Boeing 737 3

[dik]

From the Guardian UK, "The US government will order emergency checks of dozens of Boeing 737s after Southwest airlines was forced to ground planes over safety concerns.

Southwest cancelled hundreds of flights after a five-foot tear opened in the fuselage of a plane 20 minutes after taking off from Phoenix on Friday.

The Federal Aviation Administration has now ordered fuselage inspections of older 737-300, -400, and -500 models, involving what could be time-consuming and repetitive electromagnetic checks.

"This action is designed to detect cracking in a specific part of the aircraft that cannot be spotted with visual inspection," FAA administrator Randy Babbitt said.

Boeing will offer its own advice to airlines outlining specific inspection steps."

Dik

 

[twalter]

Anyone else have a habit of looking at the date codes on the aircraft as you board?

I'll still fly on them, as Aloha Airlines Flight 243 showed just how much damage a 737 can withstand and still remain airborne. That was adhesive failure, if I remember correctly.

 

[monkeydog]

About 5 years ago, I was flying from Chicago to Buffalo on a 737, got bumped up to 1st class and was enjoying my little perk. At the last minute this guy just makes onto the plane, obviously ticked-off, mumbling and grumbling to himself.
His seat was next to me, before he sits down, he stops, looks around, then mumbles to himself "piece of s--t airplane". Takes his seat, and puts his well worn binder down, on the cover its has the Boeing logo.

 

[msquared48]

When I looked at the failed section, I had two structural questions:

1. Other than for cost reasons, why were the rivets not staggered to increase the length of the line of any laminar tearing failure, which occurred?

2. Not sure, but it appeared that the skin panels were not staggered in placement, but started and stopped at each airframe rib. If so, why? This significangtly reduces the strength of the fuselage skin diaphragm.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[rb1957]

1) the stringer flanges are typically only wide enough for a single rivt row. particularly for older Boeing designs which used an inverted "top-hat" stringer section, with te frames "floating" on the inner strnger flanges (and shear ties to the fuelage skin). also rivet pitch is limits the compression strength of the skin, more than 6-8 D you reduce the 30t effective width.

2) i doubt that each frame bay is a separate panel. longitudinal joints tend to be straight. They could be off-set, but that'd be loat more complicated to fit together. Generally panels will have aligned edges longitudinally, their circumferential edges will be off-set.

 

[KENAT]

msquared, it can probably be summarized by, mass, cost, common practice at the time of original design.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[msquared48]

Not to be fecetious, but for the rivet spacing I see, and I do understand about the max spacing to provide support for the skin, it seems like a zipper might work about as well. The end result would be the same in a failure, but it could be quickly repaired.

I guess that the aviation industry is a lot like Pre-Engineered Metal Buildingss, cost and weight driven, and to the limit, unfortunately in my opinion.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[blakmax]

Does anyone know if these lpa splices are adhesively bonded or just installed with sealant? I know that Aloha 243 was manufactured using a room temperature curing film adhesive. That had to be used frozen or it would prematurely set. When exposed to the atmosphere condensation formed on the surfaces and that would inhibit the bond and trap water causing corrosion. They stopped using that material at about LN 290. What do they do now?

Regards
Blakmax

 

[DHambley]

I don't know if the culture at Boeing had changed for the 737-300 from that of the 737-200. For the earlier airplane, when the rivet holes of metal skin could not line up with the holes in the ribs, the mechanics found a way to stretch and pull on the sheet until the holes lined up, then the rivets were installed under great stress. They did this instead of asking engineering for a solution. In fact, engineering was not even told about this "solution". A lot of us only found out about this manufacturing "solution" years later after Aloha Airlines flt 243.

 

[rb1957]

i think that needs some clarification.

1) Manufacturing have some inherent authority to "adjust" things so they go together. If they have to exceed their allowance then Liaison disposition the repair with their own authority, or with production support engineering. of course, i know there is a real world out there and corners get cut. we're relying on a combination of professionalism and self-interest ... the self-interest comes in

2) it would be very unusual (= poor?) design to create full size holes in both pieces prior to installing the rivet. typical would be pilot holes, which give alot more tolerance to misalignment.

 

[thruthefence]

"In fact, engineering was not even told about this "solution". A lot of us only found out about this manufacturing "solution" years later after Aloha Airlines flt 243"

I have read a great deal about the aging aircraft problem, and specifically the Flt 243 incident. I have never read, or (before now) been made aware about a manufacturing defect of this sort leading to the failure. Can you cite a source for this information?

If true, I would think it would be the subject of intense media scrutiny.

It would also call into question the whole question of "aging aircraft" failing due to cycles & old age, which assumes proper design, & proper manufacture in the first place.

Further, if this is simply "water cooler talk",without substantive evidence, you do a great disservice to the designers AND technicians who build the aircraft, by passing it along as "inside information".

Where may I find documentation on this issue?

 

[moon161]

AFAIK Aloha 243 was deterioration of on an epoxied lap joint by the salt air. Didn't know to inspect for it till they had that particular failure.

 

[WKTaylor]

Guys... in the absence of any hard "why-it-failed-data"... there are (3) [or possibly more] serious issues at hand. Simplified...

Good/Good for Boeing. The skin failed due to maintenance induced damage [scribe-lines, maintenance impacts, etc] induced the skin damage, but the cabin roof opened/depressurized as anticipated...NOT zipper-failing like the Aloha AL B737-200, significantly beyond the area of fatigue damage.

Bad/good for Boeing. The skin failed locally in fatigue [thru fasteners] but did not zipper-fail significantly beyond the area of fatigue damage.

Bad/Bad for Boeing. The Skin failed in fatigue and zippered-open far more than predicted by analysis or testing.

Meaning...

Good/good = predictable/stable DADTA and catastropic failure modes/models... except for unpredictable [critical] maintenance issues.

Bad/good = fatigue damage that was NOT anticipated by the DADTA models, but the failure mode was stable/contained.

bad/bad = fatigue damage and failure-modes/models are proven to be grossly unacceptable, bringing into question all other similar Boeing analysis.

Regards, Wil Taylor

Trust - But Verify!

We believe to be true what we prefer to be true.

For those who believe, no proof is required; for those who cannot believe, no proof is possible.

 

[MikeHalloran]

I'm wondering it it's a design/usage mismatch.

When the 737 first appeared, it was used/ sold/ designed(?) for short flight legs, where it would not usually reach a high altitude. Other, larger, planes were used on longer legs. No airplanes were utilized to 100 pct load for 100 pct of the time, either.

Then the world changed...

Now, Southwest uses 737s for _everything_, including long legs at high altitude, and everybody runs with all seats full nearly all of the time, and keeps their airplanes airborne for much more of the day than was the case when the 737 was designed.

I just wonder if the fatigue cycle calcs included the large number of cycles per day that are typical now, and cycles to high altitude also?


Mike Halloran
Pembroke Pines, FL, USA

 

[moon161]

I think 37's crash & fail because people fly the crap out of them everywhere. A large amount of failures are in them because a large portion of takeoffs are in them.

 

[hokie66]

This plane had almost 40000 cycles. Boeing figured it was good for 60000 before comprehensive inspection. Figuring was wrong.

 

[blakmax]

Moon161: AFAIK Aloha 243 was deterioration of on an epoxied lap joint by the salt air.

The salt air may have accelerated the problem, but it was laways going to happen to Aloha 243 or some other of the earlier 737s. To add to my posting a few ahead of yours, I can explain how metal-epoxy bonds work. There are chemical bonds (mainly covalent) formed at the time the adhesive is cured. These bonds form between the metallic oxides and hydroxides on the surface of the metal and the epoxy molecules. These chemical bonds provide the strength for the adhesive to metal bond.

In the case of Aloha 243 and before about ship 290, they used a room temperature curing adhesive film material, and as previously posted, the moisture that condensed on the surface inhibited the formation of those chemical bonds and also led to moisture entrapment which in turn led to corrosion under the lap-splice. A second factor in bond performance is the ongoing integrity of the chemical bonds, because these may under certain cercumstances be susceptible to hydration which leads to disbonding. The mechanism is the presence of water (see TOPOGRAPHIC AND SURFACE CHEMICAL ASPECTS OF THE ADHESION OF
STRUCTURAL EPOXY RESINS TO PHOSPHORUS OXO ACID TREATED
ALUMINUM ADHERENDS by Gary Alan Nitowski) and since the room temeprature curing film adhesive trapped water, the mechanism for bond failure was readily available.

The real error was the selection of a room temperature curing adhesive system.

 

[msquared48]

So, are you saying that the glue is the primary structural connector and not the rivets? If so, then, are the rivets only there to provide a uniform compression force along the section to obtain a good bond?

If so, where is the extra structural capacity for overload, the safety factor, the structural duplication against failure? It seems to me like a failure just waiting to happen.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[hokie66]

Scares the hell out of me to depend on glue. But then, if I designed one, it would be too heavy to get off the ground.

 

[msquared48]

Me too, Hokie. Me too.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask