March 2018
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What an Old Airplane Can Teach Us About Stress Testing

Finding out what it takes to make the wings fall off is important and can be rather surprising

Back in the early days of jet travel, the British got off to an early lead with the first commercial jet airliner, the de Havilland Comet. Metal fatigue is one of the reasons you may never have heard of either the aircraft or its producer. The Comet suffered a series of catastrophic crashes, crashes where the main culprit was metal fatigue.

The investigation team that discovered the problem did so by recreating the conditions that caused the accident: they stressed the aircraft until the wings fell off. It turned out that the forces to which jet travel subjected the frame were much higher than originally projected and the team’s work had a big effect on the future of aircraft design.

The plane may not have been a commercial success, but the method used to uncover the Comet’s weaknesses can be used to great effect elsewhere. The Comet’s designers built an aircraft capable of withstanding what they imagined the aircraft would have to withstand. To put it another way, they made a series of assumptions. This was and is completely normal, except that their assumptions were spectacularly wrong.

The Comet’s engineers are not the first people to have made bad assumptions and will not be the last. What we can learn from them, however, is that stress-testing something until its wings fall off – while it is empty and parked on the ground – beats having the wings come off when it is full of people and thousands of feet up in the air.

Fast forward to 2008

Stress tests figured prominently in the news following the financial crisis in 2008. In the United States, banks were given scenarios against which to test themselves. The aim of the scenarios was to see if they could survive further turmoil – and here is where the Comet plays in.

The scenarios were designed to test the banks survivability – not to see what it would take to break them. The scenario-makers, like the Comet’s original engineers, made a series of assumptions about the future. The data these tests produced told the government many things, but there was much that it did not.

Organizations large and small make predictions every day because they must. Assumptions are extremely useful and we cannot do without them. Both need to be tested, however, and finding out where they are wrong is often more important than finding where they are right.

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