Load redistribution with missing fasteners
Load redistribution with missing fasteners
(OP)
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
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





RE: Load redistribution with missing fasteners
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
RE: Load redistribution with missing fasteners
Steven Fahey, CET
"Simplicate, and add more lightness" - Bill Stout
RE: Load redistribution with missing fasteners
Other times, the use of a blind fastener is required for temporary return to service.
Brian
RE: Load redistribution with missing fasteners
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.
RE: Load redistribution with missing fasteners
They could be rattling around inside the compartment, just waiting to jam something important.
RE: Load redistribution with missing fasteners
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
RE: Load redistribution with missing fasteners
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
RE: Load redistribution with missing fasteners
Steven Fahey, CET
"Simplicate, and add more lightness" - Bill Stout
RE: Load redistribution with missing fasteners
Isn't this the PIC's call?
RE: Load redistribution with missing fasteners
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
RE: Load redistribution with missing fasteners
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