De Havilland Comet-Disaster Project Group 6 Background The de Hallivand Comet of Britain was a revolution in the civil aviation industry It was the first commercial airliner to use turbojet engines, to have glued skin panels (Redux), to have a highly pressurised cabin (8.25 psi) and to use high pressure re-fuelling. However the De Havilland name is relatively unknown compared with market leading Boeing, who’s 707 only made its first test flight more than 2 years after Comet’s first commercial flight! So what happened? The Comet's story began in 1929, when a Royal Air Force officer Frank Whittle proposed powering an airplane with a gas turbine engine. Rather than applying power to a propeller, this engine would force exhaust gases out the rear of the engine, generating enough force to drive the aircraft forward. Eleven years of research and experimentation produced Britain's first jet fighter, the Gloster Meteor. By 1952, the de Havilland Comet, the world's first commercial jetliner went into service. Rather than lumbering through storms, the Comet flew above the weather, eight miles up in the stratosphere. Its air-conditioned, fully pressurized cabin provided passengers with a quiet, smooth ride previously unheard of in commercial aviation. Design The Comet was an all-metal low-wing cantilever monoplane powered by four jet engines. The first Comet would carry 36 passengers at a cruising speed of 725 kph. The clean, low-drag design featured many unique design elements; a swept leading edge, integral wing fuel tanks. Two pairs of Halford’s Ghost 50 Mk1 turbojet engines were buried in the wings close to the fuselage. This configuration avoided the drag of podded engines and allowed a smaller fin and rudder. The hazards of asymmetric thrust were reduced. The engines were mounted higher in the wings. This reduced the risk of ingestion damage, a major problem for turbine engines. These benefits were compromised due to increased structural weight including armour for the engine cells in case of an engine explosion and a more complicated wing structure. The Comet's thin metal skin was composed of advanced new alloys and was chemically bonded using the adhesive Redux and riveted. This saved weight and reduced the risk of fatigue cracks spreading from the rivets. Failures On 8 April 1954, the Comet G-ALYY ("Yoke Yoke"), on route from Rome to Cairo, crashed into the waters near Naples. 60 structural items that were thought to be suspect were reinforced. However it was only after the second major crash (Comet G-ALYY) were thorough tests carried out. The Comet (Yoke Uncle) was tested in Farmborough for fatigue failure. The cabin was supported in water to avoid extraneous weight effects and used to pressurize the cabin. Calculations indicated that the energy release under cabin rupture using air pressurisation would be similar to a 2.3kN bomb exploding in the cabin Under this form of pressurisation the Comet failed after 3057 cycles, (1221 actual and 1836 simulated) well short of its design cycles of 18,000. What went wrong? This discrepancy was attributed to the manner in which the Comet was tested. Instead of the full-scale fuselage structure, only a part of the cylindrical fuselage fixed at the rigid steel wall was initially provided for the internal pressurization fatigue test. When designed, the pressure tests were carried out as static tests with 114kPa (twice operating pressure) being applied. This did not take into the account the true loading that would be applied to the Comet during flights. In Farmborough the fuselage was hydraulically pressurised in cycles as well as the wings being flexed with jacks to simulate flight loads. Under this true loading, failure would occur at a much earlier stage, i.e. around 3000 cycles. This underestimation of the true loads involved meant that there was a gross error in the design life of the Comet. Technical Failures The final RAE report determined that the crashes were a result of: Fatigue cracks associated with the stress concentrations at the corner of the windows (due to their sharp edges) with stresses of 315MPa at the edge of the window and a bolthole around the window of 70MPa stress at the bolt position. The windows were almost rectangular, with small radius at each corner. Consequently, the stress concentration factor was high, and fatigue cracks initiated readily. The possible contribution from out-of-plane bending loads (bi-axial stresses). Improperly understood failure mode assessment procedures necessitated by new technology. Poor configuration due to wing root engine placement affecting updating potential, fire hazard, and structural integrity in the event of engine disintegration. Full-scale water tank testing of a comet (left). The fuselage of the comet (right). Note the square windows. Managerial and Ethical failures A decision was made to set up the production line before the prototype had flown. Before it flew, agreements to supply 14 aircraft to the British Overseas Airways Corporation and two to the Ministry of Supply had been signed. Usually prototypes went through 7 years of testing before widespread implementation could begin. The comet was rushed through in a mere 3 years. It took several accidents before the decision was made to ground the fleet. Many lives could have been saved if De Havilland had fully investigated the problems. Though never conclusively proved it has been suggested that De Havilland knew there could be problems with the planes take-off performance and was relying on the skill of the pilot to over-ride this. Lessons learned The Comet’s pressure cabin had been designed to a safety factor comfortably in excess of that required by British Civil Airworthiness Requirements (2.5 x P as opposed to the requirement of 1.33 x P) 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 far higher than had been appreciated, especially around sharp-cornered cutouts, such as windows. As a result, future jet airliners would feature windows with rounded corners, the curve eliminating a stress concentration. From these accidents we have learned how to better prepare against fatigue failure and we have a better understanding of the interaction of stress concentrations with fatigue Technological Outcomes: Full-scale and dynamic testing of aircraft structures utilised in future aircraft. Better understanding of fatigue testing achieved, i.e. match service and test loads (no previous over-pressurisation cycles first). Attention drawn to detect ability/critical size issues for fatigue cracks in aircraft structures. Concept of ‘one-bay’ crack tolerance in fuselage probably formulated. (Water tank to encase a concept of ‘one-bay’ crack tolerance in fuselage. First large scale reconstruction, use of underwater cameras and medical forensics as part of an accident investigation. For the development of new aircraft, two full-scale structures must be provided for testing: one is for the static load test, and the other is for the fatigue test including the pressurization test In conducting the full-scale fatigue tests, in addition to the effect of the magnitude and number of fatigue loads, the effect of the load sequence must be taken into consideration. Stress concentration factors of cut outs and notches must be correctly evaluated, and they must be minimized in the design of the structure as much as possible. Conclusion At the time of the accident investigation, the effect of pressure overloads on fatigue life had not been correctly recognized. To make progress engineers must be prepared for the possible risks involved and endeavour to foresee and prevent them. They will invariably be unable to foresee and prevent them all. However, there were significant errors in the management of the project, which resulted in the needless loss of more lives References www.worldiq/definiton/De_Havilland_Comet http://www.rafmuseum.org.uk http://www.tech.plym.ac.uk/sme/Interactive_Resources/tutorials/FailureCases/sf2.html P A Withey (1997) Fatigue failure of the De Havilland Comet 1 Engineering Failure Analysis Vol. 2 No. 4 J F Lancaster (1996) Engineering catastrophes: causes and effects of major accidents Woodhead Publishing. http://www.hq.nasa.gov/office/pao/History/SP-468/ch13-2.htm http://surf.to/comet http://www.dlyoung.freeserve.co.uk/DH106/comet_1inq.htm www.geocities.com/CapeCanaveral/Lab/8803/fcometcr.htm www.wordiq.com/definition/De_Havilland_Comet www.geocities.com/Heartland/Ridge/1747/timessumm.htm http://www.theknownuniverse.com/comet.htm http://www.tech.plym.ac.uk/sme/Interactive_Resources/tutorials/FailureCases/sf2.html http://shippai.jst.go.jp/en/Detail?fn=0&id=CB1071012& http://www.aerospaceweb.org/aircraft/jetliner/comet/