For want of a washer… Boeing 737 checks ordered after Okinawa fire

The US Federal Aviation Administration (FAA) has ordered emergency inspections of wing leading-edge slat tracks on all new Boeing 737s, after investigators discovered a loose bolt had pierced the fuel tank in the China Airlines 737-800 that was destroyed by fire at Okinawa Island on 20 August. (See: Loose bolt confirmed as cause of China Airlines plane explosion in Okinawa)

The FAA's emergency airworthiness directive issued on 25 August requires operators to inspect main slat track downstop assemblies and slat track housings within 24 days, as well as every 3,000 flight cycles thereafter. It applies to 737-600, -700, -800 and -900 series aircraft, including the -900ER.

The planes' slats are attached to the front edges of both wings and are deployed during takeoffs and landings to increase lift. The order comes after a China Airlines 737 erupted in a fireball after landing on the Japanese island of Okinawa on 20 August.

All 165 people on board escaped unhurt. Investigators say the fire started after a loose bolt pierced the right wing's fuel tank, causing fuel to spill and ignite.

They believe the bolt came loose because a worker did not re-install a washer on the slat's downstop assembly during a maintenance check that took place in the last few months, sources familiar with the probe said.

Without the small washer, the assembly's bolt came loose and punctured the fuel tank, apparently when the slats were being retracted after the plane landed, the sources said. It has recommended that a torque of 50 to 80in per lb must be applied to the nut on the downstop assembly.

The FAA's directive did not mention the China Airlines accident by name but explained the circumstances in detail. It also described a second incident in which a loose nut pierced a wing tank, causing fuel to leak. But it did not lead to a fire, and the order did not name the carrier.

Boeing has received four reports in the past about loose nuts in slats. The airplane maker has issued several bulletins since December 2005, urging airlines to ensure the nuts on the downstop assemblies are properly installed and tightened. The most recent bulletin was sent last month, a company representative said. The matter becomes critical because so many next-generation 737s are in service.