(2)
Airplanes with 20 Joule AC Twin Pack Ignition System: The 20 Joule twin-pack system is powered by 115 V, 400 Hertz voltage at two ignition exciters. Each exciter incorporates one power input connector and one output connector. The output connectors supply high tension voltage through an exciter cable to the igniter plugs.
(a) Ignition exciter
1) Airplanes with 20-4 Joule DC Starting and Continuous Ignition System: The 20-4 ignition exciter is a capacitor discharge system designed to provide ignition for the JT8D Turbofan Engine. This ignition exciter serves the dual purpose of providing intermittent duty starting ignition and continuous duty ignition which are used as required after starting. Two different input voltages are required for the exciter. Both circuits of the ignition exciter are contained in one compact housing with one input power connection and two output connections. The dual pack system has two input power connections and two output power connections.
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Oct 20/91 BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 74-00 Page 1
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569 Table of Leading Particulars
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2) Airplanes with 20 Joule AC Twin Pack Ignition System: The 20 Joule Twin-Pack ignition exciter system consists of two independent 20 Joule, AC-powered intermittent duty capacitor discharger circuits. Each circuit is contained in a completely separate exciter housing with individual power input and high tension output connections. These outputs are electrically connected through the shielded high tension cables to individual igniter plugs. Each 20 Joule circuit can thus be replaced without removing the components of the other circuit.
3) See Fig. 1 for ignition exciter.
4) See Fig. 2 for ignition exciter table of leading particulars.
5) See Fig. 3 for ignition exciter wiring schematic.
3. Distribution
A. High tension distribution
(1) The high tension distribution system delivers high voltage from the ignition exciter to the combustion chambers by means of high tension leads and igniter plugs. The high tension voltage from the exciter ionizes the gap at the plug and the result is a spark of very high energy capable of igniting fuel.
(a)
High tension leads (Fig. 4)
1) The igniter plug lead assemblies are installed between the ignition exciter and the igniter plugs. The lead assemblies transfer high frequency, high voltage from the ignition exciter to the igniter plugs.
(b)
Igniter plugs (Fig. 5)
1) There are two igniter plugs which are mounted on the lower front of the combustion chamber outer case. One projects into the number four combustion chamber and the other projects into the number seven combustion chamber.
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P&WA 74-00, Page 1, May 01/91
4.
Switching (Ref 74-31-0 D&O)
5.
Operation (Fig. 6)
A. Airplanes with 20-4 Joule DC Starting and Continuous Ignition System: All ground starts or inflight airstarts should be made with the use of the 20-Joule DC exciter (firing both igniter plugs). For optimum life of ignition system components, the operating duty cycle is 2 minutes ON, 3 minutes OFF, 2 minutes ON, and 23 minutes OFF.
NOTE: If the 20-4 Joule single pack exciter unit has been operated beyond the recommended duty cycle, the unit must be removed and checked per overhaul instructions. The integrity of components within the exciter may have been compromised due to overheating (Ref 74-11-11 R/I).
The dual pack exciter installed by SB 5880 has continuous duty. This exciter has no duty cycle limits.
B. Airplanes with 20 Joule AC Twin Pack Ignition System: Ground starts can be made with the use of only one of the two ignition systems. Pratt and Whitney Aircraft recommends that both ignition systems be used for ground starts and air starts. Using both ignition systems during a start ensures better flame propagation which in turn, results in rapid, dependable starting. For optimum life of ignition system components, the operating duty cycle is 10 minutes ON, 20 minutes OFF.
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