Airlines that operate Boeing 737s face a rigorous new emergency inspection regime following the fuselage rupture of a Southwest Airlines 737-300 over Arizona on 1 April. The new requirement will see each aircraft undergo an 8h inspection every 500 cycles.

Within three days of the incident, Boeing issued a service bulletin covering the operators of about 175 737-300/400/500s, requiring inspections of fuselage structure to begin within five days for aircraft with more than 35,000 cycles, and within 20 days for those with 30,000 to 34,999 cycles.

Meanwhile, the Federal Aviation Administration has issued an emergency airworthiness directive requiring repeated examinations every 500 cycles.

The inspection will search for sub-surface cracks in the lap-joint of a subset of 737 Classic aircraft. The cracks have been found to develop far earlier in the aircraft's service life than had been anticipated, says Boeing's 737 Classic chief project engineer Paul Richter.

The US airframer issued the service bulletin late on 4 April, requiring 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. This covers about 15.2m (50ft) - 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 between 1993 and 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 criterion, with 80 operated within the USA, almost exclusively by Southwest Airlines. Most of the others operate 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 to 2552 inclusive, which were required above 50,000 cycles.

Richter, who is chief project engineer for Boeing's out-of-production aircraft, including 737 Classics, says Boeing had anticipated some level of structural cracking in the relevant area, but had expected it to occur around 60,000 cycles, far later than the 39,871 cycles of the aircraft involved in the 1 April incident.

The design change increased the spacing of the tear strap frames from 250mm to 500mm (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.

Richter says the service bulletin "currently does not have a repetitive inspections listed in it", but the FAA mandates a 500-cycle repeat interval under its emergency airworthiness directive as a precautionary measure. Richter says the requirement will be reviewed "through the course of our analysis for adjustments as required down the road".

He says 500 cycles is a "rare interval to impose - it is quite frequent and has been used before with similar concerns". He adds that Boeing and the National Transportation Safety Board have suggested a precautionary and conservative interval until the root cause of the 1 April structural failure is established.

The examinations, which will look for disruptions in a magnetic field, indicating signatures for cracks in the base metal, will take about 8h per airframe with two mechanics in a maintenance environment, taking a further 8-16h to repair any cracks, says Richter.

Following the 1 April incident and emergency landing at Yuma, Southwest removed 79 other 737-300s from service and conducted Eddy-current structural inspections on them. Apart from the event aircraft, five other 737s have been removed from service after cracks were discovered in the lap-joints.

Richter says Boeing has not yet issued guidance to airlines on how to repair the lap-joints if they are found to be cracking, pointing out that later revisions of the service bulletin will address this, along with any preventive measures.

He says Boeing is "completely confident" that no such lower-row cracking issue exists with the lap-joints on any 737 Next Generation models that have been in service since 1997.

In November 2010, the FAA required airframers to set life limits for almost 4,200 transport aircraft, giving them between 18 and 60 months to determine how many cycles or hours an aircraft type can safely accumulate.

FAA administrator Randy Babbit said in a 6 April US congressional testimony that the FAA would take a more active role in incorporating the Southwest accident into its rule-making. "I want to make absolutely certain that what we learn from this accident gets incorporated into our requirements for reviewing ageing aircraft," he said.

"I am asking my team to review our ageing aircraft programme to ensure we are asking the right questions and taking full advantage of all available data."

Source: Flight International