Beyond understanding – pushing technology too far

Recently I joined a tour of the Science Museum’s huge collection stored near Swindon. Among the 300,000 objects there was this section of the roof of a Comet 1 airliner (registration G-ALYP).

The de Havilland Comet was the world’s first jet airliner, unveiled in 1949. It could fly higher and faster than piston-powered planes, and cut journey times massively. Power from the Halford Ghost turbojets was limited, so to achieve a decent power to weight ratio novel materials and methods were needed to keep the aircraft as light as possible. And of course, to fly at high altitudes it had to have a pressurised cabin.

The section was recovered from the wreckage of G-ALYP which crashed into the Mediterranean Sea in January 1954. In April that year another Comet crashed into the sea near Naples, and the whole fleet was grounded.

Investigations and tests on a complete aircraft in a giant water tank revealed that both aircraft had experienced explosive decompression due to metal fatigue. On G-ALYP the flexing of the structure as the plane expanded and contracted through pressurisation cycles caused tiny cracks to form at the corners of an aperture surrounding the automatic direction finder. Over further cycles these worked their way from rivet hole to rivet hole until they violently spread and tore open the fuselage. Look closely at the picture and you’ll clearly see what happened.

When the Comet was designed aircraft engineers knew about metal fatigue, and the risks from repeated pressurisation cycles. In fact, extensive metal fatigue testing was carried out to prove the design was safe. The trouble was, it was known about, but not completely understood. The post-crash investigations expanded that knowledge, and other firms – notably Boeing and Douglas in the US – were able to develop the first generation of dependable jet airliners.

All that happened seventy years ago, yet there are more recent examples of where technology was not quite as fully understood as we thought it was. Remember the Millennium Bridge in London, a low-slung pedestrian suspension bridge across the River Thames?

Everyone knows that Roman soldiers broke step as they crossed a bridge to avoid setting up a natural vertical vibration frequency that might cause the bridge to collapse. The Millennium Bridge was designed with hundreds of years of that know-how in mind. However, when people started walking over the bridge it set up a pattern of horizonal oscillations which made them feel nauseous. Getting to the cause of this motion and fixing it was expensive, but also pushed bridge technology forward.

Any project is a step into the unknown to some extent. And these examples show that however skilled and careful we are there is a danger that solving a problem in a new way will lead to an unexpected crisis. What are the downsides of using proven technology versus the potential prize (or price) of being a pioneer? Learn more at 8 – Handling risk – Zavanak