at FL350 and have a ripping tail wind than be up high just to be up high.
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CanadianNorth
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RE: 41,000 Feet -- Will There Be A Higher Standard?

Sat Dec 22, 2007 6:40 pm



Quoting LongHauler (Reply 3):
have always found it interesting that any passenger transport aircraft would be licenced in North America without oxygen masks at the passenger seats.

Makes perfect sence, as there are a selection of jets (as you stated), and many many turboprops without passenger oxygen masks. Just limit it to altitudes low enough so you don't need them, and your set.


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longhauler
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RE: 41,000 Feet -- Will There Be A Higher Standard?

Sat Dec 22, 2007 7:52 pm



Quoting CanadianNorth (Reply 5):
Makes perfect sence, as there are a selection of jets (as you stated), and many many turboprops without passenger oxygen masks. Just limit it to altitudes low enough so you don't need them, and your set.

You are right, as most turboprops are restricted to FL250 as they have either no masks, or only one air cycle machine. But I look at jets like the BAC111 which has no masks, (but two AC packs), and it is not altitude restricted. On the jet transports I have flown, if we had no masks available, (for whatever reason) we would be restricted to FL250 regardless of the number of packs, or ACMs.

I just wonder how the BAC111 got licenced in Canada and the US.
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Viscount724
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RE: 41,000 Feet -- Will There Be A Higher Standard?

Sat Dec 22, 2007 8:32 pm



Quoting LongHauler (Reply 3):
I have always found it interesting that any passenger transport aircraft would be licenced in North America without oxygen masks at the passenger seats. (Some versions of the Trident also were not equipped with masks!)

If memory correct, many Caravelles also lacked passenger oxygen maks.
 
flipdewaf
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RE: 41,000 Feet -- Will There Be A Higher Standard?

Sat Dec 22, 2007 8:56 pm



Quoting LongHauler (Reply 3):
This in mind, I have always found it curious that some (but not all) BAC-111s did NOT have a passenger oxygen system installed at all! I can only assume that they had a very fast descent rate to lower altitudes. I have always found it interesting that any passenger transport aircraft would be licenced in North America without oxygen masks at the passenger seats. (Some versions of the Trident also were not equipped with masks!)

The bac1-11s were able to deploy the reversers in flight, much like a C-17can, and achieve desent rates of over 10,000fpm. I would think that this was the reason that it dpesnt have oxygen masks.

Quoting QantasHeavy (Reply 2):
I wonder how Concorde handled this. Assume one pilot had to be on O2 and if there had been a depressurisation of that cabin at 55,000 feet it could make the descent without overspeeding because it was a high-speed aircraft.

IIRC Concorde had enough bleed air and was able to remain pressurized at 55,000ft even if one (or maybe two) of the windows blew out. I think this was one of the reasons for the small windows.

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cptspeaking
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RE: 41,000 Feet -- Will There Be A Higher Standard?

Sat Dec 22, 2007 9:33 pm



Quoting LongHauler (Reply 3):
This in mind, I have always found it curious that some (but not all) BAC-111s did NOT have a passenger oxygen system installed at all!

Did you know that on the ATR, there may be ~70 seats, but there are only 14 oxygen masks in the back? Better be able to scrap for them!!  Smile Interesting little fact to keep in mind next time you're on one listening to the FA briefing on the use of the masks...

Lack of a passenger oxygen system isn't that much of an issue in my opinion, especially if you can get down fast. Worst case, a couple people pass out and wake up again when the fun is over...
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RE: 41,000 Feet -- Will There Be A Higher Standard?

Sun Dec 23, 2007 2:28 am



Quoting QantasHeavy (Reply 2):
I wonder how Concorde handled this. Assume one pilot had to be on O2 and if there had been a depressurisation of that cabin at 55,000 feet it could make the descent without overspeeding because it was a high-speed aircraft.

Concorde had a special exemption. Also at those altitudes I belive that the crew not only had to be on O2, but I believe it had to be pressure breathing.
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Bellerophon
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RE: 41,000 Feet -- Will There Be A Higher Standard?

Sun Dec 23, 2007 4:21 am

RSBJ

...One more item; one pilot must put on his oxygen mask above FL410...

Not always, it depends on the rules of the regulatory authority under which you are operating. I believe you are correct for operations under FAA regulations, but there is no such requirement if operating under UK CAA/JAA regulations.


QantasHeavy

...I wonder how Concorde handled this. Assume one pilot had to be on O2...

You know what they say about assumptions.  Wink

No such requirement on Concorde, as it operated under UK CAA regulations and certification.


PilotPip

...Also at those altitudes I belive that the crew not only had to be on O2, but I believe it had to be pressure breathing...

Correct. The flight crew had rapid-donning pressure breathing masks immediately available to them, and were tested and timed once a year by an AvMed doctor, in the correct fitting and use of them.


Merry Christmas to all

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RE: 41,000 Feet -- Will There Be A Higher Standard?

Fri Dec 28, 2007 1:45 pm



Quoting CptSpeaking (Reply 9):
Did you know that on the ATR, there may be ~70 seats, but there are only 14 oxygen masks in the back?

WTF?? Why just the 14 oxygen masks in the back?? Seems really really strange.!!!
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SuseJ772
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RE: 41,000 Feet -- Will There Be A Higher Standard?

Fri Dec 28, 2007 5:28 pm



Quoting CptSpeaking (Reply 9):
Lack of a passenger oxygen system isn't that much of an issue in my opinion, especially if you can get down fast. Worst case, a couple people pass out and wake up again when the fun is over...

I have always thought this as well. As long as the pilot has access to the masks, I feel comfortable that he can get the plane down to an altitude where I'll recover from lack of oxygen before any brain damage.

After all, last I checked with physics, getting planes down was rarely a problem  wink 
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Max Q
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RE: 41,000 Feet -- Will There Be A Higher Standard?

Sat Dec 29, 2007 4:42 am

Been to 430 in a 767 200, one of the advantages that altitude can give you is to climb above the worst of the headwinds.
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jhooper
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RE: 41,000 Feet -- Will There Be A Higher Standard?

Sat Dec 29, 2007 12:01 pm

Also, once you start climbing into the stratosphere, the temperature of the air stops decreasing while the air gets thinner, and climbing higher loses it's value. For some reason, they put 36,000 feet on the ATP written test as the "optimum" altitude, but obviously there are variables to consider.
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txkf2010
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RE: 41,000 Feet -- Will There Be A Higher Standard?

Tue Jan 22, 2008 4:44 am

Ive been on a 757 @ FL430
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A342
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RE: 41,000 Feet -- Will There Be A Higher Standard?

Tue Jan 22, 2008 1:13 pm



Quoting SuseJ772 (Reply 13):
I have always thought this as well. As long as the pilot has access to the masks, I feel comfortable that he can get the plane down to an altitude where I'll recover from lack of oxygen before any brain damage.

Try that over the Himalaya.  Wink
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474218
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RE: 41,000 Feet -- Will There Be A Higher Standard?

Wed Jan 23, 2008 10:12 pm



Quoting LongHauler (Reply 6):
But I look at jets like the BAC111 which has no masks, (but two AC packs), and it is not altitude restricted. On the jet transports I have flown, if we had no masks available, (for whatever reason) we would be restricted to FL250 regardless of the number of packs, or ACMs.

I just wonder how the BAC111 got licenced in Canada and the US.

The BAC 1-11 has is certified to 25,000, 35,000 or 40,000 feet depending on the model and the type of supplemental oxygen system installed.
 
2H4
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RE: 41,000 Feet -- Will There Be A Higher Standard?

Wed Jan 23, 2008 10:17 pm



Quoting 474218 (Reply 18):
The BAC 1-11 has is certified to 25,000

Would would the range be for that model?

2H4
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RE: 41,000 Feet -- Will There Be A Higher Standard?

Wed Jan 23, 2008 11:33 pm



Quoting CanadianNorth (Reply 5):
Makes perfect sence, as there are a selection of jets (as you stated), and many many turboprops without passenger oxygen masks. Just limit it to altitudes low enough so you don't need them, and your set.

I was on a Saab 340 flying for ATA back in 2002 and my best interpretation of the safety card indicated that there was only one mask for 2 passengers on one side. I told the guy next to me he could have the mask, if its bad enough we needed the mask I wanted to be unconscious.
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474218
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RE: 41,000 Feet -- Will There Be A Higher Standard?

Wed Jan 23, 2008 11:36 pm

2H4, the list of serial numbers allowed to operated at 35,000 or 40,000 feet is available in TCDS, A5EU Notes 10. and 11. I assume those seral numbers not listen in the notes are restricted to 25,000 feet.
 
2H4
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RE: 41,000 Feet -- Will There Be A Higher Standard?

Wed Jan 23, 2008 11:46 pm



Quoting 474218 (Reply 21):

No, I meant range in terms of distance. At 25,000 feet, I can't imagine an already thirsty 1-11 would go very far.

2H4
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474218
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RE: 41,000 Feet -- Will There Be A Higher Standard?

Thu Jan 24, 2008 12:46 am



Quoting 2H4 (Reply 22):
No, I meant range in terms of distance. At 25,000 feet, I can't imagine an already thirsty 1-11 would go very far.

Sometime I don't really understand what I read. As for the range, I have know idea, but agree it would not be far. I read somewhere that the original design was to make an aircraft that was profitable on routes of 100 to 1000 miles. I had a real good friend (a Brit) and he thought the BAC 1-11 was a great aircraft. I would tease him about the "wonder jet".
 
thegeek
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RE: 41,000 Feet -- Will There Be A Higher Standard?

Thu Jan 24, 2008 4:08 am

In answer to the OP, I don't understand why not either. Even with the temperature not dropping off at higher altitudes, the pressure/density will certainly drop, and this is the most important thing. Perhaps it is just too difficult to design engines which are efficient both at sea level and above FL410. And the one pilot wearing an oxygen mask when operating under FAA rules would be highly annoying.

The reduction in winds at high levels would be highly advantageous, as it would allow a given plane to fly longer routes - the direction with the headwinds is the most important one. When in the other direction, you can simply fly lower and probably slower in airspeed terms.

Regarding the descent profile, couldn't you just put on a bigger speed brake, or certify the thrust reversers for in flight use?

Seems a rather obvious method of saving fuel, but it hasn't been done yet.
 
PhilSquares
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RE: 41,000 Feet -- Will There Be A Higher Standard?

Thu Jan 24, 2008 4:39 am

In response to the original question. On the 744, the max altitude is 45,100 feet. I've been up there several times mainly to avoid weather. In everyday operation, there isn't too much reason to go up that high. Normally, you're in the Trop so the temp is fairly constant, so the fuel flow will be the same, the winds are pretty constant at that altitude so you really just burn the fuel to climb that high and don't get much savings for that. Another consideration is the wing it self. Part of the cruise altitude determination is the weight of the aircraft. At normal cruise altitudes airlines will have a 1.2 or 1.3% margin on stall speed. That normally becomes the limiting factor. At high gross weights the 744, for example, could climb higher than it's optimum/max altitude but if you encounter any turbulence then you run the risk of stalling due to G loading on the aircraft.

Unless you're light for the entire flight, you're better off just staying at FL390-410.

Quoting Thegeek (Reply 24):
Regarding the descent profile, couldn't you just put on a bigger speed brake, or certify the thrust reversers for in flight use?

That becomes a very non-fuel efficient way of descending. Normally, the descent planning is done with the thurst at or close to idle (in an ideal world). To stay at cruise altitude then use drag devices to descent isn't fuel efficient.
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2H4
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RE: 41,000 Feet -- Will There Be A Higher Standard?

Thu Jan 24, 2008 5:09 am



Quoting PhilSquares (Reply 25):

That becomes a very non-fuel efficient way of descending. Normally, the descent planning is done with the thurst at or close to idle (in an ideal world). To stay at cruise altitude then use drag devices to descent isn't fuel efficient.

I believe the original descent question was in reference to emergency descents....not routine ones.

I wonder if there are any aircraft types limited to an artificially low ceiling due to unoptimized speed brakes.

2H4
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PhilSquares
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RE: 41,000 Feet -- Will There Be A Higher Standard?

Thu Jan 24, 2008 5:23 am



Quoting 2H4 (Reply 26):
I believe the original descent question was in reference to emergency descents....not routine ones.

I quickly reviewed the thread and couldn't find it so I just tried to give a general answer. The problem with making speed brakes larger is it requires a heavier structure in the wing thus adding to the weight of the aircraft. Not the best solution.
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2H4
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RE: 41,000 Feet -- Will There Be A Higher Standard?

Thu Jan 24, 2008 5:40 am



Quoting PhilSquares (Reply 27):
I quickly reviewed the thread and couldn't find it

Ah, my mistake. I must have just interpreted it that way.

Quoting 474218 (Reply 23):
I would tease him about the "wonder jet".

Ha, excellent.  biggrin 

2H4
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thegeek
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RE: 41,000 Feet -- Will There Be A Higher Standard?

Thu Jan 24, 2008 6:44 am



Quoting 2H4 (Reply 28):
Quoting PhilSquares (Reply 27):
I quickly reviewed the thread and couldn't find it

Ah, my mistake. I must have just interpreted it that way.

Actually, it is there, in Reply 3. Emergency descents are what I was referring to in Reply 24.

All design decisions involve trade offs. It could be that heavier speed brakes could be a price worth paying. I'm sure it would be if the engines didn't reduce efficiency at higher altitudes.
 
PhilSquares
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RE: 41,000 Feet -- Will There Be A Higher Standard?

Thu Jan 24, 2008 7:13 am



Quoting Thegeek (Reply 29):
All design decisions involve trade offs. It could be that heavier speed brakes could be a price worth paying. I'm sure it would be if the engines didn't reduce efficiency at higher altitudes.

But it's not an engine efficiency issue. The real problem becomes stall protection. Speed brakes the size of barn doors won't alleviate that problem. On the 744, for example, the aircraft with speedbrakes has no problem getting down at Vmo/Mmo. Making the speedbrakes larger won't do any good and in fact you would lose payload. That's the trade off.
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thegeek
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RE: 41,000 Feet -- Will There Be A Higher Standard?

Thu Jan 24, 2008 8:04 am



Quoting PhilSquares (Reply 30):
But it's not an engine efficiency issue. The real problem becomes stall protection.



Quoting PhilSquares (Reply 25):
Normally, you're in the Trop so the temp is fairly constant, so the fuel flow will be the same

Umm, if the air temperature and the engine efficiency are constant, the fuel flow will be proportional to the air density assuming the same drag profile. Perhaps that's where the problem comes in: as the air density drops the lift required remains constant. If you sit with the same attitude in the sky and the same speed you don't produce enough lift. You could, of course, fly faster with the same drag and fuel flow.

The stall protection problem you mention would be an aerodynamic issue that might be possible to engineer out if it were designed in from the start? Of course, it's only in the last few years that fuel burn has been a big issue, so that explains for my money why it hasn't been tried.
 
PhilSquares
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RE: 41,000 Feet -- Will There Be A Higher Standard?

Thu Jan 24, 2008 9:00 am



Quoting Thegeek (Reply 31):
The stall protection problem you mention would be an aerodynamic issue that might be possible to engineer out if it were designed in from the start? Of course, it's only in the last few years that fuel burn has been a big issue, so that explains for my money why it hasn't been tried.

Fuel burn has always been a big issue. Ask anyone who has ever flown LH/ULH. There is a constant battle between payload and fuel.

As I said, the issue is the wing. As you go higher into the trop, the temp remains constant but the air density gets les and less. That's where the problem is.

Quoting Thegeek (Reply 31):
Umm, if the air temperature and the engine efficiency are constant, the fuel flow will be proportional to the air density assuming the same drag profile

Ok, please re-read what I wrote. I will try to give you an example. A 744 weighing 220 Tons at ISA +10, the optimum altitude is 430 (LRC). The fuel burn is 1861 Kgs/hr/eng. If we are cruising at 410 and decide to go up, the criuse FF at 410 will be 1860 Kgs/hr/eng IMN is .849 at 430 and .845 at 410. The fuel used climbing from 410 to 430 would almost 500Kgs total. You use 500Kgs to climb 2000' only to save 1kg/hour/eng?
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astuteman
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RE: 41,000 Feet -- Will There Be A Higher Standard?

Thu Jan 24, 2008 9:19 am



Quoting PhilSquares (Reply 32):
The fuel used climbing from 410 to 430 would almost 500Kgs total. You use 500Kgs to climb 2000' only to save 1kg/hour/eng?

Echoing a discussion in CivAv - do you recover the bulk of that 500kg back through having a longer descent?

Regards
 
PhilSquares
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RE: 41,000 Feet -- Will There Be A Higher Standard?

Thu Jan 24, 2008 9:26 am



Quoting Astuteman (Reply 33):
Echoing a discussion in CivAv - do you recover the bulk of that 500kg back through having a longer descent?

Not really, the descent will begin about 6 miles earlier at 430 v. 410 or about 45 seconds earlier. Fuel on the descent will be about be about 60 kgs/min so you will be short about 400 kgs. That's why it's not always worth going higher and higher even if you are in your optimum FL as you climb.
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thegeek
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RE: 41,000 Feet -- Will There Be A Higher Standard?

Thu Jan 24, 2008 10:37 am

Quoting PhilSquares (Reply 32):
Ok, please re-read what I wrote. I will try to give you an example. A 744 weighing 220 Tons at ISA +10, the optimum altitude is 430 (LRC). The fuel burn is 1861 Kgs/hr/eng. If we are cruising at 410 and decide to go up, the criuse FF at 410 will be 1860 Kgs/hr/eng IMN is .849 at 430 and .845 at 410. The fuel used climbing from 410 to 430 would almost 500Kgs total. You use 500Kgs to climb 2000' only to save 1kg/hour/eng?

I have no doubt that you are correct for the case of the 744. It is not designed to fly at altitudes this high. But take the case of the Concorde. If you flew at FL450 in a Concorde, wouldn't you use more fuel? What I question is what's the reason why it doesn't make sense to design subsonic planes to fly into the thinner air? Ineffectiveness of oxygen masks at higher altitudes could be one issue I can think of.

I'd like to add that I don't see the relevance of air temp on cruise aerodynamics. I thought density was the only really relevant factor here.

[Edited 2008-01-24 03:07:38]
 
PhilSquares
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RE: 41,000 Feet -- Will There Be A Higher Standard?

Thu Jan 24, 2008 11:35 am



Quoting Thegeek (Reply 35):
I'd like to add that I don't see the relevance of air temp on cruise aerodynamics. I thought density was the only really relevant factor here.

Density is affected by temperature and pressure.

Quoting Thegeek (Reply 35):
I have no doubt that you are correct for the case of the 744. It is not designed to fly at altitudes this high. But take the case of the Concorde. If you flew at FL450 in a Concorde, wouldn't you use more fuel? What I question is what's the reason why it doesn't make sense to design subsonic planes to fly into the thinner air? Ineffectiveness of oxygen masks at higher altitudes could be one issue I can think of.

It's the efficiency of the wing vs. the weight of the aircraft, assuming you're not trying to climb to where you're thurst limited. In addition, there is a structural issue of the pressure differential. The 787 will have a lower cabin altitude compared to a conventional constructed aircraft. The 744 at 39-410 will be around 8000' cabin altitude the cabin differential will be around 8.2 or so, the normal high end is 8.6. The 787 will have a higher differential pressure limit.
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tdscanuck
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RE: 41,000 Feet -- Will There Be A Higher Standard?

Thu Jan 24, 2008 5:26 pm



Quoting Thegeek (Reply 24):
In answer to the OP, I don't understand why not either. Even with the temperature not dropping off at higher altitudes, the pressure/density will certainly drop, and this is the most important thing. Perhaps it is just too difficult to design engines which are efficient both at sea level and above FL410.



Quoting Thegeek (Reply 35):
What I question is what's the reason why it doesn't make sense to design subsonic planes to fly into the thinner air?

Beyond the wing problems, fatigue is a factor. Most of the fuselage on current jetliners is fatigue critical. Increasing altitude increases the differential pressure on the fuselage, which lowers the fuselage life. The advent of composites should reduce the impact of this issue, but it's certainly a factor for the current fleet.

Quoting Thegeek (Reply 35):
I'd like to add that I don't see the relevance of air temp on cruise aerodynamics.

In general, Mach number. Sonic velocity is a function of temperature only. As the outside temperature drops, sonic velocity drops so, for a given cruise speed, Mach number increases. Transonic drag is a much stronger function of Mach number than speed. Above ~35,000 the temperature stays pretty constant so the density becomes your driver.

Tom.
 
BAE146QT
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RE: 41,000 Feet -- Will There Be A Higher Standard?

Thu Jan 24, 2008 6:01 pm



Quoting SuseJ772 (Reply 13):
After all, last I checked with physics, getting planes down was rarely a problem

I'm guessing that the problem might be getting the plane down without ripping the control surfaces off. There's a 747SP out there that is a marvellous object lesson in this.

Quoting Thegeek (Reply 24):
Perhaps it is just too difficult to design engines which are efficient both at sea level and above FL410.

General questing - Isn't it more the case that it would violate some laws of physics? You want an engine that is responsive through all flight regimes, develops enormous power at takeoff, can operate in dense and thin air, and is highly efficient in the cruise. Oh - and it needs to be as light as practicable, so you can't introduce variable incidence on the fan blades.

All of those things seem to contradict each other. So as TheGeek says, you get trade-offs.

Quoting PhilSquares (Reply 30):
The real problem becomes stall protection.

From an old SlamClick post, I know that there are times when Vs and MMo are highly convergent, meaning you only have a window of a few knots to fly in. But are there aircraft (commercial or otherwise) that will happily put themselves in a situation where Vs is higher than MMo?

Quoting PhilSquares (Reply 32):
Ok, please re-read what I wrote. I will try to give you an example. A 744 weighing 220 Tons at ISA +10, the optimum altitude is 430 (LRC). The fuel burn is 1861 Kgs/hr/eng. If we are cruising at 410 and decide to go up, the criuse FF at 410 will be 1860 Kgs/hr/eng IMN is .849 at 430 and .845 at 410. The fuel used climbing from 410 to 430 would almost 500Kgs total. You use 500Kgs to climb 2000' only to save 1kg/hour/eng?

As unlikely as it may seem, that has clarified the situation immensely. Thank you.  Smile
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tdscanuck
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RE: 41,000 Feet -- Will There Be A Higher Standard?

Thu Jan 24, 2008 7:37 pm



Quoting BAe146QT (Reply 38):
But are there aircraft (commercial or otherwise) that will happily put themselves in a situation where Vs is higher than MMo?

If that were the case, wouldn't you have just demonstrated that your MMo is too low?

Tom.
 
PhilSquares
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RE: 41,000 Feet -- Will There Be A Higher Standard?

Thu Jan 24, 2008 10:56 pm



Quoting BAe146QT (Reply 38):
But are there aircraft (commercial or otherwise) that will happily put themselves in a situation where Vs is higher than MMo?

If you follow the FMC or the performance charts, you would be protected since Vs would never be above MMo. However, there have been instances where crews have put the aircraft in that situation. Not a good idea!
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saab2000
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RE: 41,000 Feet -- Will There Be A Higher Standard?

Thu Jan 24, 2008 11:15 pm



Quoting BAe146QT (Reply 38):
From an old SlamClick post, I know that there are times when Vs and MMo are highly convergent, meaning you only have a window of a few knots to fly in. But are there aircraft (commercial or otherwise) that will happily put themselves in a situation where Vs is higher than MMo?

This is called Coffin Corner and is not a happy place to visit! The Vs and Mmo meet at that place. Your indicated airspeed is quite low and your Mach number is quite high. Low speed buffet and high speed buffet are only a few knots apart.

The CRJ I fly is certified to FL410, but it has already been demostrated with tragic results why going that high is not a very good idea unless you are 100% sure you have the performance to do so. I have been to FL390. We are now company-limited to FL370 (as are most US operators) and even that high you are getting into an envelope of narrowing performance rapidly in an aircraft as underpowered and underwinged (IMHO) as the CRJ-200. Anyway, it's just not a nice plane at those altitudes. Feels like standing on a basketball......

The folks I know who have been to FL410 in the CRJ tell me about the narrow speed band allowed and it is about 20 knots IIRC.
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BAE146QT
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RE: 41,000 Feet -- Will There Be A Higher Standard?

Thu Jan 24, 2008 11:19 pm



Quoting Tdscanuck (Reply 39):
If that were the case, wouldn't you have just demonstrated that your MMo is too low?

Agreed - either that or you're operating outside the aircraft's published/certified envelope. I should not have used 'MMo' at all, In fact, a simplified version of the question would be, "Is it possible to be at an altitude where your stall speed is so high, that your engines cannot provide enough thrust to keep you there?"

Now that I look back on it, it's a silly question since the answer should have been obvious. Of course you can get to a point where your engines don't push enough - it's a little above your absolute ceiling, though it might take some effort to loft yourself up beyond the point where the plane tells you you're being optimistic. And then Phil said;

Quoting PhilSquares (Reply 40):
However, there have been instances where crews have put the aircraft in that situation.

The example I'm thinking of is the two yahoos who took their private jet up so high that they stalled out, compounded by their ineffective attempts at a relight, followed by messily augering in.

Oh dear. It's obvious that I spend my days working behind a desk, isn't it!  Smile Nevermind. Sometimes you have to "say" these things out loud to make sense of them.
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BAE146QT
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RE: 41,000 Feet -- Will There Be A Higher Standard?

Fri Jan 25, 2008 5:26 pm

Saab2000 - you posted while I was composing mine and I didn't see yours until now. Many thanks for the answer, especially since the 'horse's mouth' view from 37000 feet is always the best one.  yes 
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cpd
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RE: 41,000 Feet -- Will There Be A Higher Standard?

Sat Jan 26, 2008 8:38 am



Quoting Thegeek (Reply 24):
In answer to the OP, I don't understand why not either. Even with the temperature not dropping off at higher altitudes, the pressure/density will certainly drop, and this is the most important thing. Perhaps it is just too difficult to design engines which are efficient both at sea level and above FL410. And the one pilot wearing an oxygen mask when operating under FAA rules would be highly annoying.

The main problem is we just don't design planes to fly at those high altitudes. Concorde had no troubles flying at those altitudes (it would go as high as available performance would allow), but it was designed for it and worked efficiently in the very cold temperatures above 50,000ft. In the regions like the middle-east, and down around Miami and those areas (and Barbados I suppose) the aloft temperatures could go below -75°C, where the climb/cruise engine rating selections would actually have some effect. Otherwise, those selections most times had no change at all.

It also had the advantage of the M1-M1.3 high drag regime that could be used to assist emergency, or otherwise, hurried descents from high altitude. The procedure involved slowing to 325kias, and then asking the AP part of the AFCS to do MACH HOLD (a separate and different system from the AT MACH HOLD), which used vertical speed to maintain the mach number. With the engines throttled back, the only way to maintain that speed is to go down - quickly. It was a very effective procedure for slowing down and descending in a very short distance/time.

Despite that, it did have a limitation of 60,000ft - even though the plane would comfortably exceed that in the right weather conditions. ALT HOLD was always primed for 60,000ft as part of normal procedures. If the plane 'bumped' into FL600, the hold would turn on to prevent it going above that, and the AT MACH HOLD would likely also switch on to prevent an overspeed.
 
APFPilot1985
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RE: 41,000 Feet -- Will There Be A Higher Standard?

Sat Jan 26, 2008 4:05 pm



Quoting BAe146QT (Reply 42):
The example I'm thinking of is the two yahoos who took their private jet up so high that they stalled out, compounded by their ineffective attempts at a relight, followed by messily augering in.

Are you talking about the guys in the CRJ? I don't know of anyone doing it in a private jet (and then crashing). I've been up to FL430 a few times in CJ's and such. Sierra industries has taken their Citation II with FJ44-3's on it up to FL530 during testing.
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saab2000
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RE: 41,000 Feet -- Will There Be A Higher Standard?

Sat Jan 26, 2008 4:17 pm



Quoting APFPilot1985 (Reply 45):
Are you talking about the guys in the CRJ? I don't know of anyone doing it in a private jet (and then crashing). I've been up to FL430 a few times in CJ's and such. Sierra industries has taken their Citation II with FJ44-3's on it up to FL530 during testing.

I was once offered a job by a guy who manages airplanes for others. One of their pilots got a shaker/pusher in the low 40s while climbing above the altitude for their weight at the temp they had. Lost a few thousand feet but did recover. He almost seemed to think it was funny because it happened under Part 91 and their is very little oversight by the FAA.

This type of thing can happen and does to pilots who don't know the real issues of high altitude aerodynamics.

Needless to say I was kind of unimpressed by this joker and some other sucker probably took the job.
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A342
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RE: 41,000 Feet -- Will There Be A Higher Standard?

Sat Jan 26, 2008 5:29 pm



Quoting Cpd (Reply 44):
Despite that, it did have a limitation of 60,000ft - even though the plane would comfortably exceed that in the right weather conditions.

That was because of pressurization issues, right?
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cpd
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RE: 41,000 Feet -- Will There Be A Higher Standard

Sun Jan 27, 2008 2:20 am

Quoting A342 (Reply 47):
That was because of pressurization issues, right?

Something like that. The plane was tested to almost FL690, but FL600 was the selected safe altitude. The higher you go, the more risky it becomes if there is a de-pressurisation of the cabin.

The highest altitudes were always usually achieved in the equatorial areas due to the very cold atmosphere in those regions - and also due to the very long flights over ocean (7000km+) where the plane near the end of the flight would still be supersonic - but also light as a feather in relative terms, so the climb performance (even above FL575) would be very good.

I want to know about these smaller-bizjets claiming cruise altitudes of FL510. How often do they actually really achieve it? Or is it just a absolutely perfect conditions once in a million chance of achieving it? I can't imagine it'd be easy to achieve, because even Concorde wouldn't go straight to FL500 in normal conditions and operating weights, it'd top out at around FL495 and sit there for a while, and then begin slowly increasing altitude at maybe a couple of hundred feet per minute, or maybe even 1000ft/min if the ISA is very nice.

[Edited 2008-01-26 18:34:01]
 
BWilliams
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RE: 41,000 Feet -- Will There Be A Higher Standard?

Sun Jan 27, 2008 5:04 am



Quoting APFPilot1985 (Reply 45):
Are you talking about the guys in the CRJ?

I think that's the one he's talking about... the ferry flight where the guys went up to 410 just for S&Gs and lost both engines.
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