By Erich Eischen
Looking at the ignition system
Nothing can be more frustrating for aircraft mechanics to troubleshoot than no-start "squawks" on today’s technologically advanced jet engines. As Murphy’s Law often dictates, most no-start occurrences happen when (a) the plane is full of revenue passengers, (b) the AOG maintenance supervisor’s favorite sayings are either "Is it fixed yet," or the inevitable "How much longer," and (c) you as the mechanic are just completing the end of a very busy shift.
Regardless of timing, if "no-start" anomalies are not identified and corrected in both an accurate and timely fashion, they can add significant expenditures to an operator’s maintenance budget in costly downtime associated with troubleshooting and parts replacement. To expedite 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) in a flightline, gate maintenance, shop, or hangar environment. Both 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
In many aircraft, main turbine engines utilize two redundant operating ignition systems, each consisting of an exciter, ignition lead, and the igniter plug. 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.
Let’s say 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 such as the Power-To-Lite (PTL), many mechanics conduct an audible check to determine if the system is 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.
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