A350 & B787 cruising altitude question
People tend to assume that the stated cabin altitude (8k for most airliners, 5 or 6k for some new ones such as the A350 and 787) is used all the time at cruise - in reality those numbers are maximums and if you do a low altitude cruise, the cabin altitude will generally be lower than that.
Only half a speed-brake
The last piece of the puzzle, easy to fill in with others already in place:
The strength (good) and weight (bad) that invariably go hand in hand are both determined by the designer's decision which altitude will be the max. And then everything is optimized (thinned-down) for that. I would hazard a guess than unlike peak static overpressure on the hull, it is the blow/vent cycles in the long run that are the most demanding on the structure. But in a sense, you indeed cannot go any higher because the margins on lifetime accumulated strain would be reduced below what's allowed (sheldonspeak for "blow up"). Yet other things would immediately come out of the woodwork as well, it's built that way on purpose.
The cynic in me would also believe, that in no shape or form was the lower cruising cabin altitude a hard target in the design, toward which any strengthening of the fuselage took place. Weight increase destroys the economics of the product, even more so on a long haul craft.
The best speculation I could come up with would be that for other physical reasons, thanks to new materials and contemporary manufacturing processes, it turned out that with long-haul expected cycles the structure would take the higher peak pressure. Surely a lot of work may have been done to reach for such perhaps not-so-low hanging fruit, but only meaningless weight increase was sacrificed if any at all.
The strength (good) and weight (bad) that invariably go hand in hand are both determined by the designer's decision which altitude will be the max. And then everything is optimized (thinned-down) for that. I would hazard a guess than unlike peak static overpressure on the hull, it is the blow/vent cycles in the long run that are the most demanding on the structure. But in a sense, you indeed cannot go any higher because the margins on lifetime accumulated strain would be reduced below what's allowed (sheldonspeak for "blow up"). Yet other things would immediately come out of the woodwork as well, it's built that way on purpose.
The cynic in me would also believe, that in no shape or form was the lower cruising cabin altitude a hard target in the design, toward which any strengthening of the fuselage took place. Weight increase destroys the economics of the product, even more so on a long haul craft.
The best speculation I could come up with would be that for other physical reasons, thanks to new materials and contemporary manufacturing processes, it turned out that with long-haul expected cycles the structure would take the higher peak pressure. Surely a lot of work may have been done to reach for such perhaps not-so-low hanging fruit, but only meaningless weight increase was sacrificed if any at all.
Last edited by FlightDetent; 14th Feb 2021 at 20:02.
In the early 777-200 models (249t), not the’IGW’ or 200ER, we would get to 430 on HKG-MNL or HKG-BKK.
Short sectors, lighter weight.
On long or full flights typically the maximum would be 370/390.
With the -300, initial climb would typically only be to 330. And as previously stated, the 300ER could only make 280/290/300 initially on a 15-hour sector!
Short sectors, lighter weight.
On long or full flights typically the maximum would be 370/390.
With the -300, initial climb would typically only be to 330. And as previously stated, the 300ER could only make 280/290/300 initially on a 15-hour sector!
From my own experience as an ATCO, general initial cruising altitude for wide body twins goes 767<777<330<787=350. Cruising speeds 767<330<777<787=350. I certainly see the 777 match the newer twins when the 777 isn't operating ULH but obviously she needs to burn more fuel to do so. Fastest I've had a commercial widebody so far was a B744 at M.090 with a medical EMG on board. I have had a B744F at FL450 once, asked them how they got up there, she was an empty positioning flight. The descended shortly after as their ground speed was higher and fuel burn lower at FL410 with a big tail wind.
I always assumed the reason the 330 gets up there so much earlier than similar vintage Boeings was that the wing needed to take all the extra weight of the 340, so when on a lighter 330 it was almost overkill. Seeing how shockingly slow the A343 in particular climbed backed up that assumption in my head
I always assumed the reason the 330 gets up there so much earlier than similar vintage Boeings was that the wing needed to take all the extra weight of the 340, so when on a lighter 330 it was almost overkill. Seeing how shockingly slow the A343 in particular climbed backed up that assumption in my head
Only half a speed-brake
Slingshot?Only half a speed-brake
Initial cruise altitude on a 15-hour sector at MTOW (350t) would give a Max altitude of around FL300, so depending on direction of flight, and temperature, optimum would be 280/290.
After an hour or so, 310 would be achievable. Step climbing as the aircraft burns fuel.
Maximum fuel on departure would be around 140t, landing with 6-8t.
We used to carry a table for maximum altitude vs weight. Now the only reference is on the FMC.
After an hour or so, 310 would be achievable. Step climbing as the aircraft burns fuel.
Maximum fuel on departure would be around 140t, landing with 6-8t.
We used to carry a table for maximum altitude vs weight. Now the only reference is on the FMC.
Last edited by awair; 14th Feb 2021 at 21:02. Reason: So it makes more sense...
Only half a speed-brake
Thanks, funny facts every day. When I started the cautious of the pool would swear by no less than 4 t on arrival. To FRA, on a -500.
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The highest I have flown on a commercial jet was on a lightly loaded EK A340-500 from Phuket up to DXB on a very clear moonlight night in Nov 2012, and we arrived in DXB about 70min early, and the place was deserted. We cruised at FL43-440 from the start as I recall, with quite a tailwind. I never did the Concorde thing, but have flown in exec jets at that height, and even a little higher, but that is my max for a commercial jet as far as I am aware.
Cheers
Mr Mac
Cheers
Mr Mac
