The US FAA is proposing that operators of approximately 135 US-registered Boeing 737-300, -400 and -500 aircraft begin performing external non-destructive inspections of a certain area of the fuselage skin for evidence of fatigue cracks every 500 flights.

The proposed airworthiness directive (AD), to be published tomorrow, is the result of an in-flight skin rupture and depressurization of a Southwest Airlines Boeing 737-300 in July.

The aircraft, enroute from Nashville to Baltimore, landed safety in Charleston, West Virginia after the incident with a 43cm (17in) by 20cm hole in the skin on the upper section of the rear fuselage between two adjacent stringers.

If finalized as written, the interim AD will make mandatory a 3 September service bulletin issued by Boeing calling for aircraft with 35,000 cycles or more to be inspected using eddy current or other non-destructive techniques every 500 cycles for signs of cracks. The incident aircraft had accumulated 42,569 cycles, according to the FAA.

Operators who have installed an external doubler in the area, as specified in a previous Boeing service bulletin, will not have to perform repetitive inspections after an initial check, providing the repair meets certain other criteria spelled out in the proposed AD.