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.
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