Accident Inquiry
The industry had experienced the effect of fatigue on wings. Working closely with the RAE, Comet wings were subjected to loads in continual cycles throughout 1949-1951. Smaller components were tested in a decompression chambers, and larger sections of fuselage were immersed in a specially-built water-tank. At the moment of failure, rather than being blown to bits in an uncontrolled explosion, the water acted as a damper, “freezing” the damage, so engineers could find the cause. However, pressure testing was about ultimate strength and leak resistance, not fatigue.
The idea that further testing of pressurised aircraft might be necessary was formed during an investigation for the RAF in 1952. In October 1952, the Ministry of Supply proposed static tests at 2P, then 1.33P, followed by continued dynamic testing to 1.25P for 10,000 cycles. This was also taken up by the Air Registration Board, who published civil requirements in June 1953, but raised the test to 15,000 cycles. In July 1953, de Havilland tested the forward section of a Comet fuselage to the new proposals. In fact it took 18,000 cycles before any damage occurred. There was still no cause for concern.
Following the tragedies at Elba and Naples, an investigation began at the RAE in Farnborough, lead by Sir Arnold Hall. A BOAC Comet was tested by continual cycles to 1.33P. After a a total of 3,060 flights, real and simulated, the fuselage failed at the corner of the forward port cabin window. This was similar to G-ALYP, except that the crack had started at the ADF antenna windows at the top of the fuselage, and run down to the forward port window.
| As a result of the inquiry, a new rig was built so a complete Comet 4 airframe could be subjected to the stresses of one flight every 4 minutes. | BAE SYSTEMS |
A court inquiry followed the investigation. They decided that the Comet fuselage had indeed been too weak. However, de Havilland had acted with exemplary diligence, exceeding certification requirements. Given the scientific knowledge at the time, what happened was not reasonably foreseeable, and de Havilland were exonerated.
What was puzzling was why this weakness had not been discovered by de Havilland’s own tests, or while following the ARB schedule in 1953. Sir Arnold Hall suggested that by applying 2P pressure initially, components had been strengthened, effectively “fatigue proofing” them. The outcome was published world-wide, causing widespread changes in how manufacturers designed for fatigue, and how it was tested.
De Havilland worked hard to rebuild their reputation. After the inquiry, a Comet 4 was announced with a complete structural redesign. This proved to be extremely robust, restoring its reputation, and enjoyed 22 years service with civil airlines, the last scheduled flight being with Dan Air in November 1980. The last Comet, XS235 “Canopus” was finally withdrawn from service with the RAF in 1997. The Comet 4 is also the basis of the Hawker-Siddeley Nimrod, the first two of which were converted directly from Comet 4Cs. The youngest Nimrod was built in August 1969, and latest estimates are that the oldest will remain in service until 2040.