De Havilland Comet Accidents Is the Importance of Proper Allowable Testing

De Havilland/ BOAC Comet Accidents 1954 No prototype Sir Geoffrey de Havilland built the first commercial jet that reached production, the Comet. On January 10, 1954 a Comet broke up in mid-air. Flights were temporarily voluntarily suspended, then resumed. On April 8, 1954, another Comet broke up in air. (Both flights were taking off Testing from Rome.) The lives of 56 passengers and crew were lost in these two incidents, as well as two planes. Additionally, the prestige of the Inadequate test performed on British aviation industry suffered a blow. (I'll consider the lost prestige of British aviation a customer service impact.) program section with AND extended life Let's look at this incident in a Cause Map. A thorough root cause analysis built as a Cause Map can capture all of the causes in a simple, intuitive format that fits on one page. Although there were two separate plane breakups, the Cause Maps are the same (based on the analysis and investigation performed after the accidents). Essentially, the two planes were lost due to a structural failure of the cabin, due to Misleading fatigue cracking. fatigue tests AND AND The fatigue cracking of the cabin occurred because the actual pressure cycles exerted on the cabin were more than the allowable (or where cracking would occur). This was because the allowable pressure cycles were miscalculated. The allowable pressure cycles were miscalculated for several reasons. First, the inadequate test program. There was no prototype, and the fatigue tests were misleading. One test used a section Testing results that was effectively pre-conditioned, extending its life. In another test, the section tested was so small that the test results were influenced by influenced by boundary conditions. boundary conditions Pressurization stresses Next, the actual stress was above the predicted stress. This occurred because the pressure stresses were distributed unevenly, and the actual distributed unequally stresses were increased in some localized areas. Additionally, while the comet was being developed, there was a general lack of knowledge Actual stress about fatigue. A last problem was that the design of the Comet stretched the bounds of experience. The comet was designed to fly at twice the above predicted speed of other airliners, at twice the height, and at twice the cabin pressure (for passenger comfort). stress AND Probably the most important lesson to come from the de Havilland Comet accidents is the importance of proper Allowable testing. Once the cause was discovered, the Comet was redesigned and flew successfully, although by then Boeing presure cycles Actual stress had mostly taken over the market share. It's tragic that these accidents had to occur before the problem was solved. miscalculated AND increased in localized areas Why? Effect Cause Lack of knowledge about fatigue Safety Goal 56 passengers, NOTE: Read the Cause Map from left to right Impacted crew killed with the phrase "Was Caused By" in place of Flew at twice Pressure cycles the speed of Structural other airliners on cabin AND failure of Fatige cracking Matls/Labor exceed AND pressure cabin Goal allowable Impacted AND Loss of 2 planes Cause Map Stretched Flew at twice Detail Level beyond bounds the height of of experience other airliners Customer Goal Lost prestige of Impacted British aviation AND Houston Office 281-489-2904 www. ThinkReliabi lity.com Twice the cabin pressure of previous aircraft Copyright ThinkReliability 2009 Pressurization Aircraft cabin and sees pressure depressurizing cycles of cabin.

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