TM 1-1520-238-10 2-72 Change 4 Section XII. AUXILIARY POWER UNIT 2.48 AUXILIARY POWER UNIT (APU). WARNING To prevent an accidental APU start, the APU circuit breaker in the aft avionics bay and the APU HOLD circuit breaker on the pilot overhead circuit breaker panel shall be out when battery or exter- nal electrical power is connected to the helicopter and unqualified personnel are in or around the pilot crew station. The APU indirectly provides hydraulic, pneumatic, and electrical power for the operation of helicopter systems whenever the engines are not driving the main transmis- sion accessory section. The APU provides the means of engine starting without the need for an aircraft ground power unit (AGPU). It is located just inboard of the right engine nacelle in the aft equipment bay. The APU consists of a gearbox, a compressor, and a turbine section, togeth- er with associated fuel, lubrication, and electrical systems. The APU controller and the APU control panel are installed separately from the APU. The utility hydraulic ac- cumulator, located on the deck of the aft equipment bay below the APU, provides hydraulic pressure to the APU hydraulic starter through a solenoid-operated hydraulic start valve. The start valve opens when the APU START switch is positioned to START and closes automatically at 60% APU speed. The utility hydraulic accumulator is dis- cussed in more detail in Section VI of this chapter. 2.48.1 Fuel System. The aft fuel cell provides fuel for APU operation and is discussed in more detail in Section IV of this chapter. The APU fuel control automatically reg- ulates fuel flow. The APU burns approximately 135 pounds of fuel per hour. 2.48.2 Lubrication System. The APU has a self-con- tained oil system. An oil filler cap is located on the left side of the unit. Oil cooling is provided by airflow over cooling fins at the compressor inlet. The oil level sight gage (fig 2-45) is located on the right side and is an integral part of the oil sump. The sight gage can be inspected through an opening just below the right engine nacelle. 2.48.3 Electrical System. The APU electrical system requires dc power which is normally delivered by the heli- copter battery when the APU control switch is set to RUN. This switch will apply dc power to energize the APU boost pump, open the APU fuel shutoff valve, and at the same time, send a signal to open the utility accumulator start so- lenoid valve to allow hydraulic pressure to turn the starter, and shut off the SDC inlet throttle valve to unload the SDC for an APU start. The APU ON caution light on the pilot caution/warning panel illuminates when the APU is oper- ating. 2.48.4 APU Controller. The APU controller provides for automatic start and operation of the APU and power take-off clutch (PTO) engagement. The controller moni- tors the APU for loss of thermocouple, overtemperature, overspeed, overcurrent, low oil pressure, percent rpm, and exhaust gas temperature. The APU controller trans- mits a shutdown signal to the APU whenever the APU control panel switch is set to OFF. The controller also transmits the shutdown signal automatically for fault detection of any of the monitored functions. The APU FAIL caution light on the pilot caution/ warning panel is il- luminated by the low oil pressure switch when automatic shutdown occurs. The PTO clutch actuation is also con- trolled by the APU controller. Normally, PTO clutch en- gagement occurs at 60% APU speed if the main rotor is below 90% speed; however, if the main rotor is above 90% speed, it will engage at 95% APU speed. For cold weather starts below 0 F (–18 C), PTO clutch engage- ment is at 95% APU speed using a cold start switch on the APU control panel. 2.48.5 APU Control Panel. The APU control panel (fig 2-41), on the pilot right console, has the following three distinct functions: a. APU Control Switch. A three-position APU con- trol switch allows the pilot to START, RUN, and shut down (OFF) the APU. This switch is spring-loaded from the START to RUN position. Pilot pressure at the START position is required for only one or two seconds to ensure electrical circuit latching. CAUTION Do not use APU 95% cold start switch when the ambient temperature is above 0 F (–18 C). Use of this switch will reduce the power takeoff (PTO) clutch life drasti- cally, and could cause premature failure of the clutch duplex bearing/needle bear- ing as well as main transmission acces- sory gear case component failure.