Load redistribution with missing fasteners

[koopas]

Good day all,

I have a fairly general question.

Suppose you have a structural member (access panel, fairing, lap/butt joint, etc.) attached with fasteners.

One or two fasteners are found to be missing, and assume that no replacements could be located. As the engineer, you have to make the call to let the plane fly, or ground it (and the a/c is ready to pushback with a plane full of pax.).

How can one substantiate leaving the condition as-is? I presume that the load would redistribute to adjacent fasteners. However, of course, that's just a gut feeling that doesn't fly...no pun intended.

Any comments or suggestions? I am looking for data "acceptable" to the admistrator to substantiate dispatch. As a sidenote, one would obviously evaluate the criticality of the structure, as I am sure missing fasteners on a PSE or SSI component are more worrisome than on a fairing that's secondary structure. On the other hand, that fairing departing into the engine would be equally alarming.

Alex

 

[aviat]

Many operators use something like a CDL (Configuration Deviation List), which is a list of structural items which may be missing etc. It is not required per regulations but aids for technical dispatch. It is made up in advance and reduces the last minute panic decisions because pilots and line maintenance can then often make the decision. Like a MEL they are operator and A/C specific. TC includes it in their notes.

As far as number of fasteners missing it would depend on on a lot of things. I've seen allowing as many as 10% of the screws missing on non-structural panels. Tou may be able to get some info from tour SRM.

http://www.tc.gc.ca/aviation/REGSERV/CARAC/CARS/cars/625e.htm#625_07

 

[Sparweb]

I'd be just as worried about why the fasteners are gone. If it's an inspection panel that wasn't completely closed up, so be it. If this is a structure that isn't normally disassembled (say, a floor beam), then I would wonder if somebody was fiddling with it, or if it came from the factory that way. Take a minute to peek at any nearby joints to see if the same omission has been comitted.

Steven Fahey, CET
"Simplicate, and add more lightness" - Bill Stout

 

[bubb375]

A common problem on older aircraft is missing rivet heads. Sometimes we can contend that the shank is intact, and can still carry shear loads.

Other times, the use of a blind fastener is required for temporary return to service.


Brian

 

[Michael996]

bubb375,

I don't subscribe to your argument about rivet heads. I'd bet the FAA doesn't either. There is always out of plane loads applied to fasteners. Especially rivets with missing heads.

 

[MintJulep]

If the fasteners are not where they are supposed to be, then where are they?

They could be rattling around inside the compartment, just waiting to jam something important.

 

[Sparweb]

Stop shooting from the hip, Bubb!

Rivet heads pop off for reasons you want to know about. Sheet metal structures are often subjected to small amounts of bending or even buckling that isn't really accounted for because designers assume the rivets are intact. Without the heads, nothing restrains the out-of-plane loads, and proper clamp-up between rivet heads is essential to achieving full rivet shear strength, too.

Blind fasteners are another bad idea unless you understand their limitations. An old Eng-Tips thread dealt with this issue in exquisite detail, and is well worth re-reading. thread16-15349

To summarize all the things that blind fasteners don't do as well as solid rivets:
Lower shear allowables
Less resistance to vibration
Less clamp-up of sheets
Less expansion into hole
Leaky
Less consistent installation
Need I go on?


Steven Fahey, CET
"Simplicate, and add more lightness" - Bill Stout

 

[bubb375]

Perhaps we should focus on the problem at hand. Koopas is asking whether or not missing fasteners are critical for flight dispatch. There is no doubt that the related structure is a critical part of the equation. However, I'm stating that in some cases it can be acceptable, and in others a temporary solution is available.

For those that believe "No Plane Can Ever Fly With a Missing Rivet Head" or "Blind Fasteners are the Devil" it has been done and with FAA approval. Perhaps the delicate balance between getting a plane off the ground and providing a minimum level of safety could be better understood by those that work for an airline.

Brian

 

[Sparweb]

A rivet that has lost its head is almost as good as no rivet at all. If you are dispatching an aircraft in this condition, you'd better know your knitting. Of course, if the rivet is fastening something non-critical, you can judge the risk easily, and if it's one rivet immersed in a row of dozens, then you've got redundancy in the joint. But that does not remove your responsibility, IMHO, to investigate root causes. A missing rivet is a problem, but it can also be a symptom of a greater problem.


Steven Fahey, CET
"Simplicate, and add more lightness" - Bill Stout

 

[MintJulep]

"As the engineer, you have to make the call to let the plane fly..."

Isn't this the PIC's call?

 

[koopas]

Hello guys,

When you say that rivet heads resist out-of-plane loads, do you mean that rivet heads resist:

1. bending (say, due to eccentricity between the sheets)?

or

2. tensile loads applied to the rivet/rivet head by the sheets?

or is both?

Gracias,
Alex

 

[Sparweb]

Koopas,

I speculated that rivet heads could pop/pull through if a sheet wrinkles, perhaps if it was overloaded, or perhaps a stiffening member is also damaged or missing. It was pure speculation solely to illustrate the point that it is a sign of tension applied to the rivet by some unknown mechanism. Rivets can support some tension, but not as reliably as shear, and it is very dependent on the quality of the driving.


Steven Fahey, CET
"Simplicate, and add more lightness" - Bill Stout