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 replacement. To assist troubleshooting of the ignition system, Unison Industries manufactures a portable ignition system tester called the Power-To-LiteĀ® (P/N 137332). It is designed to isolate faulty ignition system components (plug, lead, or exciter). Commercial and military aircraft main engine and auxiliary powerplant ignition systems can be tested on a variety of airframe applications (ranging from a Cessna Caravan to a Boeing 747).

Ignition System Basics

The exciter is the power supply of the ignition system and provides a high voltage pulse to fire the igniter plugs. A typical ignition exciter operates on a nominal input voltage of 115v AC, 400 Hz or 24v DC.

The ignition lead conducts the electrical energy from the exciter to the igniter plug. The conductor or inner lead wire is often surrounded by an outer metallic braid for protection, ground return, and the prevention of electromagnetic interference.

The igniter plug transforms the energy received from the lead to an electrical spark, which ignites the fuel/air mixture in the combustor of the engine. Igniter plugs are expendable components and are replaced or "hard-timed" as a part of most operators' maintenance programs. At a minimum, they should be inspected at regular intervals, paying close attention to such rejection factors as excessive tip erosion, cracked ceramic insulators, or any evidence of igniter shell burn-through.

When in doubt as to its serviceability, replacement of the igniter is recommended since changing it during a scheduled aircraft maintenance check is by far more cost effective than replacing it in an AOG situation.

If an aircraft is experiencing a "no-start" condition and the initial troubleshooting test sequence on the engine per the applicable airframe/engine manual has determined that the "no-start" condition is not airframe electrical or fuel-related, the next step is to isolate which part of the ignition system may be at fault. Prior to having the advantage of using an ignition system tester, many mechanics would conduct an audible check to determine if the system was indeed delivering a spark to the engine. Listening alone for the familiar "snap, snap, snap" of the ignition system arcing can, in some instances, give a false indication of system serviceability because the arcing heard may be taking place within the exciter itself or the ignition lead, and not inside the engine's combustor.

Another technique that is sometimes used but never recommended is visually inspecting the igniters for firing by removing them from the engine (with the lead and exciter still connected), suspending the components freely beneath the engine, and applying voltage to the exciter. Due to the high voltage output involved, even in a low-tension ignition system with an output voltage of up to 12 KV, bodily harm to the mechanic could occur.

Let's Start Troubleshooting

The standard tester configuration includes two remote sensors (used to accommodate specific exciter/lead output terminals, i.e. type 2, 3, 4, etc.), two adapter cables (used as interface between exciter and remote sensor), and the associated interconnecting cables. The PTL performs a functional check of the exciter's output, displayed in both spark rate (up to 250 counts at 1, 10, or 60 second gate time) and stored energy (up to 30 joules).

The display of both the spark rate (frequency) and energy (magnitude) provides the mechanic with a more accurate picture of the exciter's output under test. The output can be tested at either the exciter or lead output. When using the ignition system tester, the igniter is never actually "fired" or put under a functional test. If the functional checkout of the exciter and lead indicates that they are both serviceable, Unison recommends replacing the igniter. It is important to note that the PTL was designed to check exciters on-wing or in a shop environment prior to being removed from the engine. The PTL is not authorized per the Unison Component Maintenance Manuals for return to service testing, resulting in an FAA 8130-3 Tag.

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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