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APU Ignition and Starting System Component Location  500 
49-41-0  Figure 1 (Sheet 1)  Aug 15/67 
Page 2 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

500  APU Ignition and Starting System Component Location 
Feb 15/70  Figure 1 (Sheet 2)  49-41-0 
Page 3 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

5.  Igniter Plug
A.  The igniter plug provides the high energy spark for igniting the fuel-air mixture in the combustion chamber. The igniter plug is installed on the forward side of the combustion chamber (Fig. 1). The principal parts of the igniter plug are the outer casing, center electrode, and a ceramic insulator. The spark, that bridges the nonadjustable annular gap between the center electrode and the outer casing, is sufficiently high to break down the igniter plug air gap and produce a high energy spark to ignite the fuel air mixture in the combustion chamber.
6.  High Voltage Lead
A.  The high voltage lead conducts the output current of the ignition unit to the igniter plug (Fig. 1). The lead consists of an insulated electrical conductor encased in a braided metal conduit with insulated, threaded, connectors at each end. The lead and connectors are shielded to prevent radio interference.
7.  Operation
A.  Once an APU start is initiated, operation of the ignition and starting system is automatically controlled to correctly sequence the energizing and de-energizing of the starter motor and the ignition unit. Positioning the master switch momentarily to START energizes the starter lockout relay K6. This energizes the start relay R5. When the start relay is energized, power is supplied to starter motor, and the starter motor begins cranking the engine. As the speed increases, the oil pressure rises. When the oil pressure rises to approximately 3.5 psig, the sequencing oil pressure switch closes and energizes the ignition unit and fuel solenoid valve.
B.  When the engine has accelerated to approximately 19,000 to 21,000 rpm, the 50% contact of the electronic speed switch opens. The start relay R5 is de-energized, and the starter motor 38,900 rpm, the 95% contact of the electronic speed switch actuates and breaks the circuit to the ignition unit to terminate ignition. The engine continues to accelerate until service speed of approximately 41,000 rpm is attained. If the APU does not start within 90 seconds after the air inlet door opening, the unit will automatically shut down. This occurs because a time delay device in the APU control unit trips the start and stop relay K1. The time limit is adequate for normal start of the APU, it prevents overheating the starter motor and limits the drain on airplane battery should the APU fail to start and accelerate to service speed properly. For starting and ignition system operation electrically, refer to 49-11-0.
NOTE:  During the initial phase of the APU start, if the master switch is positioned 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. The purpose of this sequence is to prevent an APU restart after an automatic shutdown.
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49-41-0 Page 4  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  May 20/82 

STARTER MOTOR - REMOVAL/INSTALLATION
1.  Prepare for Starter Motor Removal
A.  Position BATTERY SWITCH and APU MASTER SWITCH to OFF and placard that APU maintenance is "in work."
B.  Open the circuit breaker on M280 module APU accessory unit on E3-3 electronic shelf.
C.  Open APU compartment access door latches and open access door. Install door support rods.
D.  Support lower shroud, open shroud latches, and remove shroud.
CAUTION:  EXERCISE EXTREME CARE NOT TO DAMAGE SHROUD DRAIN LINES WHEN HANDLING SHROUD.
2.  Remove Starter Motor (Fig. 401)
CAUTION:  IF STARTER MOTOR IS BEING REPLACED DUE TO A CLUTCH FAILURE, CHECK CLUTCH PORTION OF STARTER FOR MISSING PARTS. REMOVE VISIBLE CLUTCH PARTS FROM GEARBOX. IF ALL PARTS ARE NOT LOCATED, REPLACE APU. IF ALL PARTS ARE ACCOUNTED FOR, REPLACE BOTH STARTER MOTOR AND OIL PUMP ASSEMBLY.
A.  Tag and disconnect electrical leads from starter motor terminals.
CAUTION:  ELECTRICAL LEADS MUST BE IDENTIFIED WITH CORRESPONDING STARTER MOTOR TERMINALS TO PREVENT CROSSING WIRES. CROSSED WIRES MAY RESULT IN DAMAGE TO APU COMPONENTS WHEN APU START IS ATTEMPTED.
B.  Remove starter motor attaching nuts and washers.
C.  Carefully withdraw starter motor from mounting studs in a straight line.
D.  Remove and discard packing.
　
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