Boeing has issued a service bulletin that covers the operators of approximately 175 737-300/400/500s requiring a inspections on the aircraft fuselage structure to begin within five days for aircraft with more than 35,000 cycles, and 20 days for aircraft with 30,000 to 34,999 cycles, with the US Federal Aviation Administration requiring repeated examination every 500 cycles.
The requirement comes in the wake of the 1 April fuselage rupture aboard a Southwest Airlines 737-300, that developed lap-joint cracks far earlier in its service life than anticipated, says Boeing's 737 Classic chief project engineer, Paul Richter.
The US airframer issued the service bulletin late on 5 April disclosing the requirement to conduct dial frequency Eddy-current inspections on the lower row of fasteners in the fuselage lap-joint, along the left and right-hand side of the crown of the aircraft at stringer four between Station 360 - just aft of the forward passenger door - and Station 908 - a few frames ahead of the rear passenger door, covering roughly 15.2m (50ft) and almost the entire length of the passenger cabin.
The service bulletin applies only to 737 Classic aircraft with line numbers 2553 and 3132 inclusive, which were delivered from 1993 until the end of the Classic's production run in 2000. Paired with the 30,000 cycle requirement, a total of 175 aircraft worldwide meet this criteria, with 80 operating within the US, almost entirely for Southwest Airlines. The majority of the balance are operating in Europe and Asia, says Boeing.
What distinguishes this tranche of 579 737s is a design modification that was intended to eliminate the requirement for a lap-joint modification programme already in effect for line numbers 292 through 2552 inclusive, which were required above 50,000 cycles.
Richter, who serves as chief project engineer for Boeing's out of production aircraft, including the 737 Classics, says Boeing had anticipated some level of structural cracking in the area in question, though it had been expected to occur around 60,000 cycles, far later than the 39,000 cycles of the aircraft involved in the 1 April incident.
The design change increased the spacing of the tear strap frames from 10in to 20in inside the fuselage, which are intended to prevent fuselage damage from propagating across structural frames in the event of a failure. Ultimately the updated design on the newer 737 Classics was intended to increase the fatigue life of the lower row of the lap-joint.
The service bulletin "currently does not have a repetitive inspections listed in it," says Richter, but the FAA will mandate a 500 cycle repeat interval as part of its emergency airworthiness directive as a precautionary measure. Richter adds that requirement will be reviewed "through the course of our analysis for adjustments as required down the road".
Richter says 500 cycles is a "rare interval to impose, it is quite frequent and has been used before with similar concerns". He adds Boeing and the NTSB have suggested a precautionary and conservative interval until the root cause of the 1 April fuselage structural failure is established.
Richter adds examinations, which will look for disruptions in a magnetic field, indicating signatures for cracks in the base metal, will take roughly 8h per airframe with two mechanics in a maintenance environment, with another 8-16h to repair any cracks.
Following the 1 April incident and emergency landing Yuma, Southwest removed 79 additional 737-300s from service and conducted Eddy-current structural inspections on the aircraft. Separately from the event aircraft, a total of five additional aircraft have been removed from service after cracks in the lap-joints were discovered.
Richter says Boeing has not yet issued guidance to the airlines on how to repair the lap-joints if they are found to be cracking, adding that later revisions of the service bulletin will address this, along with any necessary preventive measures.
Richer says Boeing is "completely confident" that no such lower row cracking issue exists with the lap-joints on any of the 737 Next Generation models that have been in service since 1997.