What causes the metal skin on an airliner to tear apart?

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May 17, 2011

On Friday, April 1, 2011, a Southwest Airlines Boeing 737-300 experienced a rapid decompression when the metal fuselage skin tore apart, leaving a 5X1 foot hole. This kind of event is rare, but can, and has been, catastrophic when it does occur.

In 1988, another Boeing 737, a 200 model, experienced a similar, but much worse, event over Hawaii. In that case, the skin on the entire front section of the aircraft cabin was denuded, leaving the passengers sitting in the first several rows completely exposed, with the aircraft being held together by two metal spars running through the floor.

Everyone, except one flight attendant, was securely strapped in their seats by their seat belts. Consequently, and unfortunately, the flight attendant was ejected from the aircraft and fell to her death from 24,000 feet. Had there been any passengers not belted in at the time, they would have suffered the same fate. Miraculously, the flight crew was able to safely land the aircraft without further mishap.

The cause of this kind of event is metal fatigue. Over the course of the life of an airliner, the metal skin is exposed to thousands of forceful expansions and contractions when the aircraft is pressurized and depressurized with each flight. Additionally, the skin is exposed to extreme temperature variations over short periods of time, as the temperature of the air decreases by 3.5 decrees F. for each 1000 feet of altitude, on average. This further exacerbates the expansion and contraction of the metal skin.

Just as bending a metal paper clip back and forth eventually causes the metal to fatigue, crack and come apart, so did the metal skin of these aircraft upon expanding and contracting thousands of times. In response, the Federal Aviation Administration issued an AD (Airworthiness Directive) 2011-08-51, which mandates inspection of the fuselage skin of certain older Boeing 737s. The inspections require the use of "eddy current" technology, which is designed to detect micro-cracks otherwise invisible to the naked eye, together with instructions to repair any cracks found. Should these cracks not be fixed, they will propagate and eventually compromise the integrity of the metal and eventually fail.

Although a rare event, the gravity of the potential consequences should cause the FAA to step-up these kinds of inspections, making them more frequent and more thorough, instead of waiting for an emergency situation to require such safeguards. Unfortunately, the FAA is notorious for waiting until the horse gets out before closing the barn door.