PT6 Ignition

The Pratt & Whitney PT6 is a widely used turbine engine in the aviation market.

After checking the glow plugs, allow them to cool down to room temperature. Remove them from the ignition leads, install new copper gaskets, and install on the gas generator case. Torque them according to manufacturer recommendations. Then install the lead and torque it. Safety the system, and you are good to go.

Spark ignition system

Spark igniters are the most common type of ignition system on PT6 engines in service. The spark ignition system consists of an ignition exciter unit, two shielded ignition leads, and two igniters.

The ignition sequence begins at the exciter box. The exciter box transforms the incoming 28v DC main ship battery power into a high energy pulsed output through solid-state circuitry consisting of transformers, diodes, and a storage capacitor. The storage capacitor is progressively charged until the stored energy reaches about four joules. At this point, an internal spark gap in the exciter arcs and allows the energy to be discharged to the igniters.

Ignition leads carry the high voltage DC power to the igniters. The leads are wrapped in a flexible metal braiding for shielding purposes.

Like their glow plug cousins, the igniters are secured to the gas generator case at the four and nine o’clock positions. The igniter consists of a positive central electrode surrounded by a semi-conducting material. The high voltage provided by the exciter is applied across the gap between the central conductor and the igniter shell (the ground). The current continues to increase until the air between the central conductor and the shell ionizes. When this happens, high energy discharges between the electrodes. This spark always occurs somewhere in the annular space between the central conductor and the shell.

As a safety precaution, the system is designed so that if either igniter is open or shorted, the other igniter will continue to function. As an additional safety precaution, if both igniters fail to work, the capacitor discharges automatically.

Operational check

Before we go any further, a quick safety note. P&WC warns “Residual voltage in the ignition exciter may be dangerously high. Make sure the ignition is switched off and the system has been inoperative for at least six minutes before removing any ignition components. Always disconnect the coupling nuts at the ignition exciter end first. Always use insulated tools to remove the cable coupling nuts. Do not touch the output connectors or coupling nuts with bare hands.”

Hawkins adds “There are two ways you can almost guarantee a fatal injury when working around PT6 engines. One is walking into a prop with the engine running. The other is touching an energized ignitor.” Be extremely careful around the system and follow all manufacturer procedures. Because of the high voltage and current involved with igniter systems, the operational check is a little different than checking glow plugs. The test is designed to be able to check the system while reducing the risk of injury to the mechanic.

Before switching the ignition system on for this test, be sure to perform a dry motor of the engine. This will ensure any residual fuel is removed from the gas generator case.

First, disconnect the coupling nut of one ignition cable from the output connector on the ignition exciter. Then, have a co-worker switch the ignition switch on. Listen at the gas generator case for a snapping sound at the rate of about one snap per second. Turn off the ignition switch. Re-connect the coupling nut at the exciter and remove the other cable coupling nut. Again, have your co-worker switch the ignition on and listen for the appropriate snap. Re-connect the coupling nut to the exciter and safety.

If neither igniter snaps during the operational check, replace the ignition exciter, and repeat the operational check. If only one igniter doesn’t snap, replace that igniter and perform the operational check again. If it still fails, replace the exciter, and test the system again.

Igniter inspection

You want to inspect the igniters for condition. Inspect the exterior cylindrical area of the firing end of the shell for chafing wear. Some minor wear is allowed — refer to the maintenance manual. You also want to inspect the igniter shell and electrode for erosion. The maintenance manual has illustrations for reference in regards to allowable wear. Re-install igniters with new copper gaskets and torque according to the maintenance manual. Re-connect the leads, torque, and safety.

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