View Full Version : Why don't jets fly higher?

29th Jun 2000, 11:31
We all agree that it's cheaper and faster the higher you go so why is it only Concorde that gets to 50000+ feet? Why are all your latest 7XX/3XXs still fighting it out for a slot at 30000-39000 feet?

29th Jun 2000, 12:52
Probabley because a B747 operater at M.86 doesnt want to see only 208 knots on his clock at FL500, and a B737 driver doesnt want to see only 178 knots at M.75 at the same level either.
I must admit its rediculous all these new types of long-haul aircraft are still dribbling around at subsonic speeds. But you can blame the politicians, greenies, and traveling public for that.

29th Jun 2000, 16:09
ATC viewpoint...

If everybody flew concord(e)s, instead of grizzling for F350 they would all want F650...

***BIGHT ME***

Danish Pilot
30th Jun 2000, 10:25
I belive the problem is what is known as "Coffin“s coner" (sorry for the spelling)
With increase in alt the stallspeed increase, and the highspeed (Mcrit) decrease. At a point they will meet so you either fly to fast or to slow. I think the first Learjets had a problem with that, being able to fly very high. (I“m no Lear driver)

So one would have to fly supersonic, wich again means new engines, new intake design, new wings, new......

Another problem would be antennas. I belive to have heard that the Concorde is the only aircraft (comercial airliner) that does not carry TCAS, because the antennas would melt at high speed, and I would to expensive to develop new antennas for 8?? aircraft.

30th Jun 2000, 13:32
Cde carries TCAS.

9 Freighter
30th Jun 2000, 16:47
I guessing part of the reason is the pressurization of the aircraft. At FL350 I get a cabin altitude of around 8000 ft. The press controller is nearly maxed out at 7.76 psid and the emergency pressure relief valves will open at 8.06 psid, so I guess I couldn't go much higher without sucking an O2 hose. I assume designing an airplane that can handle the pressure without popping like a balloon presents it's own challenges and the fix may not be cost effective.

30th Jun 2000, 16:47
Concorde had TCAS antennae fitted later than the regulators would have liked because they did experience problems with the design.

30th Jun 2000, 19:34
The weight and dimensions of the aircraft are also factors. The amount of thrust and lift required to allow something like to A3XX to cruise higher would be enormous. There is also the increase in drag created by such large aircraft that is not as much of a problem a higher speeds with something as sleek and slender as concorde. The local mach numbers would increase and cause buffeting. Fuel consumption would be massive and increase the operating costs dramatically. The list goes on......

3rd Jul 2000, 08:29
Sparky. Boeing and Scarebus could build an SST (M1.5-2.2) of A3XX/B747 proportions (and with similar operating economics) tomorrow afternoon if they really really truly wanted to. Refer to my first post as to the reason why they wont.

3rd Jul 2000, 08:56
I took a 737 test ride to check some of these ideas. OK it was MS Flight Simulator but it's probably a satisfactory model of the effects.

I could get it to 43000 feet with 100 fpm still possible. But it was very difficult. N1 was higher than perhaps is operationally realistic and EGT was over the red line a number of times.

At this height (in ISA presumably) we were in 'Coffin Corner' with the Barber's Pole at something like 225 kts - not much margin there for turbulence etc.

More significant was the lack of power above about 35000 feet.

I still can't quite get my head around why power falls away. The plane itself just goes faster as the air gets thinner until it thinks it's the same as at low level. Why does the engine run out of puff in the same circumstances?

3rd Jul 2000, 11:39
TES, the engine runs out of puff for the same reason humans do at altitude. An engine designed to process fuel/air efficiently at 35000 will be unlikely to have the compressor/turbine aerodynamic properties, nor the fuel flow to operate in the more rarified 50000 ft alt. You can imagine the inefficiency of the Olympus on the Concorde if it were cruising at 39000 ft (the norm for most modern airliners today). It would use fuel at an alarming rate compared to a CF6/4060/211 I should imagine.

However, as alluded by 9freighter, the real reason is the pressurization loads required to keep the cabin at 8000ft would need a vastly heavy structure to cope. This is the very reason the fuselage dia on the Concorde is about the same as an F27. If you increase the internal volume of the pressure vessel at the same pressure, you have the X psi's acting on more square inches of area, requiring a much stronger structure/thicker skin to withstand the load.

For the sake of simplicity here, I am ignoring the supersonic wave form effects and surface heat a large physical size fuselage would need to have catered for it.

When all else fails, read the manual!

3rd Jul 2000, 22:28
Big aeroplane supersonic = big sonic boom = overwater (or desert) flights only.

Big aeroplane which cruises at >M1.5 not efficient <M1.0

Same aeroplane = megabucks.

Market too small to make business case.

(If you made transpacific the market it may work, except that you've now emptied the premium cabins of the transpacific a/c which is where the airlines make the money)

To date, any engine which works at high speed WILL be noisy at low speed (departure/arrival). Chapter 3 marginal, no hope of meeting regs in force at time of entry into service.

If there is ever a Conc replacement it will almost certainly be a biz-jet. 'til that time they're likely to remain king of the castle. :)

4th Jul 2000, 11:15

trust will drop more rapidly with altitude when flying above the tropopause since temperature stays constant.

4th Jul 2000, 11:42

Up to the tropopause the temperature of the air decreases, offsetting some of the loss of the lower pressures. Above the tropopause (about 36,000') the temperature stops dcreasing (and in fact begins to increase in the mesosphere above ~65,000') so the specific fuel consumption of the engine begins to increase above the troposphere, and thrust decreases.

Concorde is designed to operate at supersonic speeds. The engines need to have subsonic air at the compressor inlet, and this requires a different intake to that on fan engines fitted to subsonic airliners. The engines are not as efficient, but the speed increase is worth the loss.

gas path
4th Jul 2000, 14:15
Reading Danish pilots post reminded me, I've been to 42700 in a 767 and you couldn't get the speed bugs close enough together

Danish Pilot
4th Jul 2000, 15:29
Exactly. What were you doing at that alt anyway?? :)

5th Jul 2000, 23:37
Just a little addition to checkboard's response - all the stuff about Conc engines true except they are at their MOST efficient at M2.0. There are huge benefits in pressure recovery attained by decelerating the flow from M2.0 to M0.4 (in 10 feet!). Like a supercharger upstream of the LP compressor.

MPG at M2.0 compares very favourably with any other heavy jet in CRZ.

7th Jul 2000, 20:08
Just looking at the logistics of supersonic flight makes one marvel at the fact that Concorde was ever built at all, using the technology of 30-odd years ago, when we were driving Ford Anglias and were still watching black and white telly!

All these years on, it still works perfectly, looks divine and has a faultless safety record.

Cost a few quid, though...

8th Jul 2000, 05:24
Concorde was indeed a technological success. However at $4000US or whatever a ticket costs, it's a plaything for the rich business man/woman, rock/movie stars, etc. Economically it's a flop which is one reason the American's put a bullet in their SST program. If Concorde could make money, there'd be alot more of them flying around the world. As it is, it's only a drag on BA an AF's other transatlantic business. More than a fair chance that the A3XX will be the Concorde of the future - great airplane that can't recoup it's development costs. Sure am glad I'm not subsidizing it. Maybe the Brits and French think the prestigue points are worth it.

8th Jul 2000, 14:19
Wondering hit the nail on the head.

Higher alt = thinner air, but
Higher alt = colder air too.

As altitude increases and air density decreases, thrust reduces.
As altitude increases, temperature reduces, offsetting some of the effect this density decrease has on thrust produced.

Rate of climb is dependant on excess thrust available (that over and above the amount required for straight and level flight). Since ~FL370 is theoretically where the tropopause is, jets find that lavel their theoretical optimum, since climing higher results in thinner air with no offsetting temp decrease. As a result, excess thrust diminishes above the tropopause. Unless the aircraft has buckets of excess thrust, (ie concorde)and is more efficient AND still aerodynamically stable in the thinner air above the tropopause where aerodynamic damping is also reduced, the amount of excess grunt required to get higher, and the amount of stability augmentation required to keep you upright negate the cost benefits of reduced drag and fuel flows.

That's what I reckon.

8th Jul 2000, 15:13
If BOAC had been allowed to charge normal fares in the first place, rather than being shafted by the rest, there would have been dozens more Concordes sold.
When a chap from BAC gave us a lecture at school in 1965(?), the brochures he brought to give to us spotty schoolboys had the aircraft displayed in ALL the logos of airlines who'd indicated a willingness to buy - and there were at least 20. The US SST saga (bigger, faster and too expensive) followed, along with 'SSTs cause cancer' or other such utter rubbish whipped up by american (small a, big @rse) tree-huggers and encouraged by sour grapes types in the US aerospace industry and media.

[This message has been edited by BEagle (edited 08 July 2000).]

8th Jul 2000, 17:47
Whilst all of what's been said is true, there is some more to it.

The FAA (if I remember correctly) report that subsonic "corporate" jets now cruise at up to 51000 feet.

I guess the main factor is engines. The power required to maintain higher altitudes is not cost effective for scheduled airlines.

8th Jul 2000, 21:05
Just to nail one common misconception - the Concorde fleet does indeed make a profit for BA. I believe that for two of the last eight quarters it was the only fleet to do so.

(Yes, I know this disregards acquisition costs, but of course they would anyhow have long been amortised on any fleet which had been in service for this amount of time).

Mister Proach
9th Jul 2000, 03:37
ROADTRIP. You were way off the mark. Concorde is actually BAs most profittable fleet, A/C for A/C. It makes a lot of cash and breaks even across the pond with less than half its seats full.

The yanks killed supersonic flight because they couldn't do it. \the environmental impact od spaceflight is far greater but thats OK because they can do that!

Dan Winterland
11th Jul 2000, 22:27
Back to the original topic, what you need is more thrust. Plenty of spare power will get you nice and high. Handley Page's finest bomber, the Victor could get to FL600 with no problems, I flew one of the Tanker versions with it's shorter span up to FL520, and we could cruise at M.9 at FL470. There was nothing to match us in those days, but you can find some GIVs and Citation Xs up there now.

12th Jul 2000, 09:16
Can't find the specs on the Boeing site but I think the B747-SP (early 80's?) wud cruise at FL50. PanAm had a few and maybe South African.

12th Jul 2000, 09:17
So - back to the question as you rightly point out - if GIVs and Citations can do it why have Airbus and Boeing elected not to? If they decide they want more power they just ask RR,GE etc, don't they?
Or put the question the other way round - if Airbus and Boeing don't think it's sensible what is the attraction to the corporate jet manufacturers?

12th Jul 2000, 09:40
Finding a couple of engines with XS power for a "light" corporate jet is no problem, and giving them F500 capability will get them direct routing.

For a commercial airliner, if you can find extra power, then you make more money by increasing the take-off weight of the aircraft to maximise revenue & range.

12th Jul 2000, 12:18
Hey folks... very interesting discussion.

Just thought I'd fire up PS1 (exact replication of 744) on the PC and see what alt. I'll get out of it.

I'll post the results shortly...

14th Jul 2000, 16:13
Hey Turbo!

You've been in the climb for 48 hours now. I think you're going to have to accept that wherever you've got to is your service ceiling.

Let us know.

a is dum
14th Jul 2000, 17:22
A few point on the original question.
The pressuredifferential between inside and outside the cabin is the main item. the aircon/pressurisation system will be able to maintain a reasonable (FAR/JAR certifiable that is) pressure diff. and the hull and windows can be designed to cope with it. Once an aircraft model has been designed for a certain pressure diff. it is not easy to increase it so you are faced with what the pressurisation system can do after modifications and also what the structure (incl. windows and doors) can stand.That's for the cabin area. As a large part of your fuel/hydraulics etc. systems run outside the pressure hull. this must also be able to cope with the increased pressure diff. (now greater from within the pipes/boxes to the outside of them).
Then also the engines as said before. A lot more puff needed in thinner air.
The whole lot then needs to be re-certified. Lots of work for the manufactutrer and the authorities.
In the end... Is it worth it is some case? Need a good market to get your money back. the Airlines will often not accept to pay much for it.

A is dum (Is A?)

19th Jul 2000, 02:25
If Concorde is so economically successful, why aren't there more updated versions being built and being flown on more diverse routes? Granted, Concorde is held hostage in the never-ending Euro-US pissing contest. As far as making a profit goes - I'll bet the books wouldn't stand up to close scrutiny -- Maintaining a fleet of 8 very specialized aeroplanes has got to be even more costly than even gold-plated tickets allow. I would imagine that Concorde is kept in impeccable maintenance condition.

Old Dog
20th Jul 2000, 23:21
By-pass ratio of a jet engine determine its thrust-altitude lapse rate. To fly very high altitudes, you need low by-pass turbo-jets. To be economical, you would want high by-pass fan jets.

22nd Jul 2000, 06:35
There will never be another SST, not in our lifetime. This has probably been mentioned here before, maybe even by me, but when it takes 90 minutes to get from the west end of London to checkin, saving an hour or two on the flight times is not worth the extra cost. The average speed of traffic in London is slower than horse-and-cart Victorian times - it's 6 mph. Plus there are huge parts of the planet (the bits with people on) which are a no-go area for anything that generates a sonic boom. Planes piddle along at subsonic speed not because of restrictive gov'ts or whatever, but because the market for a profitable line of aircraft doesn't exist. There are few routes suitable and few passengers willing to pay the $$$ to save an hour or two. I wouldn't pay - is 5hrs45 en route NYC-LON so horrible that it's worth paying not £250 in economy on a 747 but maybe £1000 on a NG SST to fly the same distance in 3 hours? No, especially when you're not halving your journey time, but maybe cutting it by 15% (counting 90 minutes each end for ground transportation, 60 minutes check-in and 60 minutes baggage reclaim and customs / immigration) - journey times are 747: 10hrs45, SST 8hrs.

A SST biz jet would make slighly more sense.

23rd Jul 2000, 02:53
I used to regularly cross the Atlantic on 747s, now I regularly cross in an SST. Believe me, the difference is enormous in contradiction of the above.

The reason that there will not be an airliner successor to the Conc is twofold: As already stated, the market will not support enough machines to make a viable manufacturing case. Secondly - the problem of noise in T/O and landing phases. Chapter 111 is barely achievable, 1V would be impossible.

I agree that the next civil SST will be a bizjet for both these reasons.

25th Oct 2000, 13:56
whenever i come back from sydney,
in a qantas 747-400, we always fly
at 41000ft.

25th Oct 2000, 15:41
Do we wanna be that high, with radiation and all ?

25th Oct 2000, 15:52
Did Turbofan end up in orbit or crash and burn?
Will we ever find out?
Rest in Pieces Turbofan!

25th Oct 2000, 17:47
Hey Roadtrip you realy do have a fly up your whatsit!

As previously mentioned, Concorde was embraced by an awful lot of US airlines in it's early days and made great economical sense. Just when two small countries bit the bullet and past the point of no financial return (based on all those airlines support), along comes Boeing. When the US realised their Mach 3 rival was going to bust the bank.... they said, 'Oh well, it aint possible, it's therefore impossible for anyone else.

Despite all the doubts, the tree hugging (a lot misplaced) and the finer politics of New York Port Authority, etc, the bird flew commercially for two and a half decades, and many American people enjoyed the experience too.

To date no other country large or small has operated a commercially sound aircraft across the oceans at the speed of Concorde other than France and UK ....

...and I guess thats the bit that hurts.

I suggest a book to read called Concorde and the Americans.

Me, I'm happy to cruise at 35,000 at any speeed, just so long as I don't have to try and enjoy sour grapes with my in flight meal.

[This message has been edited by buck-rogers (edited 25 October 2000).]

25th Oct 2000, 21:29
I have flown the DC-8-62 at FL410, there was no problem with "coffin corner".

Handflew Learjets 24/25 routinely at FL450, I believe the margin is somewhere around 40kts. Flies nice at those altitudes. In testing I believe they did steep turns at those altitudes.

NASA used to fly their 23 to FL500, they also took a DC8 to 450 and did a highspeed dive attaining M 1.07. First supersonic tranport??

25th Oct 2000, 22:50
Main reason is the difference in surface area between Concorde/corp jets.Multiply the number of square inches of pressure hull by a one pound incease in max diff it subjects the fuselage to hundreds of Tonnes more pressure on a big jet.

26th Oct 2000, 01:07
Seem to remember that the Gulfstream 5 can do steep turns at FL530 or something. Why you'd ever need to beats me. Nothing to avoid up there!
Fantastic looking aircraft.
Now for a quintuple lottery rollover...

Bally Heck
29th Oct 2000, 16:12

I'm a freelance rocket scientist.

An airliners maximum cruising altitude is determined by the lowest of the following three characteristics.

1) The max certified altitude which is usually determined by the pressure load limits on the fuselage.

2) The thrust limited altitude. This being the altitude at which sufficient thrust is available to provide a specific rate of climb...normally 100fpm

3) The manoeuvre limited altitude. This is the altitude at which a specific manoeuvre margin exists prior to the onset of buffet. This varied depending on the certification authority (CAA/FAA/JAA) but is normally between 1.2g and 1.4g giving between 33 and 44 degrees bank angle prior to buffet/stick shake.

Aircraft which can fly higher than the normal 40,000 foot approx ceiling are usually either very fast, (M0.85+) or capable of flying very slowly (U2)

I shall be doing a dissertation on particle physics and the origins of the universe next.

PPRuNe Radar
30th Oct 2000, 03:38

Can't find the specs on the Boeing site but I think the B747-SP (early 80's?) wud cruise at FL50.

Only problem with that level was them catching up Islanders and the like though ;)

PPRuNe Radar
ATC Forum Moderator
[email protected]

30th Oct 2000, 10:23
Regular Public Transport aircraft are mostly altitude limited by the ability to cope with an explosive decompression.

4th Nov 2000, 15:36

Wing design is also important - look at the wingspan of the A330 & 777. The A300-600, with its rather stubby wing, has the engine performance to achieve FL410, but generally is limited to FL290 for the first few hours of flight. The wing area is insufficient to produce required lift at higher levels (at med/high AUW) without running into coffin corner.

Alex Whittingham
4th Nov 2000, 23:37
Fl 600 in a Victor? I only ever made FL550 and that was at very light weight (and in the middle of the South Atlantic with nobody watching)

6th Nov 2000, 01:58
Hey, Alex, don't you know you're supposed to be annonymous?? ;)

The Falcon 50 and 900 fly superbly at F470 - 45 degree banked turns no problem. I guess that's because of the superb wing design. Pity the engines lack the puff to get it up there till it was pretty light!

I've always understood that the airliners don't go much above 410 because it takes too long to get back down to more reasonable altitudes in the case of decompression - isn't there a CAA/JAA/FAA ruling that public transport aeroplanes have to be below F300 within 3 minutes, or something like that? (Don't know how Conc. gets away with it, though.)

6th Nov 2000, 09:37
Just to add my two cents, the American SST was killed not so much because of environmentalists (who also made a lot of noise) or size/design factors. Essentially, the American taxpayer didn't wish to foot the bill for development, especially when all the profits went to the a/c maker and the airlines. Government funding of private enterprises is just not the American way (usually).

R. J. Emery