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.
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