TM 1-1520-238-10 2-58 a. Rotor Brake (RTR BK) Switch. The rotor brake is controlled by the RTR BK switch on the pilot PWR lever quadrant (fig 2-19). The switch has three positions: OFF, BRAKE, and LOCK. When set at LOCK with full utility hy- draulic system pressure, the brake prevents the gas tur- bines from driving the power turbines when both engines are at idle. When the rotor is stopped, the switch may be set at LOCK which causes solenoid valves in the manifold to deenergize and all available utility hydraulic system or accumulator pressure to be applied to the brake. A system of interlocks prevents the rotor brake from being locked when the PWR levers are in any position except IDLE and OFF. When the switch is set at BRAKE, solenoid valves in the utility hydraulic manifold operate to actuate the brake. With the switch at OFF, the only hydraulic pressure to the brake is 30 psi from the pressurized air system which, when operating, pressurizes the return side of the utility hydraulic system. b. Rotor Brake Solenoid and Pressure Switch. Rotor brake electrical solenoids and the pres- sure switch receive 28 vdc from the emergency dc bus through the RTR BRK circuit breaker on the pilot over- head circuit breaker panel. If this circuit breaker is open, or if helicopter emergency electrical power is lost for any reason, the rotor brake, if previously set at LOCK, will re- main locked as long as accumulator pressure is available. 2.40.4 Tail Rotor Drive Shafts. There are four tail rotor drive shaft sections. Three tail rotor drive shafts lead from the transmission to the intermediate gearbox. Two are of equal length. The last shaft is installed on the vertical sta- bilizer between the intermediate and tail rotor gearboxes. Hanger bearings support the longer shafts. They are cov- ered by aerodynamic fairings which may be opened for maintenance and inspection. The two equal-length shafts incorporate friction dampers and antiflails. Flexible cou- plings, attached to the shaft ends, are capable of accom- modating shaft misalignments throughout the power range. CAUTION Prolonged out of ground effect hovering (20 – 30 min.) with outside air tempera- tures above 75 F (24 C) may cause the intermediate gearbox to overheat. 2.40.5 Intermediate Gearbox. The intermediate gear- box, at the base of the vertical stabilizer, reduces the rpm and changes the angle of drive. A fan mounted on the gearbox input shaft draws air from an inlet on the vertical stabilizer. This air cools both the tail rotor gearbox and the intermediate gearbox. Four thermistors monitor tempera- ture and an accelerometer measures vibration limits. The intermediate gearbox is a grease-lubricated sealed unit. a. Intermediate Gearbox Caution Light Indica- tors. The four thermistors and the accelerometer pro- vide crewmembers with temperature and vibration caution lights. Both crew stations have TEMP INT and VIB GRBX light segments. CAUTION Prolonged out of ground effect hovering (20 – 30 min.) with outside air tempera- tures above 75 F (24 C) may cause the tail rotor gearbox to overheat. 2.40.6 Tail Rotor Gearbox. The tail rotor gearbox, mounted on the vertical stabilizer, reduces the output rpm and changes the angle of drive. The tail rotor output shaft passes through the gearbox static mast. All tail rotor loads are transmitted to the static mast. The output shaft trans- mits only torque to the tail rotor. Lubrication of this gear- box is identical to that of the intermediate gearbox. a. Tail Rotor Gearbox Caution Light Indica- tors. The four thermistors and the accelerometer func- tion in the same way as for the intermediate gearbox. The associated caution light on the pilot and CPG caution/ warning panels are labeled TEMP TR and VIB GRBX. (Both the intermediate and tail rotor gearboxes activate the VIB GRBX light segment).