Ignition System Troubleshooting: Inspecting turbine engine ignition systems

No-start squawks on today's jet engines can be frustrating to troubleshoot. If "no-start" anomalies are not identified and corrected in both an accurate and timely fashion, they can lead to costly downtime associated with troubleshooting and parts...

To begin testing, the mechanic connects the appropriate remote sensor to the ignition lead output at the lead to igniter interface. A supplied sensor coax cable is then connected from the remote sensor to the PTL tester. After the PTL is turned on, the tester completes an internal self-check. When completed, the LCD will give the mechanic the following prompt: "Enter P/N to Test." Simply enter the exciter part number under test using the tester keypad. In most cases, the part number entered will be identical to the Unison part number listed on the identification plate of the exciter. To avoid an erroneous reading induced by voltage drop-off sometimes found when testing longer leads, correction factors have been added to software logic of the tester. In these cases, a suffix has been added to the exciter part number. The specific part numbers, engine applications, testing limits, and correct sensor/cable accessories are listed in the back of the supplied PTL operation and maintenance manual.

After loading the part number under test, energize the ignition system and simply press "Enter" on the keypad. The system is now under test. When completed (approximately 15 seconds), a spark rate measurement, energy measurement, and a "Pass," "High," or "Low" result will be displayed on the tester's LCD. If a "Pass" indication is received, the exciter and lead can be eliminated as the cause of the "no-start" condition. If a "High" or "Low" indication is present, then the test sequence should be repeated by connecting the remote sensor with the aid of an adapter cable directly to the output of the exciter. Test results from the exciter will help determine whether to remove the exciter or the lead. For exciters with two outlet terminals, a supplied shorting plug is installed to the outlet not under test.

The PTL may be powered from its own internal battery supply (six 1.5-volt alkaline "C" cell batteries) or from an external power source of 115 volts at 47 to 420 Hz. Battery life averages approximately 24 hours of tester usage and the tester's LCD indicates a "battery low" warning when battery replacement is required.

The PTL tester utilizes two program libraries: a protected library and a user library. The protected library, which can only be changed by Unison, contains programs for exciters that are factory-set. Memory storage allows programming of up to 100 part numbers. The user library can be entered and edited and allows the user to add or customize part numbers. Should tester programming software require reloading or revision, a "Tester to Tester" program transfer can be completed on-site, keeping tester downtime to a minimum.

In closing, any piece of test equipment or specialized tool is not effective unless used in conjunction with a knowledgeable and experienced mechanic. The PTL was designed to aid the mechanic in troubleshooting the ignition system, saving both time and money in getting the aircraft's powerplant started and the aircraft flying again. After all, that is the bottom line.

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