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-Lite® (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.