When the GEN BRG caution legend remains on for more than 1 minute, make an entry on the DA Form 2408-13-1.
2.59.1 Generator Control Units (GCU). The GCUs monitor voltage from the No. 1, No. 2 and APU generators and take the generator(s) off-line where malfunctions occur. Underfrequency protection is disabled in .ight by the WOW switch.
2.60 AUXILIARY AC POWER SYSTEM.
An auxiliary ac power system (Figure 2-17), is a backup ac power source, providing electrical power for ground checkouts. The system consists of a 115 vac three-phase, 400 Hz 20/30 kVA, air-cooled generator mounted on and driven by the APU, a current transformer and a generator control unit. If the primary ac generators are not operating, the auxiliary ac power output will be applied through con-tactors to the No. 2 ac primary bus and through contactors and current limiters to the No. 1 ac primary bus. An advi-sory legend on the caution/advisory display will go on, indicating APU GEN ON when the APU generator is oper-ating, and the APU generator switch is ON. APU GEN ON legend will be on only when supplying power to the sys-tem, it will be off at any time either No. 1 generator or No. 2 generator is supplying power. The generator control switch on the upper console (Figure 2-6), has marked posi-
tions of TEST, OFF/RESET, and ON.
NOTE
If the APU generator is the sole source of ac generated power, all equipment may be operated, except that when the backup pump is on, the windshield anti-ice is prevented from being used.

2.60.1 Generator Control Switches. Generators are controlled by a three-position generator switch on the upper console (Figure 2-6). The switch ON position energizes the generator and permits connection of generator ac output to the ac loads. TEST permits you to test the generator ac output without connecting to the generator loads. OFF/ RESET deenergizes the generator and permits generator recycling if the generator is disabled and disconnected from its loads. The control switch is manually placed to RESET and then back to ON.
2.60.2 External AC Power System.

CAUTION
Do not connect a source of dc power to the external ac connector.

An external ac power connector, on the right side of the helicopter (Figure 2-1), accepts ground source of 115 vac, three-phase, 400 Hz power. The system is controlled by a switch on the upper console (Figure 2-6), marked EXT PWR-RESET-OFF and ON. External power will be intro-duced into the system if acceptable external power is con-nected, the EXT PWR switch is ON, and no other gener-ating source is operating. An advisory legend on the caution/advisory display will go on, indicating EXT PWR CONNECTED, whenever external power is connected to the helicopter.
Section XII AUXILIARY POWER UNIT
2.61 AUXILIARY POWER UNIT (APU) SYSTEM.
The auxiliary power unit system (Figure 2-19) consists of an auxiliary power unit (APU), accessories, controls, a monitoring system, and a starting system. The APU system provides pneumatic power for main engine starting and cabin heating, and electrical power for ground and emer-gency in-.ight electrical operations.
NOTE
The APU is not quali.ed for normal in.ight use.

APU system accessories include a prime/boost pump, hydraulic accumulator, hydraulic handpump, hydraulic start motor, and ac generator. The hydraulic accumulators and handpump, in the aft midsection cabin ceiling (Figure 2-5), provide the hydraulic pressure for driving the APU starter. If the APU does not start, the hydraulic accumulator can be recharged by pumping the hydraulic handpump. The hydraulic utility module and backup pump, on the left for-ward deck within the main rotor pylon, will automatically recharge the depleted hydraulic accumulator for the next APU start. The APU controls are in the cockpit on the upper console. Indicator legends on the caution/advisory display provide cockpit monitoring of the APU. An indica-tor panel in the cabin will indicate reason for APU shut-down on BITE indicators. The BITE indicators are incor-porated in the APU electronic sequence unit (ESU), and will indicate reasons for APU shutdown. Those indicators can be monitored during APU operation without interrupt-ing normal operating systems. During a start, the ESU com-pares input signals from speed, time, and temperature sen-sors on the APU to speci.ed values stowed in the ESU memory, and performs functional steps as a result of the comparison. The system also provides for APU protective shutdown in case of turbine overspeed, underspeed, high exhaust temperature, low oil pressure, or loss of electrical power or sequence failure. Each major sequence step will have a visual indication of go/no-go. The ESU samples predetermined parameters of exhaust temperature, turbine speed and oil pressure. If any one of the predetermined values are exceeded, the APU will shut down, and appro-priate BITE indication is made. If a momentary malfunc-tion occurs (i.e., a power interruption other than switching of the APU CONTR switch) the APU will shut down and the APU CONTR switch must be placed at OFF and then back ON, to restart the APU. There is also an output signal to the caution/advisory display to turn on the APU ON advisory legend, indicating the APU is operating. Power to operate the APU and ESU is provided from the battery bus through a circuit breaker marked APU CONTR INST.
　
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