The FAA has issued an Emergency Airworthiness Directive (AD) against Boeing 737-300, -400, and -500 series airplanes.
The emergency AD was prompted by a crack that occurred on 737-300 airplane. The lap joint at stringer S-4L between body station (BS) 664 and BS 727 cracked. The FAA reports that the cracking was located in the lower skin at the lower row of fasteners. This is suspected to be the result of fatigue cracking, although further inquiry is necessary before a final conclusion can be reached.
Boeing has published Alert Service Bulletin 737-53A1319 (April 4, 2011). The service bulletin describes procedures for external eddy current inspections of the lap joints at stringers S-4R and S-4L, along the entire length from body station (BS) 360 to BS 908. If a crack indication is found, the service bulletin specifies either confirming the crack by doing internal eddy current inspections, or repairing the crack. As an alternative to the external eddy current inspections, the service bulletin provides procedures for internal eddy current and detailed inspections for cracks in the lower skin at the lower row of fasteners at stringers S-4L and S-4R.
The AD mandates the inspections. After initial inspections, recurrent inspection will be required every 500 flight cycles.
The AD requires that repairs be approved by the FAA. It also announces that the Boeing Organization Designation Authorization (ODA) has been authorized to approve these repairs.
According to the Seattle Times, Southwest’s inspections have found five other 737s with fatigue cracks. Boeing has provided guidance repairs, which entail replacing about 18 inches of the affected lap joint.
Fatigue may become a bigger factor in PMAs in the future, like:
- How PMAs may stand up to fatigue over time;
- How PMAs can address fatigue-related issues in aircraft; and
- How the PMA market may be affected by retirement of older aircraft due to fatigue-lifing programs