BOAC Flight 781 - Metal Fatigue

Metal Fatigue

Initial examination and reconstruction of the wreckage of G-ALYP revealed several signs of inflight break-up:

• Shreds of cabin carpet were found trapped in the remains of the Comet's tail section
• The imprint of a coin was found on a fuselage panel from the rear of the aircraft
• Smears and scoring on the rear fuselage were tested and found to be consistent to the paint applied to the passenger seats of the Comet

When most of the wreckage was recovered, investigators found that fractures started in the roof of the cabin, a window then smashed into the elevators, the rear fuselage then tore away, the outer wing structure fell, then the outer wing tips and finally the cockpit broke away and fuel from the wings set the debris on fire.

To find out what caused the first failure, BOAC donated G-ALYU ("Yoke Uncle") for testing. The airframe was put in a large water tank, the tank was filled, and water was pumped into the plane to simulate flight conditions. The experiment occurred 24 hours per day, 7 days per week. It could possibly have taken as long as five months. A Seconds From Disaster documentary episode described this task as "grueling". If the hypothesis had been tested in the 2000s, investigators would have used computer technology to determine the effect of pressurization.

After the equivalent of 3,000 flights investigators at the RAE were able to conclude that the crash had been due to failure of the pressure cabin at the forward ADF window in the roof. This 'window' was one of two apertures for the aerials of an electronic navigation system in which opaque fibreglass panels took the place of the window 'glass'. The failure was a result of metal fatigue caused by the repeated pressurisation and de-pressurisation of the aircraft cabin. Another fact was that the supports around the windows were riveted, not glued, as the original specifications for the aircraft had called for. The problem was exacerbated by the punch rivet construction technique employed. Unlike drill riveting, the imperfect nature of the hole created by punch riveting caused manufacturing defect cracks which may have caused the start of fatigue cracks around the rivet. The investigators examined the final piece of wreckage with a microscope.

The Comet's pressure cabin had been designed to a safety factor comfortably in excess of that required by British Civil Airworthiness Requirements (2.5x P as opposed to the requirement of 1.33x P and an ultimate load of 2x P, P being the cabin 'Proof' pressure) and the accident caused a revision in the estimates of the safe loading strength requirements of airliner pressure cabins.

In addition, it was discovered that the stresses around pressure cabin apertures were considerably higher than had been anticipated, especially around sharp-cornered cut-outs, such as windows. As a result, future jet airliners would feature windows with rounded corners, the curve eliminating a stress concentration. This was a noticeable distinguishing feature of all later models of the Comet.

52 years after the incident, Paul Withey, an aviation expert, went to the Science Museum in London to examine the aircraft parts with 2000s technology. The aircraft piece was mounted on a plate. Withey received an impression of the aircraft part using a silicone-based putty. At Imperial College London in London Withey used an electron microscope to examine the putty impression of the fatigue crack. He found a manufacturing defect when zooming to 800 times and confirmed that Sir Arnold Hall's investigation was correct.