Worldwide inspections ordered in the wake of a fire that engulfed a China Airlines Boeing 737-800 has turned up signs of a wider safety issue and at least one clue that the cause may be traced back to Boeing's assembly line.

Investigators have determined that the ruptured fuel tank that triggered the non-fatal China Airlines fire on 20 August was caused by loose bolt in the wing leading-edge slat-track assembly.

On 25 August, the US Federal Aviation Administration ordered all Next Generation 737 operators to perform borescope inspections within 10 days, reporting multiple cases of loose or missing parts where the 737's slat-track meets the downstop. The FAA has also ordered a one-time torquing of the nut and bolt in the downstop within a 24-day window.

In Japan, investigators said on 30 August they had found a similar missing part in an Air Nippon 737-700. The absent washer meant that the bolt in the slat-track downstop could be pried loose, creating the same conditions that led to the CAL 737 fire, according to Japan's land, infrastructure and transport ministry.

Air Nippon says the washer may have been missing when the aircraft was delivered in January, implying that the fault lies with Boeing's assembly system.

Boeing is, meanwhile, continuing to collect data from operators' borescope inspections, it says. Following company procedure, the investigation is not limited by aircraft model or the potential source of the part problem - whether it may be a fault of design, production or maintenance.

Source: Flight International