BOAC Flight 781BOAC Flight 781Fatigue failureFatigue failure

By By Kandagatla vinay kumarKandagatla vinay kumar

sandeepsandeep

INTRODUCTIONINTRODUCTION BOAC Flight 781 is an air craft of de Havilland DH.106 BOAC Flight 781 is an air craft of de Havilland DH.106

Comet 1.Comet 1.

INCIDENT: The incident took place in 10 January 1954,INCIDENT: The incident took place in 10 January 1954, when it took off from Ciampio airport in when it took off from Ciampio airport in

Rome, Rome, Italy and on the way to Heathrow Italy and on the way to Heathrow airport in airport in London, England. London, England.

GENERAL ASSUMPTIONSGENERAL ASSUMPTIONS

After approaching 27000 feet high the aircraft suffered an explosive decompression and crashed into the Mediterranean sea.

Gerry Bull, a former BOAC engineer, said that when he inspected the aircraft in Rome he looked for "incidental damage". He did not find any, so he believed Flight 781 was fit to flight.

INVESTIGATIONINVESTIGATION

To find out what caused the first failure, BOAC donated G-ALYU ("Yoke Uncle") for testing.

The airframe was put in a huge water tank and water was pumped into the plane to simulate flight conditions.

Comet testing tankComet testing tank

TESTING PROCESSTESTING PROCESS In order to probe the origin of cabin explosion the cabin is In order to probe the origin of cabin explosion the cabin is

subjected to alternate pressurization and De-pressurization subjected to alternate pressurization and De-pressurization over 57 kpa over atmospheric pressure by repeatedly over 57 kpa over atmospheric pressure by repeatedly pumping water in to it and removing it. During such pumping water in to it and removing it. During such simulated cabin pressurization the wings of an aircraft were simulated cabin pressurization the wings of an aircraft were also stressed by hydraulic jack to mimic wing loading also stressed by hydraulic jack to mimic wing loading during typical flight conditions after about 3000 during typical flight conditions after about 3000 pressurization cycles a fatigue crack originating in the pressurization cycles a fatigue crack originating in the cabin window until the metal skin was pierced,cabin window until the metal skin was pierced,

SECTIONAL VIEW OF CRACKSECTIONAL VIEW OF CRACK

Causes of FailureCauses of Failure Extensive testing gave the Extensive testing gave the

following reasons for failure:following reasons for failure: The failure was a result of The failure was a result of

metal fatigue caused by the metal fatigue caused by the repeated pressurisation and repeated pressurisation and de-pressurisation of the air de-pressurisation of the air craft cabin.craft cabin.

Many investigators were able Many investigators were able to conclude that the crash has to conclude that the crash has been due to failure of window.been due to failure of window.

MANUFACTURING DEFECTSMANUFACTURING DEFECTS

Another fact was that the supports around the windows Another fact was that the supports around the windows were only riveted not glued, as the original specifications of were only riveted not glued, as the original specifications of the air craft.the air craft.

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 fatigue cracks to start around the rivet. The investigators examined the final piece of wreckage with a microscope and found this defect.

CONCLUSIONCONCLUSION

To overcome these defects the design of aircraft To overcome these defects the design of aircraft components must be done by considering pressures at components must be done by considering pressures at high altitudes and it should be examined thoroughly.high altitudes and it should be examined thoroughly.

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