The Pratt & Whitney PT6 is a widely used turbine engine in the aviation market. An important component of the engine is the ignition system. There are two types of ignition systems that are used in the engine, glow plugs, and spark igniters. We will discuss both systems and share some tips on proper inspection and maintenance of these components.
I began my research on this subject matter by contacting Ralph Hawkins, Chief Engineer for Northstar Turbines, LLC. Ralph is very knowledgeable about PT6 engines, and was able to share his knowledge with us. Any information you find valuable in this article, you can thank Ralph for. If you find anything wrong, blame me. In addition, we couldn’t tackle an article like this without reference to Pratt & Whitney Canada’s (P&WC) maintenance manuals and training manuals. Anyway, on to the subject at hand — PT6 ignition systems.
Glow plug system
The glow plug system consists of an ignition current regulator with a selectable circuit to the two sets of tubes, two shielded cables, and two glow plugs.
The ignition current regulator contains four ballast tubes. Each tube is made up of a pure iron filament surrounded by helium and hydrogen gases and enclosed in a glass envelope. Resistance in the iron filament increases with temperature increase (caused by current flow). This provides a stabilizing effect on the current that is passing through the tube. This controls the current across the four tubes to a nearly constant value over a wide range of voltages.
Each glow plug is wired in series with two parallel-connected ballast tubes. Either glow plug can be selected during engine light up. Hawkins adds “Normally both glow plugs are selected during engine start. During flight into adverse weather etc., the glow plugs are normally energized to prevent flame out, and are selected alternately to minimize wear on the ignition system.”
The ballast tubes allow an initial current surge when they are switched on which stabilizes to a constant value in about 30 seconds. This allows for a rapidly heated glow plug for fast light ups.
The glow plugs are secured to the gas generator case at the four and nine o’clock positions. The glow plug consists of a heating element that is fitted into a short conventional-type plug body. This heating element is a helically wound coil that sits slightly below the end of the plug body. There are four equally spaced holes along the outside of the plug body that lead into an area below the coil. During the starting process, the fuel that is sprayed by the fuel nozzles runs along the combustion chamber wall liner into this area of the glow plug. The fuel is then vaporized and ignited by the hot coil element (by hot, we mean a yellow hot that is more than 2,000 F). Four air holes bleed compressor air from the gas generator case into the plug body. This goes past the hot coil into the combustion chamber liner producing a hot streak or torching effect which ignites the remainder of the fuel. The air also cools the glow plug coil elements when the engine is running with the plugs switched off.
Glow plug inspection
Disconnect both ignition cables from the glow plugs. Then, remove the glow plugs and inspect them. You want to inspect the glow plug element for evidence of carbon buildup and clean as necessary. Check the element for the presence of fused area. Check manual for fuse area limits. Elements that exceed fused area limitations must be replaced. Inspect the threads on the body of plugs for condition.
Replace any bad plugs. Re-connect the glow plugs to their respective leads, move them away from the engine, and proceed with the operational check.
Glow plug operational check
With glow plugs, the operational check is pretty straightforward. With the glow plugs removed from the engine and connected to the leads (moved away from the engine) have someone switch the ignition on. Ensure that the glow plugs reach an acceptable orange-yellow color within about eight seconds.
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.
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.
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.
Keep them clean
When working with igniter systems, be sure to keep all the central conductors of the connections clean. Lubrication contamination of these central conductors can result in a high resistance path that can generate heat and oxidation, leading to the welding of the electrical components.
This has been a brief primer on PT6 ignition systems. Be sure to refer to the maintenance manual whenever performing any maintenance on the engine.