How does the right wing slat bolt pierce the fuel tank of a Boeing 737?

Question

Okinawa/737 explodes on runway last week.All level inspection.

Answer 1

What I read, which may or may not be true, is the slat track runs through the tank. It runs through a tube to keep the tank sealed. When the stop bolt was found, it still had the nut attached without the necessary washers, which you can see in drewpie's picture. What is suspected is that the nut is smaller than the hole the bolt goes through and needs the washers to keep the nut side from going through the hole. You can see in the pic the shaft of the bolt coming through the hole in tube and the nut at the end, which appears to be very small compared to the bolt.

They think when the slats were retracted after landing, the bolt, which probably fell out earlier, punctured the tank and fuel started leaking out. When the plane was marshaled into it's parking spot the mechanic noticed the fuel leak, notified the flight crew and by that time the fuel leaking on the hot engine parts or hot brakes ignited.

FAA AD http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgAD.nsf/0/E8AEEF3D61BB907F862573430015B30A?OpenDocument

Answer 2

It has nothing to do with high time or overlength bolts, it was definately a case of the part came loose and was subsequently pushed through the back of the slat can.

The photo is misleading. The slat can is right in the middle (top to bottom) of the fuel tank, so the protruding bolt would be at a point below half, closer to a third of the distance from the bottom. And the closer to the wing root, the more full the tank will be, due to the dihedral of the wing.

The FAA has issued instructions that all domestic operators of 737-600, -700, -800, -900 aircraft must inspect them within 10 days to make sure they have not come loose and must have the bolts retorqued within 24 days.

Answer 3

This is the Slat Stop bolt that came loose fell on the track and was shoved by the Slat hydraulics through the back wall of the Slat Can (area that contains the slat mechanism that is located inside the fuel tank). This was all part of the normal cycling of the Slats backward in the "flaps-up clean-up" routine from the previous flight.
Boeing sent everyone all 737-NG owners a SL (service letter) back in 2005 to be careful of the stop bolts coming loose.

This aircraft (B-18616) had the slats removed for inspection July 6th where the Slats are removed (requiring the slat stops to be removed). It looks like they can't find the locks and washers now. Without this the nut would be subject to vibration and movement. Given the timing of the inspection to the accident it looks bad for the maintenance tech.

They also had some early warnings to this. Flights on August 4th and August 5th during landings the pilots reported the Slat position warning indicated the Slats were not in the position the handle was set at. Not sure what action was taken on the ground after these warnings. Or why during refueling they didn't notice fuel coming out of the leading edge of the wing.

http://www.msnbc.msn.com/id/20373668
Picture of inside the fuel tank

Answer 4

Well they are saying that it was incorrectly fitted. The slats and the wing tank are only a few inches apart.

Slats are hydraulically operated and can easily shear a bolt that gets in the way and punctures it into the tank. ( 100% speculation on my part)

Answer 5

Wrong bolt length wears a hole in the wing structure. But don't assume that was the only thing wrong with that high hours aircraft. FAA and Other adgencys will release there findings in about 9 months. Aircraft Accident data base. Check there

Answer 6

Yesterday, mechanics find a bolt without washer on one
of its 737, is it possible such defected part were used
at the first place, can anyone answer? thanks.