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.
What is the standard for other carriers in recognizing and/or inspecting such a condition? How did SWA not reasonably have knowledge of the potential for such a condition?
ReplyDeleteThe FAA establishes the minimums standards for aircraft inspections. In a classic tail wags the dog scenario, manufacturers and carriers significantly influence the inspection standards issued by the FAA. Even small increases in inspection frequency increase operating costs. Legal liability will attach if other carriers in this instance ignore the new inspection recommendations set by Boeing.
ReplyDeleteSWA probably did not know of the metal fatigue condition. The crack developed over the wing in the roof of the fuselage. It developed from the exterior. The captain or co-pilot conducts a pre-flight inspection, but that is from the ground.
An addition from Jerry Sterns, (www.trial-law.com) counsel to many victims of commercial crashes for many years that further explains the duty and difficulty to detect metal fatigue early:
ReplyDelete"Metal fatigue is always a possibility and must be keenly watched for in any sort of machine that has cyclic stress. An airplane is of course a very prime example. And very vunerable to this phenom. And not all initiation points or small cracks are visble on nonaided inspection from the outside. Full checks require Xray, dye penetrant, and other methods, which are time consuming and expensive, so the airlines would like to avoid them if possible."