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Is it FOD?

Is it FOD?

Is it FOD?

On inlet cowls, we often get in panels that have had blind fasteners removed and replaced over the years so that they begin to sound like maracas. The blind rivet butts are loose so they can rattle around in the honeycomb panels, but they can not escape, they are completely trapped.

When faced with this situation does it make more sense to remove the currently installed blind fastener, creating another rivet butt in the panel so that adhesive/sealant could be added to capture the rivet butts so they can't move around or leave it as is, with the rivet butts loose. The primary concern was with relative movement of the rivet butts causing damage to the honeycomb core. On the other hand, we're adding weight from the additional butts and sealant, and reducing the acoustic properties of the panel.

The question I have been asked is "Is this FOD, since it is trapped and can't be ingested by the engine without some more serious event to release them.



Kirby Wilkerson

Remember, first define the problem, then solve it.

RE: Is it FOD?

Rattling fasteners could generate abrasion particles that could easily get sucked into the engines. While they might not cause catastrophic damage, the continued ingestion of potentially abrasive particles could degrade the performance of turbofan blades.

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RE: Is it FOD?



RE: Is it FOD?

thanks... Dik

RE: Is it FOD?


I was using it as a term for Foreign Object Debris.


The fasteners are blind fasteners so there is no re-tightening. To gain access, the head of the previous blind fastener is drilled out and the butt is pushed in/aside to separate components. Then when being reassembled at installation a new fastener is used.


Even if the skin was only perforated, the size of the particle would need to be below 0.050", and the amount of material liberated would seem quite minimal as well. But in this case the panel is covered in wire mesh, in that instance would you still hold the abrasive damage possibility? The hole size of the wire mesh is vanishingly small. Also we are mostly drilling on the back side of a panel with septumized core, though not every fastener is behind a septum layer.

Thanks for everyone's responses.


Kirby Wilkerson

Remember, first define the problem, then solve it.

RE: Is it FOD?

It can't be FOD, since the rivet butts are not 'foreign' to the aircraft.

It could be AOD, 'Autogenous Object Damage'.

... which might placate or confuse an inquiring party, but doesn't resolve the problem.
... if it is a problem.
... which might be a useful way to define the existence of a problem.
I.e., if anyone asks, it's a problem, and conversely.

... but that feels irresponsible, doesn't it? I.e. the panel left the factory with no extra rivet butts and no rattles.

Evaluating damage to honeycomb core from repair detritus doesn't sound like something a grad student would do out of intellectual curiosity, and it could be an expensive course of investigation. It is the sort of thing a honeycomb supplier might do, quietly, but they would have no great incentive to publicize the results. So they might bury it in the forest, e.g. at sae.org.

Mike Halloran
Pembroke Pines, FL, USA

RE: Is it FOD?

Another definition of FOD = Foreign Object Debris

Generally speaking FOD that is 100% trapped and is light weight [fastener tails, chips/shavings etc] and is non-hazardous and may remain-in-place indefinitely... preferably with a sealant coat to make it stick together/structure and to 'silence the maracas'.

The problem with FOD [chips/shavings, fastener tails, etc] in honeycomb is that it generally has to be liberated by 'drilling-thru' or 'punching-thru' so another fastener can be installed. This action generally damages the HC and opens a moisture pathway. In this case It is advisable to pot the small cavity with adhesive or potting resin [filled adhesive] to stiffen the area and seal it. At BARE minimum, new fastener [etc] 'liberally wet' with sealant to assure environmental protection [seal].

In some cases, a small cavity can be accessed thru a relatively small 'dime/quarter sized hole' with a small diameter vacuum cleaner tube or hose to suck-out a majority of the FO. The small hole is then patched by a 'dollar patch'[fastened or bonded].

In fighter wings, where integral fuel tank cavities are thin/narrow and inaccessible W/O skin removal, and a repair was required for a skin crack or puncture [etc], I have been forced to enlarged repairs beyond optimum so that a FOD retrieval hole... deliberately cut in skin areas where old fasteners are being dilled-out/punched-thru and new fasteners are being drilled for the basic repair. An extra-large doubler/patch is needed for the basic repair + prophylactic repair [FO retrieval hole]. These God-awful repairs are necessary to keep loose FO [noted] from finding its way toward an engine [screen-filter, micron-filter, pump, and piping].

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

RE: Is it FOD?


I'm all for dazzling them with brilliance, and when that fails, baffling them with BS. I'll have to remember Autogenous Object Debris/Damage as masterfully confusing BS.


The other problem with sealant/potting is we may fill up the cavity, so installing any fastener is impossible without repairing the area to remove them all. Based on your comments I will recommend that if we have the fastener out for a repair to use sealant to entrap any butts that can't be removed, but don't drill out a fastener, creating another butt, just so you can entrap a previous butt that was not captured during a previous repair. This sounds like the most reasonable solution to me.


Kirby Wilkerson

Remember, first define the problem, then solve it.

RE: Is it FOD?

"She swallowed the spider to catch the fly..."

Another problem with pouring adhesive into the cavity is whether it plugs up any drain holes at the same time.
It sounds like this is a cavity at risk of filling with moisture.
It will work best with an adhesive that can be applied as a spray, if possible. Then it can be sprayed with a thin straw through any available fastener hole.
I know of a light aircraft operator in the tropics that sprays their aircraft internal cavities with a corrosion preventive liquid that would do exactly what you want. Stays gummy for a while to catch the debris that doesn't get caught right away. I think they just mixed a blob of PRC into a bottle of MEK, but you may find something a little less awful to handle.


RE: Is it FOD?

Apropos quote SparWeb!



Kirby Wilkerson

Remember, first define the problem, then solve it.

RE: Is it FOD?

Your welcome, but don't read too much farther down the poem... bad news for you if your situation gets to the next verse!


RE: Is it FOD?

This is an interesting question.

One thing about removing structural bulb type blind rivets (NAS 1097, Cherrymax, etc.) installed into a closed structure like the aluminum honeycomb panel described, is that these rivets often use an alloy steel stem. Drilling out this type of rivet stem could leave ferrous metal chips inside the honeycomb cells, where there can also be moisture/humid air present. This would seem to present a potential corrosion issue.

I was reading thru the FAA AMT Airframe Handbook to see what it had to say about the subject and there was this somewhat related bit in chapter 4, p.47:
[i]"Blind rivets shall not be used:
1. In fluid-tight areas.
2. On aircraft in air intake areas where rivet parts may be ingested by the engine.
3. On aircraft control surfaces, hinges, hinge brackets, flight control actuating systems, wing attachment fittings, landing gear fittings, on floats or amphibian hulls below the water level, or other heavily stressed
locations on the aircraft.
NOTE: For metal repairs to the airframe, the use of blind rivets must be specifically authorized by the airframe manufacturer or approved by a representative of the Federal Aviation Administration (FAA)."

RE: Is it FOD?

Guys...OH, NO... only inject/fill tiny cavities in honeycomb core [skin-core-skin] panels due to drill-thru or fastener tail punch-in. Obviously large spaces never intended to be filled should be left intact... and moisture drain paths must remain open.

Repairing these structures always entails decisions, such as remove or don't remove FOD. Biggest problem with FOD retrieval, that I alluded to above, is that a hole big enough to allow retrieval [vacuum or shake-out] requires its own/unique secondary repair which can amount to a really big deal.

So: better to leave small bits?... or retrieve these bits and increase repair complexity dramatically? Judgement!
RE potential for steel stem/bits rusting in cavities. IF structure is properly finish-coated/sealed then affect should be minimal for years. On the other hand new structure tends to avoid steel entirely in-favor of aluminum, coated CRES or coated titanium fasteners.

NOTE. My major rules for using blind fasteners in not-so good locations, thus...

Corrosion protection/resistance and moisture sealing is essential.

ALWAYS design to tension and shear YIELD values for component/repair ultimate [1.5 X Limit or destructive] loads... which can be very low... but help fasteners remain intact/functional with a long service life.

ALWAYS specify hole and countersink size [degrees x depth] for mechanics... never leave it to their 'best judgement'. Tighter/closer fits are universally better for durability. Allowing minor head protrusion above flush makes for a better installation than a head set below flush.

ALWAYS [try-to] specify a BR or BB with a strong tail element that PULLS UP structure TIGHTLY when setting the fastener.


NOTE. When You work on fighters with thin wings and tight cavities, blinds are inevitably installed 'en'masse'. Since they are designed-in from the beginning, it is easier to understand usage rules and policies, specific to that Jet. And there can be an amazing variety with necessary features IF YOU know Your 'blinds' in detail.

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

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