(c)  
Close external power dc circuit breakers.

(2)  
To disconnect external power:

(a)  
Open external power dc circuit breaker.

(b)  
Close battery charger circuit breaker.

(c)  
Disconnect external dc power supply.

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Nov 15/77  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  49-11-0 Page 1 

B.  APU start is initiated when the master switch is moved through ON to START position and released. After being released, the master switch assumes the ON position and the APU dc boost pump is energized. The starter lockout relay K6 locks in the start signal from the airplane master switch until the air inlet door has fully opened, and locks out the R5 start relay until a start is called for. The K6 relay closes through the normally closed contacts of the start relay R5 (Fig. 1). The starter lockout relay K6 then locks in through its own contacts. The start and stop relay K1 CLOSE coil is energized if the fire shutdown relay K4 is de-energized, the overspeed relay K2 is in RESET position and the air inlet door switch is in the NOT FULLY OPEN position. The high oil temperature light and the low oil quantity light circuits are armed and the low oil pressure light illuminates. The high oil temperature automatic shutdown becomes operable. The APU fuel valve opens. After the APU fuel valve opens, power is transferred from the fuel valve to the air inlet door actuator, and the door opens.
NOTE:  If the master switch is positioned initially to ON and the air inlet door fully opens before the master switch is moved to START, the CLOSE coil of the start and stop relay K1 will not be energized and the APU will not start.
C.  When the air inlet door switch is in the FULLY OPEN position, power is supplied to the sequencing oil pressure switch and to the 60 -second timer to start the timing operation (Fig. 1). The start relay R5 closes. This trips the starter lockout relay K6 and completes the starter motor circuit. The starter motor begins cranking the engine.
NOTE:  On airplanes incorporating SB 49-1035, the APU control unit (M280) has a 90-second start circuit timer to improve APU start capability.
D.  As the engine rotates, the oil pressure rises. When the oil pressure reaches approximately 4.5 psig, the sequencing oil pressure switch closes and completes the circuit to the fuel solenoid valve and to the ignition unit (Fig. 1). The fuel solenoid valve opens and permits fuel flow to the combustion chamber. The ignition unit causes the igniter plug to fire. The engine lights up and begins to accelerate to service speed. When the engine speed reaches approximately 14,500 to 16,500 rpm, the 35% switch in the centrifugal switch opens. This de-energizes the timer and the start relay R5. With the start relay R5 de-energized, the starter motor circuit is broken. The starter motor disengages, slows and stops. When the engine oil pressure reaches 55 psig, the low oil pressure switch opens and the low oil pressure light is extinguished. The engine continues to accelerate. When the engine speed reaches approximately 37,300 to 38,900 rpm, the 95% switch actuates. This de-energizes the ignition unit, arms the bleed air valve solenoid circuit, arms the pneumatic solenoid valve circuit, and energizes the APU GEN OFF BUS light and on airplanes incorporating SB 49-1032, de-energizes the APU dc boost pump. The APU low oil pressure automatic shutdown becomes operable at this speed.
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E.  If the APU does not start as described in par. D, the engine does not reach 50% service speed and the 90-second time delay device remains energized. After 90 seconds have elapsed, the time delay device completes TRIP coil circuit of start and stop relay K1, and the relay trips (Fig. 1). This de-energizes relay K7. The power is removed from the start relay R5, ignition unit, 90-second time delay device, and the fuel solenoid valve. The starter motor and ignition unit cease operation, and the fuel flow ceases to the combustion chamber.
F.  The APU is stopped by positioning the master switch to OFF. Power is supplied to the overspeed oscillator in the electronic speed switch to simulate an engine overspeed. The normally closed 110% switch contact opens. This breaks the fuel solenoid valve circuit. The normally open 110% switch contact energizes the TRIP coil of the start and stop relay K1. With K1 tripped, relay K7 is de-energized to completely isolate the fuel solenoid valve, which was initially de-energized when the 110% switch was opened. As soon as the fuel solenoid valve closes, fuel ceases to flow to the combustion chamber, and the engine slows and stops. Simultaneously with the actuation of the 110% switch, power is supplied to the APU fuel valve and the valve closes. When the valve is fully closed, power is transferred to the air inlet door actuator, and the door closes.
　
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