One of the first commercial jet aircraft was the British de Havilland Comet, which first flew in 1952. By 1954, seven of the Comets had crashed under unknown circumstances. Since only 21 had been built, the one-third failure rate was catastrophic. The impact of the Comet crashes was the loss of Britains early leadership in commercial passenger jet aircraft.
One plane crashed about a half hour after takeoff from Rome, with the wreckage falling into the sea between the islands of Elba and Monte Cristo. The Tyrrhenian Sea was shallow enough for the British Royal Navy to recover the wreckage.
While the investigation was continuing, fifty structural modifications were made to the Comets. They were cleared to fly again, but within two weeks another crashed.
Researchers assembled and examined the wreckage. A large number of theories including sabotage and pilot error were considered and discarded. The hypothesis that best fit the circumstances was metal fatigue.
The Comet had square windows, like an earlier unpressurized propeller-driven passenger planes. The square windows had severe stress concentrations at the corners, which increased stresses and reduced fatigue life. Examination of the wreckage showed cracks initiating at the corners of the windows and propagating into the plane’s fuselage. Stress concentrations and fatigue loads, as at the Point Pleasant Bridge, make a deadly combination. Today, aircraft windows are round or oval, without sharp corners.
The solutions are fatigue resistant design, coupled with an extensive program of inspection and maintenance. Bridges, aircraft, and other structures subjected to fatigue are designed to eliminate stress concentrations as much as possible. Smooth transitions are used where lines of stress change direction. Connections and details prone to fatigue are identified, and maintenance schedules are followed to inspect or replace these components periodically.
See Chapter 3 of the book Beyond Failure: Forensic Case Studies for Civil Engineers, Delatte, Norbert J., ASCE Press 2009.
The Comet crashes are covered in one chapter of Levy and Salvadori (1992, pp. 121-126). In addition, a chapter of Freiman and Schlager (1995, pp. 78-86) covers the incidents. This case study is also featured on the History Channel Modern Marvels Engineering Disasters 4 videotape/DVD.
