Go Back  PPRuNe Forums > Misc. Forums > Spectators Balcony (Spotters Corner)
Reload this Page >

A350 & B787 cruising altitude question

Spectators Balcony (Spotters Corner) If you're not a professional pilot but want to discuss issues about the job, this is the best place to loiter. You won't be moved on by 'security' and there'll be plenty of experts to answer any questions.

A350 & B787 cruising altitude question

Old 20th Jan 2021, 18:06
  #21 (permalink)  
 
Join Date: Jul 2013
Location: Everett, WA
Age: 66
Posts: 3,224
Originally Posted by Captain Capstan View Post
As the aircraft climbs the cabin climbs to maintain the differential pressure so the cabin will never burst.
Eventually you could get to an altitude where - at that constant differential pressure - the cabin altitude got too high (10k) and the masks drop - which would obviously be bad.
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.
tdracer is offline  
Old 20th Jan 2021, 19:03
  #22 (permalink)  

Only half a speed-brake
 
Join Date: Apr 2003
Location: Commuting home
Age: 43
Posts: 3,295
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.

Last edited by FlightDetent; 14th Feb 2021 at 20:02.
FlightDetent is offline  
Old 14th Feb 2021, 08:22
  #23 (permalink)  
Ant
even ants need some lovin'
Thread Starter
 
Join Date: Jan 2001
Location: Kent, UK.
Posts: 148
Here's a 767 at unusually high cruising altitude FL430, showing that it's not always the 787s and A350s up that high!



Ant is offline  
Old 14th Feb 2021, 16:04
  #24 (permalink)  
 
Join Date: Jan 2005
Location: NA
Posts: 99
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!

awair is offline  
Old 14th Feb 2021, 16:54
  #25 (permalink)  
 
Join Date: Jun 2013
Location: Róisín Dubh
Posts: 1,262
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

Una Due Tfc is offline  
Old 14th Feb 2021, 20:04
  #26 (permalink)  

Only half a speed-brake
 
Join Date: Apr 2003
Location: Commuting home
Age: 43
Posts: 3,295
Originally Posted by Ant View Post
Here's a 767 at unusually high cruising altitude FL430, showing that it's not always the 787s and A350s up that high!
Slingshot?
FlightDetent is offline  
Old 14th Feb 2021, 20:06
  #27 (permalink)  

Only half a speed-brake
 
Join Date: Apr 2003
Location: Commuting home
Age: 43
Posts: 3,295
Originally Posted by awair View Post
the 300ER could only make 280/290/300 initially on a 15-hour sector!
Just wondering, what exactly do you mean by initially ... on a 15 hr sector?!
FlightDetent is offline  
Old 14th Feb 2021, 20:39
  #28 (permalink)  
 
Join Date: Jan 2005
Location: NA
Posts: 99
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.

Last edited by awair; 14th Feb 2021 at 21:02. Reason: So it makes more sense...
awair is offline  
Old 14th Feb 2021, 21:02
  #29 (permalink)  

Only half a speed-brake
 
Join Date: Apr 2003
Location: Commuting home
Age: 43
Posts: 3,295
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.
FlightDetent is offline  
Old 15th Feb 2021, 16:33
  #30 (permalink)  
 
Join Date: Jun 2007
Location: Beyond the Blue Horizon
Age: 60
Posts: 869
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
Mr Mac is offline  

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Thread Tools
Search this Thread

Contact Us - Archive - Advertising - Cookie Policy - Privacy Statement - Terms of Service - Do Not Sell My Personal Information -

Copyright © 2021 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.