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Aircraft Dimensions Accuracy?  
User currently offlineariis From Poland, joined Sep 2004, 418 posts, RR: 1
Posted (3 years 9 months 3 weeks 3 days 13 hours ago) and read 3588 times:

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


FAO - Flight Activities Officer
12 replies: All unread, jump to last
 
User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 1, posted (3 years 9 months 3 weeks 3 days 12 hours ago) and read 3567 times:

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.


User currently offlineFly2HMO From , joined Dec 1969, posts, RR:
Reply 2, posted (3 years 9 months 3 weeks 2 days 18 hours ago) and read 3407 times:

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


User currently offlinevikkyvik From United States of America, joined Jul 2003, 9393 posts, RR: 27
Reply 3, posted (3 years 9 months 3 weeks 2 days 14 hours ago) and read 3354 times:
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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.



"Two and a Half Men" was filmed in front of a live ostrich.
User currently offline474218 From United States of America, joined Oct 2005, 6340 posts, RR: 9
Reply 4, posted (3 years 9 months 3 weeks 2 days 13 hours ago) and read 3306 times:

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.


User currently offlineokie From United States of America, joined Jul 2003, 2850 posts, RR: 3
Reply 5, posted (3 years 9 months 3 weeks 2 days 12 hours ago) and read 3290 times:

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


User currently offlinedw747400 From United States of America, joined Aug 2001, 1254 posts, RR: 1
Reply 6, posted (3 years 9 months 3 weeks 2 days 11 hours ago) and read 3267 times:

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.



CFI--Certfied Freakin Idiot
User currently offlineokie From United States of America, joined Jul 2003, 2850 posts, RR: 3
Reply 7, posted (3 years 9 months 3 weeks 2 days 9 hours ago) and read 3231 times:

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


User currently offlinesimairlinenet From United States of America, joined Oct 2005, 904 posts, RR: 2
Reply 8, posted (3 years 9 months 3 weeks 2 days 8 hours ago) and read 3208 times:

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.


User currently offlinevikkyvik From United States of America, joined Jul 2003, 9393 posts, RR: 27
Reply 9, posted (3 years 9 months 3 weeks 2 days 7 hours ago) and read 3189 times:
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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?



"Two and a Half Men" was filmed in front of a live ostrich.
User currently offlineRoseFlyer From United States of America, joined Feb 2004, 9375 posts, RR: 52
Reply 10, posted (3 years 9 months 3 weeks 1 day 16 hours ago) and read 3079 times:

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!
User currently offlinekanban From United States of America, joined Jan 2008, 3210 posts, RR: 26
Reply 11, posted (3 years 9 months 3 weeks 1 day 16 hours ago) and read 3068 times:
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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. '


User currently offlineokie From United States of America, joined Jul 2003, 2850 posts, RR: 3
Reply 12, posted (3 years 9 months 3 weeks 1 day 10 hours ago) and read 3002 times:

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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