Abstract: With many advantages, ratchet clutches are widely used in the drive systems of aero engines. The engine of an aircraft couldn't start up. The failure cause was analyzed based on fault tree analysis method and the operating principle of ratchet clutch. The results show that the main cause of the startup failure of the engine was the failure of the two-speed drive system. The main cause of the two-speed drive system failure was the fatigue cracking of the pawl. Manufacture defects and excessive sulphur promoted the initiation of the fatigue crack of the pawl. Some suggestions to prevent this failure were put forward.