15.13 Start & Ignition Systems Flashcards
What is the primary function of the engine start system?
To ensure that a gas turbine engine starts satisfactorily by rotating the compressor and igniting the air/fuel mixture
The system consists of two separate systems: one for compressor rotation and the other for ignition.
What are the two systems required for starting a gas turbine engine?
- Compressor rotation system
- Ignition system
True or False: The starter must continue to assist the engine until it reaches self-sustaining speed.
True
What happens if the starter assistance is cut off below self-accelerating speed?
The engine may fail to reach idle speed or could decelerate.
What is a cartridge starter?
A starting method that uses high velocity gases from a burning cartridge to rotate the engine.
Describe how a cartridge starter is ignited.
An electrically fired detonator initiates the burning of the cartridge charge.
What is the primary advantage of air starters?
They are light, simple, and economical to operate.
What pressure range is typically used for air starters?
30–50 pounds per square inch (psi)
Fill in the blank: The starter turbine is rotated by air taken from an _______.
[external ground supply, auxiliary power unit, or cross-feed from a running engine]
What does the sprag clutch assembly do in an air starter?
Engages automatically as the rotor starts to turn and disengages when the drive coupling turns faster.
What is the function of the pressure regulating and shutoff valve?
It regulates air pressure and controls the air supply to the starter.
What are the two types of electric starting systems used on gas turbine engines?
- Direct cranking electrical systems
- Starter-generator systems
True or False: The direct cranking electric starter must disengage after the engine has started.
True
What is the advantage of a starter-generator system?
It combines the functions of both starter and generator, reducing weight and components.
What are the four field windings in a starter-generator?
- Series field (C field)
- Shunt field
- Compensating field
- Interpole or commutating winding
What is the typical voltage and peak amperes required for starting with a starter-generator?
24 volts and 1500 peak amperes
What is the role of the undercurrent controller in a starter-generator?
To ensure positive action of the starter and keep it operating until the engine reaches sufficient speed.
What must be checked if the starter does not operate?
- Air supply
- Electrical continuity in cutout switch
- Starter drive coupling
How can low air supply affect the starter?
It may prevent the starter from accelerating to normal cutoff speed.
What does the presence of metallic particles on the magnetic drain plug indicate?
Small fuzzy particles indicate normal wear; larger particles indicate internal difficulty.
What is the role of the shunt field in a conventional voltage control circuit for a generator?
It is connected to provide voltage control.
What do compensating and commutating or interpole windings provide?
Almost sparkless commutation from no load to full load.
What does the undercurrent controller do in a starter-generator?
Controls the starter-generator when used as a starter and assures positive action of the starter.
What is required to start an engine equipped with an undercurrent relay?
Close the engine master switch.
What action does energizing the throttle relay perform?
Starts the fuel pumps.
What three relays close when the battery and start switch is turned on?
- Motor relay
- Ignition relay
- Battery cut-out relay
Why is opening the battery circuit necessary during the starting process?
To prevent damage to the battery from the heavy drain of the starter motor.
What happens as the motor builds up speed in a starter-generator circuit?
The current draw of the motor begins to decrease.
What is the action taken by the undercurrent relay when current decreases to less than 200 amps?
It opens the circuit from the positive bus to the relay coils.
What occurs when the undercurrent relay de-energizes?
Stops the start operation sequence.
What can be used to break the circuit from the positive bus to the main contacts of the undercurrent relay if the engine fails to reach sufficient speed?
The stop switch.
What is a probable cause for the engine not rotating during a start attempt?
Low supply voltage to the starter.
What should be checked if the power switch is suspected to be defective?
Check the switch for continuity.
What should be done if the ignition switch in the throttle quadrant is defective?
Replace the switch.
What should be checked if the starter relay is suspected to be defective?
Check for 48 volts DC across the starter relay coil.
What indicates a defective starter?
Improper voltage at the starter with the start circuit energized.
What should you do if the starter drive shaft is sheared?
Replace the engine.
What is a probable cause if the engine starts but does not accelerate to idle?
Insufficient starter voltage.
What should be done if the engine fails to start when the throttle is placed in idle?
Check the ignition system for proper operation.
What is the primary purpose of the engine turbine ignition system?
To ignite the fuel in the combustion chamber during engine starting and to provide continuous ignition under certain conditions.
Continuous ignition is used during take-off, landing, and emergency situations.
How does the turbine engine ignition system operate during the starting sequence?
It operates for a short period to ignite the fuel and is switched off once combustion is initiated.
What is continuous ignition and when is it used?
Continuous ignition is a lower voltage mode used to relight fuel and prevent engine flameout during conditions like take-off, landing, or abnormal situations.
What type of ignition system do most turbine engines use?
A high-energy, capacitor-type ignition system that is air-cooled by fan airflow.
What components are typically included in a turbine engine ignition system?
- Two identical independent ignition units
- Two exciter units
- Two transformers
- Two intermediate ignition leads
- Two high tension leads
What is the role of the generator in the ignition system?
The generator produces power any time the engine is turning, supplying the ignition system with energy.
What is the maximum charge that the storage capacitor can assume in a typical ignition system?
Approximately 4 joules.
What is the significance of the high voltage supplied to the igniter spark gap?
It provides a high degree of reliability for ignition under varying conditions of altitude and temperature.
What is a key feature of the exciter unit in a capacitor-type ignition system?
It is hermetically sealed to protect components from adverse conditions and prevent flashover at altitude.
Describe the function of the igniter plug in a turbine engine ignition system.
It ignites the fuel and must withstand much higher energy currents than a conventional spark plug.
What distinguishes an annular gap igniter plug from a conventional spark plug?
Its electrode gap is larger, allowing it to arc more easily due to lower operating pressures.
What maintenance tasks are essential for the turbine engine ignition system?
- Inspection
- Testing
- Troubleshooting
- Removal and installation
What is a common issue if there is no igniter spark when the system is ON?
Possible issues include ignition relay problems, transformer unit failure, or issues with the high tension lead.
What should be inspected on the ignition lead terminals?
They should be free of arcing, carbon tracking, and cracks.
What is the recommended wait time after shutdown before handling high voltage components?
At least 20 minutes.
What are the signs of a faulty igniter plug?
- Cracked insulator
- Damaged semiconductor
- Hot electrode erosion
What should be done before disconnecting the ignition lead from the igniter plug?
Disconnect the low-voltage primary lead from the ignition exciter unit and wait at least one minute.
What is the danger associated with the turbine ignition system?
It uses very high voltages that can cause serious injury or death if safety precautions are not followed.
Fill in the blank: Continuous ignition is used to relight the fuel and keep the engine from _______.
stopping
True or False: The igniter can be checked by listening for a clicking noise as the engine begins to turn.
True
What should you always refer to when performing maintenance on the starting system?
The relevant engine maintenance manual for the specific engine type.
What must be done to circuit breaker switches before starting work on the starting system?
They must be open/off and tagged to indicate not to operate.
What hazard zones must be observed when testing the starting system?
Engine intake and exhaust hazard zones.
What type of gloves should be used when handling hot parts?
The correct hand gloves (PPE).
What is essential to follow when working on engine ignition systems?
All manufacturer’s instructions.
What can residual high voltage and current in the ignition exciter cause?
Injury or death if released.
What is the most likely way for an accidental discharge to occur?
By touching the igniter.
What is the standard procedure for performing igniter maintenance?
Disconnection of the igniter lead coupling nuts at the exciter end first.
What type of tools must be used when working on the ignition system?
Insulated tools.
What should not be touched with bare hands during ignition system maintenance?
Coupling nuts or connectors.
What can touching the ignition units without taking safety precautions result in?
Serious injury or death.
What should be ensured before touching the ignition units?
That there is no electricity left in the ignition units.
How long should you wait after opening the applicable circuit breakers before touching the ignition units?
At least three minutes.
