Yesterday, the FAA released AD 2011-08-51 in response to the in-flight fuselage rupture experienced by Southwest Airlines flight 812 an April 1st. The AD calls for NDT inspections of 737 classic models (-300, -400, -500) with more than 30,000 flight cycles, and every 500 flight cycles after that.
So, what does that mean? A flight cycle is essentially the same as a flight. When an aircraft takes off, climbs to cruise altitude, descends and lands, that is all counted as one flight cycle. So this AD only applies to aircraft that have performed 30,000 flights or more (the Southwest aircraft involved in the fuselage rupture incident had accumulated 39,781 flight cycles).
What about the NDT inspections? When checking for cracks in metal that cannot be detected by the naked eye, a technique called eddy current testing is performed. Essentially what happens is a low electrical current is put into the metal at one point, and the current is read at another point. By knowing what the current should be in a new piece of metal, the tester can determine if there are any flaws or cracks in the metal being tested, and the severity of any flaws or cracks.
Why weren’t these aircraft already being checked for cracks? Well, they were. They just weren’t being checked in the location where the problem occurred last Friday. Why not? The simple answer is that no aircraft had experienced problems with fatigue cracks in that area before, so no one thought to look there. It is unfortunate, but a lot of the regulations involving aviation are written in blood. Typically someone has to be hurt or killed, or placed in a dangerous situation as happened on Friday, for attention to be drawn to a previously unknown and unconsidered problem. Thankfully Friday’s Southwest flight ended without any injuries, and hopefully the inspections required by the FAA’s AD 2011-08-51 will catch any more problems before they get to the point where the tops of aircraft are peeling open in flight.