The document to the left was in my “in box” on 7 April, one week after a Southwest Boeing 737-300 experienced rapid decompression after a large section of its cabin crown ripped away in flight. The suspected cause? Widespread Fatigue Damage (WFD), a problem that started near the fuselage lap joints.
What’s a lap joint? Funny you should ask that, and fortunate the FAA should publish an expanatory document on the topic almost simultaneous with the event (as with most government organizations, the WFD story would have been in the works long before its 7 April publish date).
As the FAA explains on pages 5-7 in the article “Widespread Fatigue Damage: You can hardly see it”, the repeated pressurization cycles on an airframe cause the fuselages skin to want to stretch outward, which over time causes small cracks to form near the rivets of the lap joints.
Over time, the cracks get bigger. “Cracks that start this way can’t be seen until they’ve grown to an unsafe length,” the agency says in the report. The FAA in the Southwest incident issued an emergency directive for eddy current inspections of hundreds of older 737s to sniff out what the eye cannnot.