plug in the transmission drain opening attracts any ferrous  particles that may be in the oil. The starter uses turbine oil,  the same as the engine, but this oil does not circulate through  the engine.  the off position.  [Figure 5-24]  The control crank links a  pilot valve that meters pressure to the servo piston, with  the bellows connected by an airline to the pressure-sensing  port on the starter. Turning on the starter switch energizes the regulating valve  solenoid. The solenoid retracts and allows the control crank  to rotate to the open position. The control crank is rotated  by the control rod spring moving the control rod against the  closed end of the bellows. Since the regulating valve is closed  and downstream pressure is negligible, the bellows can be  fully extended by the bellows spring. The ring gear housing, which is internal, contains the rotor  assembly. The switch housing contains the turbine switch  and bracket assembly. To facilitate starter installation and  removal, a mounting adapter is bolted to the mounting pad  on the engine. Quick-detach clamps join the starter to the  mounting adapter and inlet duct.  [Figure 5-22]  Thus, the  starter is easily removed for maintenance or overhaul by  disconnecting the electrical line, loosening the clamps, and  carefully disengaging the drive coupling from the engine  starter drive as the starter is withdrawn. The air path is directed through a combination pressure- regulating and shutoff valve, or bleed valve, that controls all  duct pressure flowing to the starter inlet ducting. This valve  regulates the pressure of the starter operating air and shuts off  the air supply to the engine when selected off. Downstream  from the bleed valve is the start valve, which is used to control  air flow into the starter.  [Figure 5-23] The pressure-regulating and shutoff valve consists of two  subassemblies: pressure-regulating valve and pressure- regulating valve control.  [Figure 5-24]  The regulating valve  assembly consists of a valve housing containing a butterfly- type valve.  [Figure 5-24]  The shaft of the butterfly valve is  connected through a cam arrangement to a servo piston. When  the piston is actuated, its motion on the cam causes rotation  of the butterfly valve. The slope of the cam track is designed  to provide small initial travel and high initial torque when the  starter is actuated. The cam track slope also provides more  stable action by increasing the opening time of the valve. The control assembly is mounted on the regulating valve  housing and consists of a control housing in which a  solenoid is used to stop the action of the control crank in  As the control crank rotates to the open position, it causes  the pilot valve rod to open the pilot valve, allowing upstream  air, which is supplied to the pilot valve through a suitable  filter and a restriction in the housing, to flow into the servo  piston chamber. The drain side of the pilot valve, which  bleeds the servo chamber to the atmosphere, is now closed  by the pilot valve rod and the servo piston moves inboard.  [Figure 5-24]  This linear motion of the servo piston is  translated to rotary motion of the valve shaft by the rotating  cam, thus opening the regulating valve. As the valve opens,  downstream pressure increases. This pressure is bled  back to the bellows through the pressure-sensing line and  compresses the bellows. This action moves the control rod,  thereby turning the control crank, and moving the pilot valve  rod gradually away from the servo chamber to vent to the atmosphere.  [Figure 5-24]  When downstream (regulated) pressure reaches a preset value, the amount of air flowing  into the servo through the restriction equals the amount of  air being bled to the atmosphere through the servo bleed; the  system is in a state of equilibrium. When the bleed valve and the start valve are open, the  regulated air passing through the inlet housing of the starter  impinges on the turbine causing it to turn. As the turbine  turns, the gear train is activated and the inboard clutch gear,  which is threaded onto a helical screw, moves forward as  it rotates; its jaw teeth engage those of the outboard clutch  gear to drive the output shaft of the starter. The clutch is  an overrunning type to facilitate positive engagement and  minimize chatter. When starter cut-out speed is reached, the  start valve is closed. When the air to the starter is terminated,  the outboard clutch gear, driven by the engine, begins to turn  faster than the inboard clutch gear; the inboard clutch gear,  actuated by the return spring, disengages the outboard clutch  gear allowing the rotor to coast to a halt. The outboard clutch  shaft continues to turn with the engine. Air Turbine Starter Troubleshooting Guide Butterfly air control valve Pneumatic valve operating mechanism Figure 5-23.  Regulating and shutoff bleed valve. 5-16 The troubleshooting procedures listed in  Figure 5-25  are  applicable to air turbine starting systems equipped with a 