Operators of certain Boeing 737 Classics will need to perform eddy current inspections on parts of their fuselages every 500 flight cycles, forcing interruptions to their service at much more frequent intervals than anyone had originally envisioned. The mandate stems from an April 5 emergency AD issued by the FAA, published four days after a five-foot-long piece of a Southwest Airlines 737-300 tore open during a revenue flight between Sacramento and Phoenix, forcing an emergency landing in Yuma, Ariz.
The AD calls for inspection of a section of lap joint for fatigue cracking in certain airplanes that have flown 30,000 flight cycles. Only about 15 years old, the Southwest airplane that developed the tear in its fuselage had accumulated 39,781 flight cycles and 48,740 flight hours.
Although engineers expect high-cycle operating environments to place extra strain on airplane skins due to repeated pressurization and depressurization, the incident appears to have surprised Boeing. In fact, its maintenance requirements called for such inspections at 60,000 hours.
The AD came after Southwest voluntarily grounded 79 of its 737-300s and found fatigue cracking in five of them. Apparently, the airplanes came from a “subset” of airplanes manufactured between 1993 and 2000. The inspections apply only to models with a specific lap joint design no longer used in production, wrote Boeing Commercial Airplanes marketing vice president Randy Tinseth on his blog, Randy’s Journal.
As other airplanes with the same design approach 30,000 life cycles, they’ll also undergo the same inspections, he added. Although the AD immediately covers only 175 airplanes, it will eventually apply in total to 571, of which 31 now sit in temporary or long-term storage, according to the FAA. Eight other airplanes affected by the AD have left service completely.
The question remains, with all of Boeing’s experience and expertise in metal fatigue science: how could it have so wildly miscalculated the interval at which inspections of this particular area of fuselage should occur? It might takes months before anyone knows for sure, but it seems clear that the incident stands to heighten concerns about the scores of narrowbody and regional jets that have become the workhorses of the air transport industry.