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2.62 APU.
The auxiliary power unit (Figure 2-19) consists of a gas turbine shaft power section, a reduction gear drive, and appropriate controls and accessories. The accessory gear box provides an axial pad with a 12,000 rpm output drive for the APU ac generator, rpm pad for mounting the APU start motor, rpm drive pad for the APU fuel assembly. A magnetic pickup mounted on the accessory gear box senses engine speed. The APU is lubricated by a self-contained oil system. Refer to Figure 2-23 for servicing.
2.62.1 APU Controls. The APU control, on the upper console (Figure 2-6), consists of a CONTR switch and an APU .re extinguisher T-handle. The APU CONTR switch, with marked positions OFF and ON, controls the operation of the APU. Placing the switch ON starts the APU and allows it to operate. The APU is off when the switch is OFF. The APU FAIL caution legend will be on any time the APU automatically shuts down. The APU OIL TEMP HI caution legend is on when APU oil temperature is above normal range. During ground operation at high ambient temperatures the APU OIL TEMP HI caution legend may go on. If this occurs, the APU should be shut down imme-diately to prevent damage. After a 30-minute cooling period, the oil level should be checked. If OK, the APU may be restarted. The control system receives electrical power from the battery bus through a circuit breaker marked APU CONTR INST on the lower console. When illuminated, the APU T-handle warns the pilot/copilot of a .re in the APU compartment. When the T-handle is pulled, it turns off fuel to the APU, sends a stop signal to the ESU, arms the .re extinguisher system, and sets the extinguisher direction control valve to the APU. During APU starts using battery power, if the .re extinguisher is required, FIRE EXTGH RESERVE must be used. The T-handle microswitch receives electrical power from the battery util-ity bus through a circuit breaker marked FIRE EXTGH on the lower console circuit breaker panel.
2.62.2 APU Fuel Control System (Helicopters equipped with (T-62T-40-1 APU). This system consists of a fuel pump and a control assembly. The fuel pump is protected by a .lter. Fuel pump output .ow passes through another .lter before entering the control assembly. A gov-ernor and .ow metering valve controls fuel .ow to the engine during ignition, permitting automatic starting under
FUEL CONTROL FIREWALL ELECTRICALENCLOSURE FIRE DETECTOR CONNECTOR
HYDRAULIC START MOTOR
FIREWALL
GENERATOR
OIL LEVEL BLEED.AIR
OIL FILLER PORT
SIGHT GAGE PORT
AND DIPSTICK AA0519
SA
Figure 2-19. Auxiliary Power Unit (APU)
all ambient conditions, and controls the turbine at a con-stant speed once it has accelerated to operating speed. An electronic speed sensing device provides automatic fuel .ow, ignition, and operation of the APU.
2.62.3 APU Fuel Control System (Helicopters equipped with GTC-P36-150 APU). The fuel control system includes a fuel pump and metering section. The fuel pump is protected by an integral inlet .lter. Fuel pump output .ow passes through a .lter screen before entering the metering assembly. Fuel pump discharge pressure is limited by an ultimate relief valve which, when activated, bypasses fuel .ow back to the pump inlet. Fuel metering is accomplished by the torque motor metering valve as a function of an electrical signal from the electronic sequence unit (ESU). For accurate fuel metering, a constant, pressure drop across the metering valve is maintained by the differ-ential pressure regulating valve. The fuel solenoid valve is energized by the ESU following the initiation of APU start. This allows fuel to .ow to the engine. The fuel control assembly subsequently provides fuel according to a pre-programmed schedule to effect efficient acceleration. The fuel solenoid valve will close completely without visible leakage from the minimum operating fuel pressure to 110% of the maximum operating fuel pressure.
2.62.4 APU Fuel Supply System. APU fuel is sup-plied to the APU from the left main fuel tank. The FUEL PUMP switch must be at APU BOOST for all APU opera-tion, except engine priming. The APU prime/boost shutoff valve is a two-position, open-closed unit mounted on the APU compartment .rewall where it also functions as a .re-wall shutoff valve. The valve is pilot-operated from the upper console FUEL PUMP switch as well as by the FIRE EXTGH APU T-handle. If the APU does not start and the APU ACCUM LOW advisory legend is not on, the manual override lever on the accumulator manifold should be pulled to attempt another start, and held until the APU has reached self-sustaining speed.
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