Turbine Ignition Maintenance
By Jennifer Sparks and Brad Mottier
Turbine Ignition Inspection and Maintenance
Exciters Designed to require little or no maintenance, an ignition exciter often lasts to engine TBO before recertification or overhaul of the unit is required. Repair of an exciter is costly and typically impractical because of the test equipment required to perform leak checks, tests of spark rate and insulation resistance. Due to the low cost and high reliability of exciter overhaul and exchange programs offered by the original equipment manufacturer, many airlines, as well as fleet and individual operators are taking advantage of overhaul programs. New or reconditioned units can be shipped back to customers or exchanged within a few days,making field repair of exciters more expensive and less attractive than the overhaul or exchange alternative.
Igniter plug inspections consist of a simple distance measurement from the plug shell to the center electrode. The amount of shell material necessary to survive between scheduled inspection periods is determined by the engine and plug manufacturers respectively, and will vary from engine to engine. When this gap limit is reached, the plug should be replaced. Consideration of unusual flight conditions should influence recommended plug replacement schedules as well. For example, frequent, short flights resulting in a higher percentage of exciter "on-time" relative to total engine hours may considerably shorten the expected life of a plug. In this instance, replacement of plugs —even those that pass visual and electrical scrutiny — should be determined by the technician. Many plug manufacturers offer tools and gages to help determine plug serviceability.
There is little variation in inspection and maintenance procedures between high- and low-tension plugs; however, one difference needs to be noted — low-tension plugs operate most effectively when trace amounts of carbon are allowed to remain on the plug's firing tip. Carbon residue deposited on the tip of the plug during the combustion process
sh ould not be removed during normal cleaning procedure.
Ignition leads require little maintenance and can typically be counted on to last through engine TBO. However, indications of arcing or flashover at the ignition lead connectors can typically be ttributed to trace amounts of grease or dirt in the exciter and igniter plug terminal wells. These areas must be clean before installing the ignition lead. A worn environmental seal on the lead, or an igniter plug with a breached seal, can also cause contamination of the igniter terminal well. Residual amounts of moisture, dirt, and other contaminants will ground an other-wise faultless ignition system. Clean-ing the output terminal wells of the exciter and the lead is paramount to proper ignition system performance, and usually requires little more than a lint-free cloth, and an appropriate cleaning solvent.
Flashover in the mating area of the ignition lead and the igniter plug will virtually guarantee the replacement of mating components. Inspect all connectors, paying close attention to the terminal well where the lead and plug couple. Likewise, the high-voltage discharge created by the ignition system will combine with high operating temperatures and eventu-ally compromise the integrity of the environmental seal — typically con-structed of a silicon compound — allowing moisture and other contami-nants to invade the terminal well. Replacement of the seal at every en-gine overhaul will reduce the likeli-hood of arcing, subsequent electrical failure, and costly replacement of the lead and plug. In more severe applications, seal replacement is required more frequently.
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