ariis
Topic Author
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Joined: Wed Sep 22, 2004 8:04 am

Aircraft Dimensions Accuracy?

Sun Jun 27, 2010 10:01 pm

Hi there,

Recently I got intrigued by someone's simple question: what is the amount of deviation in large commercial aircraft overall dimensions (length, wingspan) across theoretically identical brand new airplanes?

For example, B744 is said to be 70.6 meters in length, but how different particular airplanes could be? Are we talking millimeters-level assembly accuracy, or whole centimeters or even more?

If somebody has the idea or any applicable data, I would appreciate reading about it. Out of curiosity, are Airbus models due to more geographically/politically/culturally diverse production facilities more prone for such (non-critical) errors? No A vs B of course.

Thanks in advance

FAO
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tdscanuck
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RE: Aircraft Dimensions Accuracy?

Sun Jun 27, 2010 10:45 pm

Quoting ariis (Thread starter):
what is the amount of deviation in large commercial aircraft overall dimensions (length, wingspan) across theoretically identical brand new airplanes?

I would guess on the order of a few inches. It's going to be the tolerance stack-up of all the primary structure from nose to tail and that's a lot of parts, and most of it is drill-on-assembly.

Tom.
 
Fly2HMO
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RE: Aircraft Dimensions Accuracy?

Mon Jun 28, 2010 4:50 pm

Quoting tdscanuck (Reply 1):

I would guess on the order of a few inches.

I dunno. This is me just guesstimating, but me thinks it would be in the order of 1" at most in the more recently developed aircraft. I know they use lasers for precise alignment when building the 777. And with the advancement in CNC technology and what not I think the total error must be relatively small once the aircraft is finished
 
vikkyvik
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RE: Aircraft Dimensions Accuracy?

Mon Jun 28, 2010 8:56 pm

Quoting Fly2HMO (Reply 2):
I dunno. This is me just guesstimating, but me thinks it would be in the order of 1" at most in the more recently developed aircraft. I know they use lasers for precise alignment when building the 777. And with the advancement in CNC technology and what not I think the total error must be relatively small once the aircraft is finished

I agree with that as well. It's not terribly difficult to hold tolerances in aluminum or whatever to a thousandth or an inch, and aerospace parts are frequently machined to some number of ten-thousandths of an inch tolerance.

If you assume there are 200 parts that are stacked for the length of the aircraft, each with, say, 5 thousandths of an inch tolerance, you'd be out at most 1" in either direction.

With that said, I'm not intimately familiar with parts manufacturing for airplanes, so I'm just speculating.
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474218
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RE: Aircraft Dimensions Accuracy?

Mon Jun 28, 2010 10:20 pm

Quoting tdscanuck (Reply 1):
I would guess on the order of a few inches. It's going to be the tolerance stack-up of all the primary structure from nose to tail and that's a lot of parts, and most of it is drill-on-assembly.


I would have to assume that the overall dimensions are very close and repeatable. The fuselage barrel sections are jig built so the only place where stack-up tolerances would come in to effect would be in the barrel mate joints. The standard tolerance for the mate joint would be in the +/- 0.030 of an inch range. If there are five barrel sections, four joints, the max allowable difference in over all length would be +/- 0.120".

The same would hold true for the wings, with but with on two joints the overall tolerance would be even less.
 
Okie
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RE: Aircraft Dimensions Accuracy?

Mon Jun 28, 2010 10:40 pm

Quoting 474218 (Reply 4):
I would have to assume that the overall dimensions are very close and repeatable. The fuselage barrel sections are jig built so the only place where stack-up tolerances would come in to effect would be in the barrel mate joints. The standard tolerance for the mate joint would be in the +/- 0.030 of an inch range. If there are five barrel sections, four joints, the max allowable difference in over all length would be +/- 0.120".



You will end up with larger variances just with expansion and contraction with temperature change. When you are looking at say 120F heat soaked aircraft in the desert vs -40F/C cold soaked aircraft on a long flight.

I would agree with assembly techniques in this day and time in a controlled environment, +/- 0.120 would be relatively repeatable.

Okie
 
dw747400
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RE: Aircraft Dimensions Accuracy?

Mon Jun 28, 2010 11:53 pm

Quoting okie (Reply 5):
I would agree with assembly techniques in this day and time in a controlled environment, +/- 0.120 would be relatively repeatable.

How about early jet airliners, like a 707, 737-200, or 747-100? The workmanship on these old birds is impressive, but the technology and practices used to assemble them in the 50s and 60s would certainly reduce precision.
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Okie
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RE: Aircraft Dimensions Accuracy?

Tue Jun 29, 2010 2:30 am

Quoting dw747400 (Reply 6):
How about early jet airliners, like a 707, 737-200, or 747-100? The workmanship on these old birds is impressive, but the technology and practices used to assemble them in the 50s and 60s would certainly reduce precision.



I was looking over an old "Buff" the other day. It had the boards out exposing the under-wing. Just looking at the layers of chromate, and repairs over the years along with the standard waffled fuse, crusty cables to operate gear door latches and a little rust and corrosion here and there, just make you wonder how it would fly.

Okie
 
simairlinenet
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RE: Aircraft Dimensions Accuracy?

Tue Jun 29, 2010 3:21 am

Quoting vikkyvik (Reply 3):
If you assume there are 200 parts that are stacked for the length of the aircraft, each with, say, 5 thousandths of an inch tolerance, you'd be out at most 1" in either direction.
Quoting 474218 (Reply 4):
I would have to assume that the overall dimensions are very close and repeatable. The fuselage barrel sections are jig built so the only place where stack-up tolerances would come in to effect would be in the barrel mate joints. The standard tolerance for the mate joint would be in the +/- 0.030 of an inch range. If there are five barrel sections, four joints, the max allowable difference in over all length would be +/- 0.120".

Rather than looking at a maximum difference, we probably ought to be thinking about the probabilistic range using independent standard deviations. Granted, the independence can certainly be questioned depending on the origins of the parts, but this would greatly reduce the stated maximum variances mentioned above.
 
vikkyvik
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RE: Aircraft Dimensions Accuracy?

Tue Jun 29, 2010 3:58 am

Quoting simairlinenet (Reply 8):
Rather than looking at a maximum difference, we probably ought to be thinking about the probabilistic range using independent standard deviations. Granted, the independence can certainly be questioned depending on the origins of the parts, but this would greatly reduce the stated maximum variances mentioned above.

I'm not sure I entirely understand what you're saying. Are you basically saying that you calculate the probability of each part being at a certain spot within its tolerance, and then calculate the probability of the whole assembly being a at a certain spot within its tolerance, based on those individual probabilities?

If I read it correctly, then while this will certainly give you an idea of how close most of the assemblies will be to nominal, it won't tell you how far from nominal they may vary, correct?
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roseflyer
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RE: Aircraft Dimensions Accuracy?

Tue Jun 29, 2010 6:38 pm

Quoting vikkyvik (Reply 3):

I agree with that as well. It's not terribly difficult to hold tolerances in aluminum or whatever to a thousandth or an inch, and aerospace parts are frequently machined to some number of ten-thousandths of an inch tolerance.

I would not say that aerospace parts are frequently machined to ten-thousandths or an inch. You find that level of detail in bearings, motors, pumps, engines and such, but never in structure. That level of detail is very costly. Structure does not move, so it would never need that level of tolerances.

Quoting dw747400 (Reply 6):

How about early jet airliners, like a 707, 737-200, or 747-100? The workmanship on these old birds is impressive, but the technology and practices used to assemble them in the 50s and 60s would certainly reduce precision.

I haven't dealt much with the older generation of aircraft, but from what I know, dimensions and tolerances have not become much tighter with new models. Tight tolerances cost money, so even though they can be made, staying loose if it is permissible is a good thing. If there are any stress engineers here, they'd know the answer.

Quoting vikkyvik (Reply 9):
Quoting simairlinenet (Reply 8):
Rather than looking at a maximum difference, we probably ought to be thinking about the probabilistic range using independent standard deviations. Granted, the independence can certainly be questioned depending on the origins of the parts, but this would greatly reduce the stated maximum variances mentioned above.

I'm not sure I entirely understand what you're saying. Are you basically saying that you calculate the probability of each part being at a certain spot within its tolerance, and then calculate the probability of the whole assembly being a at a certain spot within its tolerance, based on those individual probabilities?

The typical method is RSS. If there are enough tolerances, then the variance is looked at since the probability of all parts being on the edge of their tolerance is low. A straight addition of tolerances is unrealistic.
If you have never designed an airplane part before, let the real designers do the work!
 
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kanban
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RE: Aircraft Dimensions Accuracy?

Tue Jun 29, 2010 7:09 pm

early on 707/727s might have a difference of 1 to 1.5 inches in length... wings were less problematic from root to tip.. the problem was increased by tooling wear... some 707/727 tools were plywood without drill bushings. newer planes built from better tooling there is less of a deviation...

there is still enough tolerance shift to necessitate hand locating some exterior fairings. the old rule was the 'accumulation of tolerances occurs at the worst possible point. '
 
Okie
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RE: Aircraft Dimensions Accuracy?

Wed Jun 30, 2010 12:42 am

Quoting kanban (Reply 11):
early on 707/727s might have a difference of 1 to 1.5 inches in length... wings were less problematic from root to tip.. the problem was increased by tooling wear... some 707/727 tools were plywood without drill bushings. newer planes built from better tooling there is less of a deviation...



While length differences adding up would not really seem to be a major issue, I would assume that the tolerances for the contour of the wing would be the most significant since that is what provides the lift.

Okie

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