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Ignition Exciter
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 A.  The ignition exciter is a sealed unit containing electronic circuitry encased in epoxy resin. It transforms the dc input into a pulsed high voltage output. When the system is energized, a capacitor on the high voltage side of the output tranformer is charged until the stored energy (four joules) is sufficient to energize a spark gap in the unit and discharge the capacitor across the two igniter plugs. If one igniter is shorted or has an open circuit the remaining igniter continues to function. This network also enables the capacitor to discharge automatically if both igniters are inoperative, or the input voltage is switched off.
Ignition Cables
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 A.  Two individual ignition cable assemblies carry the energy output from the ignition exciter to the engine mounted igniter plugs. Each lead consists of an electrical conductor constrained within a flexible metal braiding. Coupling nuts at each end of the cable facilitate connection to the respective connectors on the igniters and the ignition box.
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A

747-400
MAINTENANCE MANUAL

SEE A
FWD
APU COMPARTMENT
IGNITION 
EXCITER 
STARTER 
MOTOR 
IGNITION 
FWD  CABLE  IGNITER PLUG 
(2 LOCATIONS) 
A 

APU Ignition and Starting System
Figure 1

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L64998
A
747-400
MAINTENANCE MANUAL

APU BATTERY 
SEE  B 
ELECTRONIC 
EQUIPMENT 
RACK, E33 
SEE  A 
FWD 
ELECTRONIC EQUIPMENT RACK, E33 
(AFT EQUIPMENT CENTER) 
A 

FWD
APU BATTERY B
APU Battery
Figure 2

L65523
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A

747-400
MAINTENANCE MANUAL

APU START TRU, H123
SEE B
ELECTRONIC EQUIPMENT RACK, E33
SEE A
FWD
ELECTRONIC EQUIPMENT RACK, E33 (AFT EQUIPMENT CENTER)
A
FWD
APU START TRU, H123 B
APU Start TRU
Figure 3

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 7.  Igniter Plugs
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 A.  The igniter plugs screw directly into the gas generator case and are in the form of threaded plugs with a central positive electrode encased in an annular semiconducting material. The potential developed by the ignition exciter is applied between the central electrode and the igniter shell (ground). As the potential increases a small current passes across the air gap. This current increases until the air between the central conductor and ground ionizes at which point a high energy discharge occurs between the electrodes.
 8.  Operation
_________
 A.  The APU controller will automatically do the APU start sequence when it receives the start command signal from the airplane flight deck. As the starter motor rotates the gas generator rotor assembly, the APU controller energizes the ignition unit. The ignition unit is energized by an airframe supplied 28-volt dc source via the APU controller. The unit transforms the input dc voltage into a pulsed high voltage output which is carried to the igniter plugs by the ignition cables. The resultant high energy spark ignites the fuel/air mixture in the combustor. The ignition exciter is attached to the inlet plenum with vibration mounts and the igniter plugs are located at the 5 and 7 o'clock positions on the gas generator case. For more details on the APU ignition and starting system, refer to these wiring diagrams and functional schematics: WDM 24-31-21: APU Battery WDM 24-33-11: Main Battery WDM 49-11-12: APU Start SSM 24-31-02: APU Battery SSM 24-33-01: DC Standby Power SSM 49-11-02: APU Start.
　
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