Since turbine ignition systems are operated mostly for a  brief period during the engine-starting cycle, they are, as a  rule, more trouble-free than the typical reciprocating engine  ignition system. The turbine engine ignition system does  not need to be timed to spark during an exact point in the  operational cycle. It is used to ignite the fuel in the combustor  and then it is switched off. Other modes of turbine ignition  system operation, such as continuous ignition that is used  at a lower voltage and energy level, are used for certain  flight conditions.  Continuous ignition is used in case the engine were to flame  out. This ignition could relight the fuel and keep the engine  from stopping. Examples of critical flight modes that use  continuous ignition are takeoff, landing, and some abnormal  and emergency situations.  Most gas turbine engines are equipped with a high-energy,  capacitor-type ignition system and are air cooled by fan  airflow. Fan air is ducted to the exciter box, and then flows  around the igniter lead and surrounds the igniter before  flowing back into the nacelle area. Cooling is important when  continuous ignition is used for some extended period of time.  Gas turbine engines may be equipped with an electronic-type  ignition system, which is a variation of the simpler capacitor- type system.  exciter unit. Before the electrical energy reaches the exciter  unit, it passes through a filter that prevents noise voltage  from being induced into the aircraft electrical system. The  low-voltage input power operates a DC motor that drives  one multilobe cam and one single-lobe cam. At the same  time, input power is supplied to a set of breaker points that  are actuated by the multilobe cam. The typical turbine engine is equipped with a capacitor-type,  or capacitor discharge, ignition system consisting of two  identical independent ignition units operating from a common  low-voltage (DC) electrical power source: the aircraft battery,  115AC, or its permanent magnet generator. The generator  is turned directly by the engine through the accessory gear  box and produces power any time the engine is turning.  The fuel in turbine engines can be ignited readily in ideal  atmospheric conditions, but since they often operate in the  low temperatures of high altitudes, it is imperative that the  system be capable of supplying a high heat intensity spark.  Thus, a high-voltage is supplied to arc across a wide igniter  spark gap, providing the ignition system with a high degree  of reliability under widely varying conditions of altitude,  atmospheric pressure, temperature, fuel vaporization, and  input voltage. A typical ignition system includes two exciter units, two  transformers, two intermediate ignition leads, and two high- tension leads. Thus, as a safety factor, the ignition system  is actually a dual system designed to fire two igniter plugs.  [Figure 4-65]  Figure 4-66  is a functional schematic diagram of a typical  older style capacitor-type turbine ignition system. A 24-volt  DC input voltage is supplied to the input receptacle of the  When the spark igniter is made conductive, the storage  capacitor discharges the remainder of its accumulated energy  along with the charge from the capacitor in series with the  primary of the triggering transformer. The spark rate at the  When the charge on the capacitor has built up, the contactor  is closed by the mechanical action of the single-lobe cam.  A portion of the charge flows through the primary of the  triggering transformer and the capacitor connected with it.  This current induces a high-voltage in the secondary, which  ionizes the gap at the spark igniter. From the breaker points, a rapidly interrupted current is  delivered to an auto transformer. When the breaker closes,  the flow of current through the primary winding of the  transformer establishes a magnetic field. When the breaker  opens, the flow of current stops, and the collapse of the field  induces a voltage in the secondary of the transformer. This  voltage causes a pulse of current to flow into the storage  capacitor through the rectifier, which limits the flow to a  single direction. With repeated pulses, the storage capacitor  assumes a charge, up to a maximum of approximately 4  joules. (Note: 1 joule per second equals 1 watt.) The storage  capacitor is connected to the spark igniter through the  triggering transformer and a contactor, normally open. Figure 4-65.  Turbine ignition system components.  Ignition exciters  Exciter-to-igniter plug cable (2 locations) Igniter plug (2 locations) 4-40