There have been numerous reports of Boeing finding "foreign object debris" in the fuel tanks of its 737 Max airliners.

It's not clear what this debris is. It could range from a couple of missing screwdrivers and a blowtorch, to perhaps microscopic particles that might contaminate the fuel.

Has Boeing released any details of what these objects are? And, more generally, how unusual is such contamination?

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    $\begingroup$ I saw something that claimed it included tools, shoe covers and dirty rags but I can't remember where. $\endgroup$
    – Flexo
    Commented Feb 22, 2020 at 21:37
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    $\begingroup$ @Flexo I'm pretty sure that I did too, and now can't find it either, but 1 and 2 (for example) explain that the term "foreign objects" can be used to describe objects like those, and it's possible that reporting or editing glossed over the distinction. $\endgroup$
    – uhoh
    Commented Feb 23, 2020 at 6:35

3 Answers 3


As a generic answer, not unusual. Besides personal objects like bucking bars, clecos, rags and such left in tanks, the biggest contamination source, from my experience working on a production line many many years ago, is "swarf". Swarf is aluminum drill shavings and mountains of it are created during assembly. You will also get wire cutoffs and other similar bits, shop (bucked) heads of rivets that were drilled and punched out because the rivet was dumped when being driven and had to be removed and reshot.

All that stuff is supposed to be thoroughly vacuumed out before the sealers go in with their goo guns, but nothing is ever perfect. Even if the wing was cleaned out before the sealing was done, if the wings get modded or repaired farther down the line, where more rivets get drilled out and redone, more swarf or other contam gets introduced and may get overlooked.

The first time fuel flow test runs are done on the completed aircraft, that's when you find out how bad a job they are doing up the line as the fuel screens and filters are removed to check.

Some manufacturers will build special rotation fixtures to put the wing in (not really practical in a one piece swept wing, but easy to do with a straight one) after it comes out of the assembly jig to rotate, barbecue chicken style, while they listen for objects clattering about. It's good for catching bucking bars, clecos and such that get left inside.

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    $\begingroup$ Do fuel flow test reveal solid foreign objects like tools that don't emit fibers or particles like some rags might? I want to ask "What's the longest time that a tool has sat in a commercial aircraft fuel tank before being discovered?" but not sure if it's on-topic ;-) $\endgroup$
    – uhoh
    Commented Feb 23, 2020 at 6:36
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    $\begingroup$ What's a cleco? $\endgroup$
    – user7915
    Commented Feb 23, 2020 at 14:48
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    $\begingroup$ Those little cylindrical sheet metal clamps that look like spent cartridge casings. $\endgroup$
    – John K
    Commented Feb 23, 2020 at 18:18
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    $\begingroup$ They'll do an initial fuel flow test, using a special test rig, to flush out the system and check for leaks. All the filter screens are pulled and inspected, cleaned and reinstalled. They repeat this until the the screens are clean after a FF run. None of that stuff will actually make it into a pump or an engine, but if severe enough it could block an inlet or filter screen causing low fuel pressure indications. Things like bucking bars or other larger inanimate objects are fairly harmless because they'll just sit there taking up space in a corner of the tank (notwithstanding rust issues). $\endgroup$
    – John K
    Commented Feb 23, 2020 at 18:34
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    $\begingroup$ @BenCrowell Cleco Pins are temporary fasteners used to hold metal together as it's being riveted. The piece is clamped in place, then holes are drilled for the rivets. As each hole it drilled, a Cleco pin is put in place to keep the pieces from moving. They're left in place until each one is replaced by a rivet. $\endgroup$
    – FreeMan
    Commented Feb 24, 2020 at 19:24

In addition to Excellent answer by John K.

In 2017, Boeing Engineering team head resigned and he bought these complaints to FAA. As per him, Spanners, Rags, and the shavings of drill bits (Aluminium bits) Scraps and other tools were left as is in the wings and tanks.

As per him, he took the issue to his superiors, who ignored his complaints. Then he resigned and then filed the issue with the FAA.

Why Now? There had been some recent criticism from him in a podcast and on TV (I guess) by BBC, which was broadcasted throughout the world. As per the podcast, there were others who filed similar complaints on the brand too.

As this issue was extensively covered, The media sought some additional clarity from the brand as they are running short on Action content in the past month. Hence this issue was covered. I am not advocating any of Boeing's actions, but despite this being known to all and FAA, none cared to cover it when the incidents occurred, however the same was bought to light when they are running short on content.

  • $\begingroup$ Are you going out of your way to avoid mentioning the engineering head's name for some reason? That's the impression I get from this answer. I don't know if the answer even needs it, but it caught my attention. $\endgroup$
    – bobsburner
    Commented Feb 24, 2020 at 12:06
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    $\begingroup$ What is that last paragraph about? $\endgroup$
    – MikeB
    Commented Feb 24, 2020 at 12:42
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    $\begingroup$ Nothing for the media to report at the moment?? There's no Corona virus or something? @MikeBrockington: The suggestion is that "the media" are writing about Boeing because they have no better subjects. $\endgroup$
    – MSalters
    Commented Feb 24, 2020 at 16:56

I saw this yesterday:

Samchui.com: Foreign Objects Discovered In Boeing 737 Max Fuel Tanks

Included in the list of items discovered are rags, tools, metal shavings and other production equipment, which pose a serious safety risk in flight and on the ground in operational service.

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    $\begingroup$ I added a quote from the story, because link only answer are not considered a good practise here. $\endgroup$
    – Jpe61
    Commented Feb 25, 2020 at 22:15

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