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Do Misshaped Parts Compromise Fuselage Integrity?  
User currently offlineAzstagecoach From United States of America, joined Mar 2006, 152 posts, RR: 0
Posted (8 years 8 months 2 weeks 3 hours ago) and read 1978 times:

This question is for anyone familiar with airframe assmbly. The Washington Post is reporting today that parts of the 737 fuselage were fit into place by drilling extra holes and/or bending the aluminum into shape. Later in the article, some "airline experts" claim this is normal and won't damage the long-term reliability.

My question: is this true?

From the article:

"Prewitt's job at Wichita was to purchase parts for 737s and other jets from Ducommun and other suppliers. She said workers informed her that some pieces were coming in with inaccurate measurements beyond the margin of error. In the summer of 2000, she visited one assembly line where the aluminum ribs, known as chords, were being attached to the 737 fuselage.

As Prewitt watched, she said, one worker pulled a chord from the stack and saw its holes were in the wrong place. "I said, 'So what do you do?' She grabbed a drill and drills a hole and connects it together," said Prewitt, now 45. "We're all appalled. I sat there watching her drill, drill, drill."

She said the chord problem reinforced worries that others had raised for a year about other Ducommun parts. She had examined reports of problems with "bear straps," large pieces of reinforcing sheet metal bonded to the skin around an airliner's doorways. Prewitt said the pieces, which have four jutting corners something like a bearskin rug, were coming in short in one corner. That forced workers to drill holes for rivets closer to the edge of the piece than specified.

The whistle-blowers said they learned that some managers knew of the problem but encouraged workers to make the parts fit. For example, when Prewitt recommended tossing out 24 bear straps she considered unacceptable, a Boeing procurement manager objected. "Scrapping any bearstraps is stupid, since we've used over 300 with the same condition," the manager wrote a whistle-blower in a May 13, 1999, e-mail reviewed by The Post."


http://www.washingtonpost.com/wp-dyn.../2006/04/16/AR2006041600803_2.html

6 replies: All unread, jump to last
 
User currently offlineMD11Engineer From Germany, joined Oct 2003, 14139 posts, RR: 62
Reply 1, posted (8 years 8 months 2 weeks 1 hour ago) and read 1955 times:

In short: Yes.

The edge margin (distance of rivet/fastener hole) has been calculated to withstand a certain amount of stress. Also, drillinh additional holes will definitely weaken the part.
If I do a repair of primary structure (and door reinforcement doublers are primary structure) I have to make sure that all the rivet holes and dimensions fit the specifications given in the SRM, else the part gets scrapped.

Also, bending heat treated, stressed parts is not without danger either. If you don't know what you are doing you can introduce cracks. Many parts get formed while being in a soft heat treatment state and then heat treated to a tough condition.

Jan


User currently offlineAzstagecoach From United States of America, joined Mar 2006, 152 posts, RR: 0
Reply 2, posted (8 years 8 months 1 week 6 days 22 hours ago) and read 1923 times:

the article's quote that you can bang misshaped parts into place:

"Sheet metal parts are necessarily pretty flexible so if they don't fit perfect as delivered, it's not a big deal to shove them into place, bend them a little bit, push on them and rivet them together," said Charles Eastlake, a professor of aerospace engineering at Embry-Riddle Aeronautical University in Daytona Beach, Fla., and a former aircraft structural designer for the Air Force. "Quality control people turn purple when they see that, but it's the way it's always been."

thoughts?


User currently offlineTod From Denmark, joined Aug 2004, 1732 posts, RR: 3
Reply 3, posted (8 years 8 months 1 week 6 days 22 hours ago) and read 1915 times:

Quoting Azstagecoach (Reply 2):
"Sheet metal parts are necessarily pretty flexible so if they don't fit perfect as delivered, it's not a big deal to shove them into place, bend them a little bit, push on them and rivet them together,"

Preloading components not intended to be preloaded invalidates the structural substantiation and creates an airframe that is in noncompliance with it's engineering and certification base.

Tod


User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 4, posted (8 years 8 months 1 week 6 days 22 hours ago) and read 1914 times:

Bending sheet metal is one thing, it is designed to flex to quite a significant degree. That's one of the benefits of metal fuselages compared to composite, the tolerances can be larger and the different sections will still mate.

If you start cutting or drilling, or even bending machined parts in ways which were not included in the design... welcome to the world of flight testing. Now what do the regulations say about carrying passengers for test flights?

Cheers,
Fred



I thought I was doing good trying to avoid those airport hotels... and look at me now.
User currently offlineTod From Denmark, joined Aug 2004, 1732 posts, RR: 3
Reply 5, posted (8 years 8 months 1 week 6 days 3 hours ago) and read 1830 times:

Quoting FredT (Reply 4):
If you start cutting or drilling, or even bending machined parts in ways which were not included in the design... welcome to the world of flight testing.

Or SB's to inspect and/or replace suspect parts.
Or have your stress engineers do another structural analysis and amend your TC or STC support the existing configuration.
Either way, hope the FAA doesn't view it as a 14CFR21.3 (b) issue and tell you to park 'em until you fix 'em.


Tod


User currently offlinePurdueAv2003 From United States of America, joined Dec 2005, 251 posts, RR: 2
Reply 6, posted (8 years 8 months 1 week 5 days 15 hours ago) and read 1804 times:

Quoting Azstagecoach (Reply 2):
"Sheet metal parts are necessarily pretty flexible so if they don't fit perfect as delivered, it's not a big deal to shove them into place, bend them a little bit, push on them and rivet them together,"

I will have to disagree with this statement. One issue with aircraft structure, particularly in the 737's case, is fatigue. It is important that we, as engineers, be able to have a general idea of when a part will fail due to the cyclic loading and unloading of the fuselage. The 737 in particular has a number of AD's out related to fatigue cracking at the FWD and AFT pressure bulkheads and the lapjoints. By bending a part to make it fit, you are changing the fatigue life of the part by introducing a stress riser and possibly increasing the load on adjacent structure due to the "unique" shape of the affected part.

Prior to installing the parts, Boeing engineers should have been contacted to perform the approrpriate analysis. What may not seem like a "big deal" very well could cause a structural defect in the future. It bothers me that a fellow engineer would make the above statement. Although some of my professors in school were out of touch with the "real world" environment, they always drilled into our heads that you have to be conservative. Bending a part and installing it with no concern for possible structural defects is NOT conservative!



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