a380900 From France, joined Dec 2003, 1009 posts, RR: 1 Posted (2 years 1 month 4 weeks 1 day 13 hours ago) and read 2549 times:
I read on a thread that initials A333 were at 212t or 215t mtwow and that they are now at 235t. That's almost a 10% increase.
How can a plane gain so much capacity? Are these the consequences of an engine change? Was the engineering right in the first place?
How do manufacturers know when to stop when they start adding weight? How can they be sure? What do they look at? Is there a statistical indicator (number of incidents on the airframes caused by a too heavy landing for instance)?
It looks like there is some wiggle room. With competitive pressures all around, how do they manage to stay on the safe side. Also does it ever go the other way around (airframe which have their capacity restricted, I think it happened for the Galaxy C5, do you know of any examples in civil aviation?).
PolymerPlane From United States of America, joined May 2006, 991 posts, RR: 3 Reply 1, posted (2 years 1 month 4 weeks 1 day 12 hours ago) and read 2521 times:
I would guess from structural strengthening, mainly on the wing, especially if the MZFW doesn't change. It is probably on the same way A330-200 got its MTOW increase.
Engine is one part of the equation, but as long as the take-off performance is enough, it is not as crucial
Quoting a380900 (Thread starter): How do manufacturers know when to stop when they start adding weight?
I would guess by computer/engineering model. When the A380 failed the ultimate load test, they were able to use analysis for the wing strengthening. The same process I would guess is valid for MTOW increase. IIRC the 777LR series did not have to conduct ultimate load test.
The static and fatigue test is as much as proving the ultimate capability of the plane as getting data to show that their engineering model accurately predicts the structural behavior.
Quoting a380900 (Thread starter): Is there a statistical indicator (number of incidents on the airframes caused by a too heavy landing for instance)?
Landing is dictated by the maximum landing weight instead of MTOW
Typically for a large increase in MTOW you would need upgrated engines.
Quoting a380900 (Thread starter): How do manufacturers know when to stop when they start adding weight? How can they be sure? What do they look at? Is there a statistical indicator (number of incidents on the airframes caused by a too heavy landing for instance)?
They do the math to see if the structure can take it. They do the math to see if the wing can lift the weight.
Quoting a380900 (Thread starter): Also does it ever go the other way around (airframe which have their capacity restricted, I think it happened for the Galaxy C5, do you know of any examples in civil aviation?).
All the time. Some customers need less MTOW so will get variants with less powerful engines.
There are quite a few engine variants on the 333 as you can see.
Starlionblue From Hong Kong, joined Feb 2004, 15870 posts, RR: 66 Reply 4, posted (2 years 1 month 4 weeks 1 day 8 hours ago) and read 2434 times:
Quoting a380900 (Reply 3): Quoting Starlionblue (Reply 2):
They do the math to see if the structure can take it. They do the math to see if the wing can lift the weight.
Yes but my question would be how can "the first math" turn out to be off by 10%? It feels like a magic trick...
The "first math" is not off. It's just that most initial designs have growth "built in". That is they assume there will be demand for higher MTOW (and stretches in some cases) in the future and make provisions for this.
Just because an airliner can have a higher MTOW doesn't mean that all customers necessarily want to use that. More MTOW means more weight carried around, more powerful (=expensive) engines and so forth. So airframers have to factor in a range of MTOW from the start.
In some cases, structure such as landing gear needs beefing up for the higher MTOW, but that is also factored in.
Quoting a380900 (Reply 3): Isn't there an objective measure once the airframe is flying that leads engineers to determine that "it can take on more weight"?
Yes. As mentioned that is factored in already when they do the initial design.
[Edited 2011-03-25 19:40:47]
"There are no stupid questions, but there are a lot of inquisitive idiots." - from Citadel by John Ringo
zeke From Hong Kong, joined Dec 2006, 7725 posts, RR: 73 Reply 5, posted (2 years 1 month 4 weeks 1 day 7 hours ago) and read 2418 times:
Quoting a380900 (Thread starter): I read on a thread that initials A333 were at 212t or 215t mtwow and that they are now at 235t. That's almost a 10% increase.
Actually the lowest MTOW available on the A333 was 184t. The big difference between the light weight and heavy weight aircraft is the main landing gear, there is a placard on the heavy weight gear legs indicating that they higher MTOW capable.
Not particularly, depends on where it is operated. Somewhere cool at sea level without big hills around after takeoff you should not need any additional thrust from the engine. I often see takeoff performance numbers coming back saying the aircraft would be capable of a 250+t MTOW takeoff from a performance point of view. The takeoff limit is normally a structural one, be it MTOW or MLW.
Quoting a380900 (Thread starter):
How do manufacturers know when to stop when they start adding weight?
With the A330 they also have the A340, it is essentially a very common wing, the MTOW on the A340 is around 275t. The difference between the A330 and A340 are the way the flap and slats are used (they go to slightly differnt position when selected) and the bending moment relief provided by the outer engines on the A340.
The design regulations have specific airspeed vs load requirements that need to be met, that is the main limit. Takeoff performance on a twin is not the same as a quad, it is slightly less restrictive on a twin.
Quoting a380900 (Reply 3):
Isn't there an objective measure once the airframe is flying that leads engineers to determine that "it can take on more weight"?
Yes, when initially designing a new aircraft they make assumptions on the average sector length, number of cycles, and the sort of loads the airframe will "see" during its life. These assumptions form part of the basis for setting the basic structural airspeed vs load diagram, with in service experience the assumptions can be refined to be in line with what actually was seen by the structure.
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