The Difference Engine: Old before their time

The first to appreciate the catastrophic effects of stress-reversals were railway engineers in the 1840s. Broken axles caused countless accidents as railway lines crept across Europe and America. Being simply a rotating horizontal shaft with a heavy vertical load on it, early locomotive axles suffered severe stress reversals in their outer skins with every rotation. William Rankine, a Scottish engineer and one of the fathers of thermodynamics, was the first to explain how these repeated stress reversals could cause cracks to propagate. By the 1850s, the steam-engine pioneer James Braithwaite had coined the term “metal fatigue”.

The irony is that the lesson had to be relearned a century later. This time it was aircraft manufacturers who suffered the consequences. Their troubles began in the 1950s when they started flying higher and needed to pressurise the cabins of their passenger planes. Two de Havilland Comet aircraft—the world's first commercial jet—broke up mysteriously in mid-air in 1954. Though it all but destroyed de Havilland, the disaster gave the industry crucial insights into how metal fatigue can rip an aircraft suddenly apart. It also taught them how to prevent stresses concentrating at certain points, thereby triggering a fatal tear in the aircraft's skin.

Over the past few weeks, aircraft engineers have found they do not know quite as much about metal fatigue as they thought. The source of the problem that forced the Boeing 737-300 used on the Southwest Airlines flight 812 from Phoenix to Sacramento to make an emergency landing on April 1st, following a five-foot rent appearing in the upper-fuselage skin, has flummoxed engineers and safety officials alike.

By all accounts, it should not have happened. Admittedly, every time an airliner takes off and lands it goes through a demanding cycle of stress reversals. Like the paper-clip, the airframe and its alloy skin are stressed first in one direction as the cabin is pressurised while climbing to its cruising altitude, and then in the opposite direction when depressurised during descent for landing. On average, the short-haul aircraft used by Southwest, a budget carrier based in Dallas, do that half a dozen times a day—year in, year out.

The Boeing 737-300 in question was only 15 years old when its skin peeled open along a riveted lap-joint while flying above 34,000 feet (just over 10,000 metres) with 118 passengers on board. The failure caused the cabin to lose pressure instantly and the oxygen masks to deploy. Within minutes, the pilot had got the plane down to 11,000 feet, where the passengers could begin to breath normally again. Shortly thereafter, the plane landed at a military base without further mishap or serious injury.

Much has been made of the 737-300's age. But a commercial aircraft that is 15 years old is still in its prime of life. The real issue is the way Southwest works its fleet so aggressively, specialising in rapid turnarounds. As a result, the plane concerned had accumulated nearly 40,000 flight cycles. An aircraft of that type and age would normally be expected to have logged little more than 30,000 flights.