Southwest grounded 79 Boeing 737 to perform inspection and maintenance of the outside skin of the aircraft after Flight 812 developed a hole while at cruising altitude. The airline is concerned about a condition known as aircraft skin fatigue. The crack developed through the body of the fuselage over the wings of the aircraft.
Metal fatigue, a larger category to aircraft skin fatigue, is a known problem in modern aircraft design and maintenance. Metal fatigue is a condition that occurs in airplanes because the exterior metal skin endures expansion and contraction of the aircraft every time it reaches cruising altitude and descends again.
The strange twist involving the Southwest Boeing 737-300 is that the manufacturer did not expect any metal fatigue, nor require any inspections for metal fatigue until the aircraft reached 60,000 takeoff and landing cycles. The aircraft of Southwest Flight 812 had only about 40,000 cycles under its belt. Boeing has since lowered the threshold from 60,000 cycles to 30,000 cycles to inspect the 737's skin for signs of metal fatigue.
Boeing does not have an explanation for how the crack in the skin of the aircraft developed, nor any rational for recommending additional inspections. While many at the FAA believed Federal Regulations solved the metal fatigue problem, this recent Southwest incident raises concerns over the current inspection regimen. I once heard an aviation crash investigator make the obvious but profound comment that "aviation does not like mysteries." In other words, until we solve the mystery of exactly why the skin of the Boeing 737-300 failed on the Southwest flight over Arizona we are at risk to repeat the incident, but with far worse consequences.