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C.  Airplanes with 20-4 Joule DC Starting and Continuous Ignition System: For continuous operation, the 4-Joule system should be used in lieu of the 20-Joule DC system for protection against flameout during takeoff and prior to activating the engine inlet anti-icing system. The 4-Joule system, which can be operated continuously may also be used for protection against flameout of at any other time deemed advisable, such as during periods of moderate or severe turbulence. To conserve the life of the ignition system components, the 4-Joule ignition system should be turned OFF during normal flight conditions whenever engine operation is stable.
D.  Airplanes with 20-4 Joule DC Starting and Continuous Ignition System: The intermittent duty starting circuit requires an input of 28 VDC nominal while the continuous duty circuit requires an input of 115 VAC 400 Hertz. The intermittent duty starting circuit discharges through both outlets, firing two igniter plugs. The continuous duty circuit discharges only through the outlet marked CONTINUOUS DUTY OUTLET, firing one igniter plug. Spark gaps prevent current from following in one circuit when the other circuit is in operation.
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Ignition System Schematic  569 
74-00  Figure 6  Oct 20/91 
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BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

E.  Airplanes with 20 Joule AC Twin Pack Ignition System: Power is supplied to the input connector of the exciter from the 115 VAC 400 Hz source in the airplane. Basically each circuit converts the input AC power into high voltage DC for storage as a charge on the tank capacitor. This stored energy is delivered to the igniter plug at the specified spark rate by means of a suitable high tension cable. Provisions in the exciter circuitry enable a high voltage pulse to be delivered to the igniter plug to provide positive breakdown of the igniter plug gap under all conditions.
NOTE:  The exciter is reparable using test equipment available in most airline overhaul shops. Positive hermetic seal is assured through use of a stainless steel case weldment. The exciter capability for continuous operation will assure conformance with potential airline ignition duty cycles. Maximum exciter service life can, however, be attained by ignition utilization per recommended engine/or airplane operational procedures.
F.  The igniter plug provides a gap across which an electrical spark passes to ignite the fuel-air mixture. The igniter plug gap becomes ionized by very high voltage (22-26 KV) provided by a high tension transformer. Once the air gap becomes ionized, current stored in a storage capacitor discharges across the gap. This discharge results in a high temperature plasma arc which is capable of igniting the fuel-air mixture.
NOTE:  Pratt and Whitney recommends the use of any igniter plug regardless of type, that meets the overhaul manual examination and testing requirements. However, each operator must concern himself with determining how long the igniter plug can be expected to continue meeting these requirements while exposed to engine use within his operation. The appropriate inspection/check, cleaning/painting interval can only be established by the individual operator based on his experience.
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Nov 15/80  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  74-00 Page 9 

IGNITION SYSTEM - TROUBLE SHOOTING
1. General
A.  To find and correct ignition system troubles first study the symptoms carefully, then check each possible cause, beginning with the most probable, until the exact cause of the trouble is determined. Refer to 72-00, Trouble Shooting (P&WA JT8D Maintenance Manual) for fault isolation procedures on ignition system.
B.  Before attempting to diagnose the trouble or work on the system which has been reported malfunctioning during flight, consult the pilot's flight report and all other pertinent data for information which might help in diagnosing the trouble.
WARNING:  IGNITION SYSTEM VOLTAGE IS DANGEROUSLY HIGH. IGNITION SWITCH MUST BE IN "OFF" POSITION BEFORE REMOVAL OF ANY IGNITION COMPONENTS. ALLOW SEVERAL MINUTES TO ELAPSE BETWEEN OPERATION OF IGNITION SYSTEM AND REMOVAL OF IGNITION COMPONENTS. IMMEDIATELY UPON DETACHING IGNITER CABLE FROM IGNITER PLUG, DISCHARGE CURRENT BY GROUNDING IGNITER CABLE TERMINAL TO ENSURE COMPLETE DISSIPATION OF ENERGY FROM IGNITION SYSTEM. FAILURE TO FOLLOW THIS PROCEDURE COULD RESULT IN SEVERE INJURY TO PERSONNEL.
　
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