Air control valve Airframe pylon Engine air starter External gearbox High pressure air Exhaust air Cross feed from running engine Auxiliary power unit (A.P.U.) Ground start supply Figure 5-20.  Air turbine starters are supplied by ground cart, APU, or another operating onboard engine.  The turbine housing contains the turbine rotor, the rotor  switch actuator, and the nozzle components that direct  the inlet air against the rotor blades. The turbine housing  incorporates a turbine rotor containment ring designed to  dissipate the energy of blade fragments and direct their  discharge at low energy through the exhaust duct in the event  of rotor failure due to excessive turbine overspeed.  Figure 5-21  is a cutaway view of an air turbine starter. The  starter is operated by introducing air of sufficient volume and  pressure into the starter inlet. The air passes into the starter  turbine housing where it is directed against the rotor blades by  the nozzle vanes causing the turbine rotor to turn. As the rotor  turns, it drives the reduction gear train and clutch arrangement,  which includes the rotor pinion, planet gears and carrier, sprag  clutch assembly, output shaft assembly, and drive coupling.  The sprag clutch assembly engages automatically as soon  as the rotor starts to turn but disengages as soon as the drive  coupling turns more rapidly than the rotor side. When the  starter reaches this overrun speed, the action of the sprag  clutch allows the gear train to coast to a halt. The output  shaft assembly and drive coupling continue to turn as long  as the engine is running. A rotor switch actuator, mounted in  the turbine rotor hub, is set to open the turbine switch when  the starter reaches cutout speed. Opening the turbine switch  interrupts an electrical signal to the start valve. This closes  the valve and shuts off the air supply to the starter. The transmission housing contains the reduction gears, the  clutch components, and the drive coupling. The transmission  housing also provides a reservoir for the lubricating oil.  [Figure 5-22]  Normal maintenance for air turbine starters  includes checking the oil level, inspecting the magnetic chip  detector for metal particles, and checking for leaks. Oil can  be added to the transmission housing sump through a port in  the starter. This port is closed by a vent plug containing a ball  valve that allows the sump to be vented to the atmosphere  during normal flight. The housing also incorporates a sight  gauge that is used to check the oil quantity. A magnetic drain  5-14