Enhanced Air Turbine Starter Clutch Design

Since introduction into service, the B737-300 starter has been susceptible to clutch damage. At the onset of the problem, a team was formed to identify the causes of clutch damage and shear shaft failures and to define a corrective action. Several in-service airplanes were equipped with recording instrumentation, and data from over 10,000 start cycles was recorded. It was noted that large pneumatic duct pressure fluctuations from a variety of causes played an important role in the reason for the starter reliability problem. After understanding the problem, a design improvement program was launched to replace the current pawl-and-ratchet clutch with an inner-race overrunning sprag clutch that would be capable of withstanding resulting duct pressure fluctuations.