FAA issues emergency AD in wake of China Airlines fire
Initial investigation reveals that following retraction of the slats
after landing, loose parts of the main slat track
downstop assembly punctured the slat can of the China Airlines
737-800, which resulted in a fuel leak and a fire. A similar slat
problem was reported by another operator. This prompted the FAA to
issue an emergency AD. This AD requires a detailed inspection and test
on of each main slat track downstop assembly on to all Boeing 737-600,
-700, -700C, -800, -900, and -900ER series airplanes. (FAA)
FAA AD 2007-18-51

Washer eyed in jet fuel leak / Bolt reinstallation error may have
triggered events leading to explosion
The bolt that came loose and damaged the fuel tank of the China
Airlines jet that burst into flames at Naha Airport last Monday likely
was incorrectly installed, as a washer and other parts that should
have been installed with it have not been found, officials of a
government commission investigating the accident said.
The Construction and Transport Ministry's Aircraft and Railway
Accidents Investigation Commission has learned that the Boeing 737-800
jet underwent a scheduled periodical checkup in July.
As part of this checkup, the bolt in question--part of a slat in the
plane's right wing--was removed for examination before being
reinstalled and passing the maintenance check, the officials said.
The commission assumes the bolt was incorrectly reinstalled during the
checkup, and subsequent checks did not discover the problem. The
commission said it will continue its investigation while considering
the possibility of a maintenance error on the part of the airline.
The commission finished its inspection of the plane Saturday.
Though the commission initially suspected fuel pipes inside an engine
mount beneath the right wing caused the fuel leak, no problems were
found with these parts.
The commission concluded that the bolt dislodged and pierced the wall
of the fuel tank, resulting in the leak, according to the officials.
The two millimeter-thick wall of the tank sustained a hole 41
millimeters long and 23 millimeters wide.
According to the officials, the plane underwent the annual checkup
July 6, and parts of the rear end of the slat pillars were removed for
checks and reinstalled.
A set of such parts comprises a 42-millimeter-long bolt, a nut with an
external diameter of 10.4 millimeters, and six other components placed
between them. They were installed through a bolt hole measuring 14
millimeters in diameter at the rear end of the slat pillars.
The set also contains two washers, with an external diameter of 15.9
millimeters, to prevent the bolt from coming loose.
The plane's maintenance record showed work to replace the parts was
done in line with Boeing's instructions.
When maintenance is carried out on aircraft, after one mechanic has
completed a process, another mechanic checks whether the work has been
done properly and has followed procedure. A final check is made after
all processes are completed.
When individual maintenance steps are completed, the head mechanic
signs a record sheet. Each plane's record carries signatures for each
stage of the maintenance process, such as checks and the replacement
of parts.
Although the nut was still attached to the dislodged bolt--a washer
and two other parts are missing--it came loose and pierced the wall of
the fuel tank.
Consequently, the commission suspects a mistake occurred when the bolt
was removed and reinstalled.
One possibility is that a mechanic forgot to install one of the
washers and the bolt dropped off due to flight vibrations. Another is
that a mechanic forgot to install the complete set of parts, with the
parts being left inside a section of the wing that accommodates the
fuel tank.
A Japanese airline mechanic said: "During maintenance, we sometimes
place all the parts together and tighten the nut to prevent small
parts from becoming scattered and lost. It's possible that a mechanic
forgot to install the set while carrying out another task at the same
time."
http://www.yomiuri.co.jp/dy/national/20070827TDY02005.htm