Grounding the 737’s: SWA Flight 812

By ThinkReliability Staff

As new information comes to light, processes need to be reevaluated.  A hole in the fuselage of a 15-year-old Boeing 737-300 led to the emergency descent of Southwest Airlines Flight 812.  737’s have been grounded as federal investigators determine why the hole appeared.  At the moment, consensus is that a lap joint supporting the top of the fuselage cracked.

While the investigation is still in the early stages, it appears that stress fatigue caused a lap joint to fail.  Stress fatigue is a well known phenomenon, caused in aircraft by the constant pressurization and depressurization occurring during takeoff and landing.  Mechanical engineers designing the aircraft would have been well aware of this phenomenon.  The S-N curve, which plots a metal’s expected lifespan vs. stress, has been used for well over a century.

Just as a car needs preventative maintenance, planes are inspected regularly for parts that are ready to fail.  However, the crack in lap joint wasn’t detected during routine maintenance.  In fact, that joint wasn’t even checked.  It wasn’t an oversight however.  Often the design engineers also set the maintenance schedule, because they hold the expertise needed to determine a reasonable procedure.  The engineers didn’t expect the part to fail for at least 20,000 more flight hours.  At the moment, it’s unclear why that is.

In response to the incident, the FAA has grounded all similar aircraft and ordered inspections of flights nearing 30,000 flight hours.   Cracks have been found in 5 aircraft of 80 grounded aircraft so far.  However a looming concern is how to deal with 737’s not based in the United States, and therefore outside the FAA’s jurisdiction.