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While developing a new tyre pressure monitoring system for the landing gear of the Boeing 777 aircraft, Crane Aerospace & Electronics specified Victrex® PEEK™ polymer reinforced with short glass fibres for the hubcaps containing the system.
(Published on June 2008 – JEC Magazine #41)
PEEK was chosen because of the material’s high temperature resistance and high mechanical strength, combined with its good chemical resistance to the caustic elements found on the runway. The PolyEthylEtherKetone (PEEK) is a linear aromatic polymer which is semi-crystalline and widely regarded as the highest performance thermoplastic available today. Victrex PEEK and compounds typically have a glass transition temperature of 143°C and a melting temperature of 343°C. Independent tests have shown that Victrex PEEK exhibits a heat distortion temperature up to 315°C (ISO R75, glass fibre filled) and a Continuous Use Temperature of 260°C (UL 746B). The tyre monitoring system on the Boeing 777 is designed to ensure proper tyre air pressure for take-off and landing.
The hubcaps include a conduit which allows a rim-mounted pressure sensor to communicate with an onboard system, providing real-time data to the cockpit. Maintaining proper tyre pressure is critical to aircraft safety. Temperatures at high altitude can be as low as -54°C whereas temperatures on braking can reach more than 200°C. The landing gear must be able to withstand repeated hard landings and impacts from flying debris. In addition, it is exposed to both ozone and chemicals in jet fuel, hydraulic fluid and de-icing solutions. Weight also played a major part in the selection of Victrex PEEK polymer to replace aluminium for the hubcaps. Each hubcap now weighs less than 7 kg. With 14 wheels per plane, the weight reduction is considerable.