Engine Starting & Ignition Systems (15.13) PDF

Engine Starting and Ignition Systems (15.13) Learning Objectives 15.13.1 Explain the operation of gas turbine engine start systems and components (Level 2). 15.13.2 Explain the operation of gas turbine engine ignition systems and components (Level 2). 15.13.3 Describe the maintenan...

Engine Starting and Ignition Systems (15.13) Learning Objectives 15.13.1 Explain the operation of gas turbine engine start systems and components (Level 2). 15.13.2 Explain the operation of gas turbine engine ignition systems and components (Level 2). 15.13.3 Describe the maintenance of safety requirements of gas turbine engine starting and ignition components (Level 2). 2024-07-23 B1-15b Gas Turbine Engine Page 161 of 290 CASA Part 66 - Training Materials Only  Starting Systems Purpose of Starting Systems The starting procedure for all jet engines is basically the same, but can be achieved by various methods. To start a gas turbine engine, it is necessary to accelerate the compressor to provide suf cient air to support combustion in the combustion section. The type and power source for the starter vary in accordance with engine and aircraft requirements. There are many different methods to turn the engine for starting. Some use electrical power, while others use gas, air or hydraulic pressure, and each has its own merits. The basic types of starters in current use for gas turbine engines are direct current (DC) electric motors, starter/generators and air turbine type starters. Some previous starter systems included: Cartridge pneumatic – The starter motor is basically a small impulse-type turbine driven by high-velocity gases from burning an explosive propellant charge. It is used on military engines and provides a quick, independent starting method. Combustion starter – The starter unit has a small combustion chamber into which high- pressure air from an aircraft-mounted storage bottle and fuel from the engine fuel system are introduced. The air-fuel mixture is ignited in the combustion chamber and the resultant gas is directed onto the turbine of the air starter. Air impingement – Jets of compressed air are piped to the inside of the compressor or turbine case so that the jet air blast is directed onto the compressor or turbine rotor blades, causing them to rotate. Hydraulic starters – One of the engine-mounted hydraulic pumps is utilised and is called a pump/starter. Power to rotate the starter is provided by hydraulic pressure from a ground supply unit or an aircraft accumulator. It is used for starting some small jet engines. 2024-07-23 B1-15b Gas Turbine Engine Page 162 of 290 CASA Part 66 - Training Materials Only  Starter Motors The starter motor must produce a high torque and transmit it to the engine rotating assembly in a manner that provides smooth acceleration from rest up to a speed at which the gas ow through the engine provides suf cient power for the engine turbine to take over. Gas turbine engines are generally started by starter power input to the main accessory gearbox, which in turn rotates the compressor. On the dual-compressor gas turbine, the starter rotates the high-pressure compressor system only. On free turbine, turboprop and turboshaft engines, such as APUs with single compressors, only the compressor is rotated by the starter through the accessory gearbox. The free turbine is not coupled to the starter drive. Compressor rotation by the starter provides an engine with suf cient air for combustion and aids the engine in self-accelerating to idle speed after combustion occurs. Neither the starter nor the turbine wheel has suf cient power on its own to bring the engine from rest to idle speed, but when they are used in combination, the process takes place smoothly. The starter is normally initiated by a cockpit toggle switch, but it is often automatically terminated by a speed sensor device. At this point, turbine power alone is suf cient to take the engine up to idle. If the engine is not assisted to the correct speed, a hung (stagnated) start may occur. That is, the engine stabilises at or near the point of starter cut-off. To remedy this situation, the engine must be shut down for investigation of the problem. Any attempt to accelerate by adding fuel often results in a hot start, as well as a hung start, because the engine is operating with insuf cient air ow to support further combustion. Turboprop engines are either started in low pitch, to reduce drag on the rotor and provide more speed and air ow ( xed shaft), or con gured with a free turbine driving the propeller. This allows for low drag acceleration in that only the compressor rotor system is being turned by the starter. 2024-07-23 B1-15b Gas Turbine Engine Page 163 of 290 CASA Part 66 - Training Materials Only  Electric Starters Electric starters are not in common use on aircraft engines because of their excessive weight, although when used as a combination starter‑generator, they provide weight savings that make them feasible for use on small engines. Electric starters are, however, in common use on auxiliary and ground power units. A typical starter motor, shown below, is a 12- or 24-V series-wound motor, which develops high starting torque. The torque of the motor is transmitted through reduction gears to the clutch. This action actuates a helically splined shaft, moving the starter jaw outwards to engage the engine cranking jaw before the starter jaw begins to rotate. After the engine reaches a predetermined speed, the starter motor automatically disengages. Typical aircraft starter motor Other types of electric starters normally contain an automatic release clutch mechanism to disengage the starter drive from the engine drive when the engine has reached self-sustaining speed. The clutch mechanism also provides an over‑torque protection to protect the engine drive. At a set torque, small clutch plates inside the clutch slip and act as a friction clutch. This setting is adjustable. During starting, the friction clutch is designed to slip until engine and starter speed increase to develop less than the slip torque setting. It is important that the slip torque tension be correctly set to avoid damage to the engine drive ratchet, or slow and hot (hung) starts. 2024-07-23 B1-15b Gas Turbine Engine Page 164 of 290 CASA Part 66 - Training Materials Only  Another function of the clutch assembly is to provide an ‘over-running’ clutch. This consists of a pawl- and-ratchet assembly containing three pawls that are spring-loaded into the disengage position. When the starter is energised, inertia causes the pawls to move inwards and engage the ratchet gear on the starter drive shaft as illustrated below. The inertia used is present because the pawl cage assembly, which oats in the overrunning clutch housing, tries to remain stationary when the starter armature tries to drive the clutch housing around. The overrunning clutch housing overcomes the disengage springs and forces the pawls inwards. When the engine accelerates up to approximately self-sustaining speed, it is turning faster than the starter motor. The pawls slip out of the tapered slots of the engine drive gear and disengage under the in uence of the disengage springs. This overrunning feature prevents the engine from driving the starter to self-destruct speed. Typically, starter circuits do not contain fuses or circuit breakers because initial motor current (series-wound DC motor) can be excessive. Starter clutch 2024-07-23 B1-15b Gas Turbine Engine Page 165 of 290 CASA Part 66 - Training Materials Only  Over-running clutch Starter‑Generators Starter-generators are most commonly found on private- to small-sized jets. The Boeing 787 is the rst of the large commercial aircraft to use starter-generators as the starting system for its engines. These starting systems use a starter motor to drive the engine during starting. After the engine has reached a self‑sustaining speed, it then operates as a generator to supply the electrical system power. The starter-generator simply has a shear drive spline that is permanently engaged in the engine. Starter-generator units are desirable from an economic standpoint because one unit performs the functions of both starter and generator. Also, the total weight of starting system components is reduced and fewer parts are required. Starter generator 2024-07-23 B1-15b Gas Turbine Engine Page 166 of 290 CASA Part 66 - Training Materials Only  Starter-Generator Operation The starter-generator unit is basically a shunt generator with an additional heavy series winding. This series winding is electrically connected to produce a strong eld and a resulting high torque for starting. Pneumatic Starters Pneumatic starting is the method most commonly used on commercial and military jet engine- powered aircraft. It has many advantages over other systems in that it is lightweight, simple and economical to operate. The typical air turbine starter (ATS) weighs one quarter to one fth as much as an electric starter capable of starting the same engine. A pneumatic starter transmits its power through a reduction gear and clutch to the starter output shaft which is connected to the engine. A typical air starter is shown below. Aviation Australia Typical air turbine starter 2024-07-23 B1-15b Gas Turbine Engine Page 167 of 290 CASA Part 66 - Training Materials Only  Aviation Australia Air starter schematic The starter turbine is rotated by high-volume LP air taken from an external ground supply, an APU or bleed air from a running engine. The air supply to the starter is controlled by an electrically operated control and pressure regulating valve. This valve is operated when an engine start is selected and is automatically closed at a predetermined starter speed. A typical air starting system is shown below. Aviation Australia Typical air starting system 2024-07-23 B1-15b Gas Turbine Engine Page 168 of 290 CASA Part 66 - Training Materials Only  The air passes into the starter turbine housing, where it is directed against the rotor blades by the nozzle vanes, causing the turbine rotor to turn. As the rotor turns, it drives the reduction gear train and clutch arrangement, which includes the rotor pinion, planet gears and carrier, sprag clutch assembly, output shaft assembly, and drive coupling. The starter clutch also automatically disengages as the engine accelerates up to idle speed, and the rotation of the starter ceases. The turbine housing incorporates a turbine rotor containment ring designed to dissipate the energy of blade fragments and direct their discharge at low energy through the exhaust duct if the rotor fails due to turbine overspeed. The transmission housing contains the reduction gears, the clutch components and the drive coupling. It also provides a reservoir for the lubricating oil. Normal maintenance for air turbine starters includes checking the oil level, inspecting the magnetic chip detector for metal particles and checking for leaks. Air starter installation 2024-07-23 B1-15b Gas Turbine Engine Page 169 of 290 CASA Part 66 - Training Materials Only  Operation of Pneumatic Starters The Pressure Regulating and Shut-Off Valve consists of two sub-assemblies: The pressure-regulating valve, a butter y-type valve The pressure-regulating valve control, which contains a solenoid used to stop the action of the control crank in the OFF position. Aviation Australia Electronically-operated control and pressure regulating valve The operation of the air starter proceeds as follows: 1. Turn on the starter switch. This energises the regulating valve solenoid, which retracts and allows the control crank to rotate the pilot valve to the OPEN position. 2. The control crank is rotated by the control rod spring moving the control rod against the closed end of the bellows. 3. Since the regulating valve is closed and downstream pressure is negligible, the bellows can be fully extended by the bellows spring. 4. As the crank rotates to the open position, it causes the pilot valve rod to open the pilot valve, allowing upstream air to ow into the servo piston chamber. 2024-07-23 B1-15b Gas Turbine Engine Page 170 of 290 CASA Part 66 - Training Materials Only  5. The drain side of the pilot valve, which bleeds the servo chamber to atmosphere, is now closed by the pilot valve rod, and the servo piston moves to the right. 6. This linear motion of the servo piston is translated to rotary motion of the valve shaft. 7. This in turn rotates the butter y valve towards an open position. 8. As the regulating valve opens, downstream pressure increases and is bled back to the bellows through the pressure-sensing line. This compresses the bellows. 9. The compression of the bellows moves the control rod. 10. This turns the control crank and moves the pilot rod gradually away from the servo chamber to vent the air to atmosphere. When downstream pressure reaches a preset value, the amount of air owing into the servo chamber equals the amount of air being bled to atmosphere and the system is in a state of equilibrium. Aviation Australia Electronically-operated control and pressure regulating valve schematic When the regulating valve is open, the regulated air passing through the inlet housing of the starter impinges on the turbine in the starter motor. 2024-07-23 B1-15b Gas Turbine Engine Page 171 of 290 CASA Part 66 - Training Materials Only  When engine starting speed is reached, a set of yweights in a centrifugal cut-out switch actuates a plunger, which breaks the ground circuit of the regulating valve solenoid. This cut-out switch is located in the external gearbox. When the ground circuit is broken and the solenoid is de-energised, the pilot valve is forced back to the OFF position, opening the servo chamber to atmosphere. This action allows the actuator spring to move the regulating valve to the CLOSED position. To facilitate starter installation and removal, a mounting adapter is bolted to the mounting pad on the engine. Quick-detach clamps join the starter to the mounting adapter and inlet duct. This allows easier removal for maintenance or overhaul by disconnecting the electrical line, loosening the clamps and carefully disengaging the drive coupling from the engine starter drive as the starter is withdrawn. Aviation Australia "V" band clamps allow quick attachment and detachment of components 2024-07-23 B1-15b Gas Turbine Engine Page 172 of 290 CASA Part 66 - Training Materials Only  Air Turbine Starter Circuit With the master start switch on, pressing the start button energises the solenoid-operated air valve, which allows pneumatic pressure air to the starter. When the preset N2 speed is exceeded, the centrifugal switch in the starter releases under centrifugal force, which de-energises the air valve holding relay. This shuts off the air supply to the starter, allowing the engine to continue without the need for starter air. The system has now reset itself and is available for another engine start. Typical air turbine starter circuit Relevant Youtube link: Engine Starting (Video) 2024-07-23 B1-15b Gas Turbine Engine Page 173 of 290 CASA Part 66 - Training Materials Only  Ignition Systems Introduction to Ignition Systems A gas turbine ignition system contains three major components: A high-voltage exciter A high-voltage transmission lead An igniter plug. The exciter is powered by the aircraft electrical system, either DC, AC or PMA (Permanent Magnet Alternator). The high-voltage transmission lead connects the exciter to the igniter plug. The igniter plug then arcs (spark jumps a gap) to ignite the air-fuel mixture in the combustion chamber. After a normal start, ignition is no longer needed and the ignition system is deactivated. 2024-07-23 B1-15b Gas Turbine Engine Page 174 of 290 CASA Part 66 - Training Materials Only  Ignition System Types There two common classi cations of jet engine ignition systems: Low tension (DC voltage) High tension (AC voltage). Both low- and high-tension systems are in general use on today’s aircraft. Low-tension systems are designed to use DC and high-tension systems are designed to use AC as input power. DC-operated systems receive their power from the battery bus, and AC systems are powered from the aircraft AC bus or from a PMA. Although the operating voltages of the systems are different, both systems contain similar components as illustrated below. FADEC systems have dual-power systems to provide redundancy for operation. Aviation Australia Engine ignition system components All ignition systems can be grouped into one of two types of systems: Intermittent duty cycle Continuous duty cycle. 2024-07-23 B1-15b Gas Turbine Engine Page 175 of 290 CASA Part 66 - Training Materials Only  Duty cycle refers to the time limit placed on the operation of the ignition system by the manufacturer to prevent damage to its components. Intermittent duty cycle types draw suf ciently high amounts of current to cause overheating within their units if operated for extended periods. For this reason, they have a restricted duty cycle based on operating time, followed by a cooling-off period. For example, 2 min ON, 3 min OFF (cooling). Continuous duty types have long duty cycles or, in some cases, no limits at all. That is, they can be in continuous operation. Intermittent Duty Cycle Intermittent duty cycle ignition systems can only be used for short periods and only usually during ground starting. Once the engine has reached self-sustaining rpm, the ignition system is turned off. On other intermittent duty cycle type ignition systems, a low-tension, continuous-duty circuit is incorporated within one of the transformer units. This allows low power discharge to one igniter plug (which, again, can be selected by the pilot). This system can be operated for as long as there is a need for self-relight capability in ight. Intermittent duty cycle ignition system block diagram Continuous Duty Cycle If a continuous duty cycle main ignition system is installed, full ignition can be selected to both or either plug at the pilot’s discretion. During critical ight manoeuvres (e.g. take-off, landing and go- around), the pilot may select both igniter plugs to give instantaneous relight. The continuous duty cycle can also be selected during icing and/or turbulent conditions. High-intensity ignition systems capable of continuous operation need to be air cooled. 2024-07-23 B1-15b Gas Turbine Engine Page 176 of 290 CASA Part 66 - Training Materials Only  Ignition System Components Gas turbine engines are typically equipped with a dual high-energy ignition system. The principal components of a dual system are shown below and described on the following pages. Aviation Australia Dual high energy ignition system Ignition and Relight Switches The ignition and relight switches are located in the aircraft cabin, usually close to the throttles. They connect bus voltage to the ignition relay and HEIUs (High-Energy Ignition Units). Ignition Relay When energised, the ignition relay supplies electrical power to the HEIUs. It is contained in a control box which is usually located in an equipment compartment in the engine nacelle. High-Tension Ignition Leads The high-tension ignition leads are located on the aircraft engine, connected between the HEIUs and the igniter plugs. They conduct the high voltage from the HEIUs to the igniters. 2024-07-23 B1-15b Gas Turbine Engine Page 177 of 290 CASA Part 66 - Training Materials Only  High-Energy Ignition Units The HEIUs develop the high voltage necessary for engine ignition. In a dual-ignition system, there are always two units tted to each engine. An igniter plug is connected to each HEIU. HEIU Types The ignition system can be supplied with either AC or DC voltage, depending on the type of HEIUs tted. A DC type HEIU contains a trembler mechanism or a transistor circuit, while an AC type HEIU contains a transformer. In any case, the basic operation is similar for each of these types. HEIUs are rated in joules (1 J equals 1 W/s). They are designed to produce outputs which may vary according to requirements and are generally classi ed as either: High-joule (12–16 J) Low-joule (3–6 J). Although many engines are tted with high-joule HEIUs, low-joule units are suf cient for normal starting requirements. The high-joule units are required where it is necessary to relight the engine at high altitudes. Under normal ight conditions, HEIUs are turned off after the engines have started. But during take-off when ice, heavy rain or snow exists, the HEIUs may be operated continuously to give an immediate relight should an engine ame-out occur. This continuous operation is usually performed by low-joule HEIUs, as persistent operation of the high-joule units may reduce the life of the igniter plugs. To suit all engine operating conditions, a combined system has been developed in which one HEIU emits a high output to one igniter plug, and the second unit supplies a low-value output to the second igniter. As mentioned earlier, the basic operation of the different types of HEIUs is similar, so we will limit our discussion to the DC trembler-operated HEIU shown below. It contains the following components: Induction Coil: consists of primary and secondary windings. Trembler Mechanism: consists of a capacitor and a set of contacts which vibrate rapidly, opening and closing the primary circuit of the induction coil. Reservoir Capacitor: charges up, then discharges, supplying the HEIU’s high-voltage output. Glass-Sealed Discharge Gap: comprises two metallic contacts separated by an air gap, all encapsulated within a sealed glass tube. High-Voltage Recti er: converts the output of the induction coil to DC to charge the reservoir capacitor. Choke: an inductor which extends the time taken for the reservoir capacitor to discharge. Safety Resistor: bleeds off the capacitor to ground when the system is de-energised. 2024-07-23 B1-15b Gas Turbine Engine Page 178 of 290 CASA Part 66 - Training Materials Only  Aviation Australia Low tension HEIU Aviation Australia Low tension HEIU schematic 2024-07-23 B1-15b Gas Turbine Engine Page 179 of 290 CASA Part 66 - Training Materials Only  Igniter Plugs Due to the much-higher-intensity spark, igniter plugs for jet engines differ considerably from spark plugs used in reciprocating engines. They are normally constructed from nickel‑chromium alloy with silver-plated threads to prevent seizing. The hot end of the igniter plug is generally air cooled to keep it between 250 and 300 °C cooler than the surrounding gas temperatures. Cooling air is pulled inwards through the cooling holes in the ame tube, and over the end of the igniter, by the pressure differential between the primary and secondary combustor air ow. Glow Plugs Some smaller engines incorporate a glow plug type igniter rather than a spark igniter. This glow plug is a resistance coil of very high heat value and is said to be designed for extreme low temperature starting. A typical system is seen in some models of the Pratt & Whitney PT6 turboprop engine. The glow plug is supplied with 28-V DC at approximately 10 A to heat the coil to a yellow-hot condition. The coil is very similar in appearance to an automobile cigarette lighter. Air directed up through the coil mixes with fuel running down from the main fuel nozzle. This is designed to occur when the main nozzle is not completely atomising its discharge at low ow conditions during engine starting. The in uence of the air ow on the fuel creates a hot streak or blowtorch type ignition. After fuel is terminated, the air source serves to cool the igniter coil during the time the engine is being operated. Because this ignition system operates on low voltage at low amps, it does not have the inherent dangers and handling precautions of high-intensity capacitor discharge type ignition systems. © Aviation Australia Glow plugs 2024-07-23 B1-15b Gas Turbine Engine Page 180 of 290 CASA Part 66 - Training Materials Only  Low-Tension Igniters Igniter plugs for low-tension systems are referred to as the self-ionising or shunted gap type. The ring end contains a semi-conductive material which initially provides a path between the centre electrode and the ground electrode. As the initial current ows, the semiconductor reaches an incandescent state (glows white-hot). This heating is suf cient to ionise the air gap, and the main current ow takes this path to the ground electrode. A typical low-tension igniter is illustrated below. Low-tension igniter plug 2024-07-23 B1-15b Gas Turbine Engine Page 181 of 290 CASA Part 66 - Training Materials Only  High-Tension Igniters High-tension igniter plugs (or annular gap plugs) operate on a similar principle to a normal spark plug. The high-tension current passing through the plug initially causes the air gap between the electrodes to be ionised. This ionisation of the air gap allows the high-intensity spark to ow between the centre and ground electrodes. High-tension igniter plug Many types of igniter plugs are available, as shown below. Only one will normally suit the needs of a particular engine. Care must be taken to ensure the manufacturer’s recommended igniter plug is used. Relevant Youtube link: Igniter Testing (Video) 2024-07-23 B1-15b Gas Turbine Engine Page 182 of 290 CASA Part 66 - Training Materials Only  Igniter Plug Installation Most high-intensity igniter plugs are installed in a removable boss or adapter. The boss protects the combustion case from thread damage. The igniter and the boss are the wearing parts since they are far less expensive to replace than a combustion case. The boss is torqued to a greater torque than the igniter and is normally lock-wired to the combustion case. To prevent loosening, hold the boss with a spanner when removing the igniter. Igniter plug installation Relevant Youtube link: Ignition Systems (Video) 2024-07-23 B1-15b Gas Turbine Engine Page 183 of 290 CASA Part 66 - Training Materials Only  Ignition System Operation A schematic diagram of a basic jet engine ignition system is illustrated below. For simplicity, only one HEIU and one igniter plug are shown. When starting the engine, once the starter motor has been engaged and the engine’s rotating assembly begins to increase in speed, the aircrew closes the ignition switch. Then, 28-V DC is supplied to the ignition relay. Once energised, the ignition relay supplies voltage to the ‘ignition on’ light and to the input of the HEIU. The high-level, pulsating DC output voltage from the HEIU is conducted, via the high-tension ignition lead, to supply the igniter plug. At this stage, if the starter motor has increased the engine’s speed suf ciently to correctly mix the air and fuel supplied to the engine, ignition occurs. Once the air-fuel mixture has been ignited, the ame spreads rapidly through the engine combustion chambers; thus, the combustion is self-sustaining and the ignition system can be switched off. On many aircraft, a timer relay is employed to automatically shut down the ignition system after a predetermined time. Basic high energy jet engine ignition system 2024-07-23 B1-15b Gas Turbine Engine Page 184 of 290 CASA Part 66 - Training Materials Only  Starting Sequence Two separate systems are required to ensure that a gas turbine engine starts satisfactorily: Rotation of the compressor Ignition of the air-fuel mix. To help ensure that the engine comes on-speed quickly and without damage, it is necessary to control the sequence of events during a gas turbine engine starting cycle. The exact sequence of the starting procedure is important because there must be suf cient air ow through the engine to support combustion at the time the air-fuel mixture is ignited. The fuel rate is not suf cient to accelerate until after self-sustaining speed has been attained, and failure to correctly sequence the starting events prevents the engine from reaching this speed. The usual sequence of events during an engine start is: Select start (ignition on) High-pressure fuel on Light up Self-sustaining rpm Starter circuit cancelled Idle rpm stabilised. Graphical representation of RPM/Time/EGT during a correctly sequenced start 2024-07-23 B1-15b Gas Turbine Engine Page 185 of 290 CASA Part 66 - Training Materials Only  For ease of maintenance, it must be possible to motor over the engine without the ignition sequence initiating and to operate the ignition system without rotating the starter motor for in- ight relighting of the engine in the event of a ame-out. Engine Relight If a ame-out occurs while the aircraft is in ight, the engine continues to rotate due to the ow of air through the compressor. To re-ignite the air-fuel mixture in the engine combustion chamber, only a source of ignition is necessary. This is achieved by selecting the relight switch. With this switch closed, 28-V DC will be supplied directly to the HEIU. In some cases, however, a low-joule HEIU is tted and operated continuously, providing automatic relight. 2024-07-23 B1-15b Gas Turbine Engine Page 186 of 290 CASA Part 66 - Training Materials Only  Testing, Inspection and Maintenance Purpose of Testing, Inspection and Maintenance Maintenance of the turbine engine ignition system consists primarily of inspection, testing, troubleshooting, removal and installation. The following instructions are typical examples of inspection procedures that you may be required to perform. CAUTION: Prior to performing maintenance on an ignition system, always consult the relevant technical publication for all applicable safety precautions, maintenance procedures and speci cations. Maintenance warning Igniter Plugs The igniter plugs are inspected visually for burning and erosion of the electrode or shell, cracking of the ceramic insulator, and damage to the threads or ange. If damage is visible, the igniter should be discarded. Install with the proper anti-seize on the engine threads in accordance with the manufacturer’s instructions. An incorrect anti-seize may cause the igniter plug to weld into the boss. 2024-07-23 B1-15b Gas Turbine Engine Page 187 of 290 CASA Part 66 - Training Materials Only  High-Tension Ignition Leads The ignition leads are cleaned with an approved solvent and inspected for worn or burned areas, deep cuts, fraying and general deterioration. The ignition leads’ connectors are visually inspected for damaged threads, corrosion, cracked insulators, and bent or broken connector pins. The continuity of the leads is checked with a multimeter, and insulation properties are checked with a ‘Megger’ in accordance with speci cations laid down in the relevant technical publication. Operational Test Some aircraft servicing may require an operational test of the ignition system to check the serviceability of the HEIUs, high-tension leads and igniter plugs. In this test, the engine starter-motor is disabled so the engine will not rotate, preventing engine start. When the battery and relight switches are closed, sparking from the igniter plugs is clearly audible. This enables assessment of the ignition system’s serviceability. Another method is to simply start the engine. 2024-07-23 B1-15b Gas Turbine Engine Page 188 of 290 CASA Part 66 - Training Materials Only  Safety Precautions High-energy infers that a lethal charge is present and turbine engine ignition systems require special maintenance and handling. The manufacturer’s instructions and engine maintenance manuals should be fully understood and followed when handling any component of a jet engine ignition system. Some typical precautions are as follows: WARNING Dropping an igniter plug can fracture the internal ceramic or glass insulators. If the plug has been dropped, it should be rejected. Ensure that the ignition switch is turned off before performing any maintenance on the system. To remove an igniter plug, disconnect the transformer input lead, wait the time prescribed by the manufacturer (usually 1–5 min), then disconnect the igniter lead and ground the centre electrode to the engine. The igniter plug is now safe to remove. Exercise great caution in handling damaged transformer units. Some contain radioactive material (e.g. caesium‑barium 137). Unserviceable igniter plugs containing aluminium oxide and beryllium oxide, a toxic insulating material, should be disposed of properly. Before performing a ring test of igniters, the tter must ensure that the combustion chamber is not fuel-wetted, as a re or explosion could occur. Do not energise the system for troubleshooting if the igniter plugs are removed. Serious overheating of the transformers can result. 2024-07-23 B1-15b Gas Turbine Engine Page 189 of 290 CASA Part 66 - Training Materials Only