15.13 Start & Ignition Systems

Question 1

What is the primary function of the engine start system?

Answer 1

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.

Question 2

What are the two systems required for starting a gas turbine engine?

Answer 2

• Compressor rotation system
• Ignition system

Question 3

True or False: The starter must continue to assist the engine until it reaches self-sustaining speed.

Answer 3

True

Question 4

What happens if the starter assistance is cut off below self-accelerating speed?

Answer 4

The engine may fail to reach idle speed or could decelerate.

Question 5

What is a cartridge starter?

Answer 5

A starting method that uses high velocity gases from a burning cartridge to rotate the engine.

Question 6

Describe how a cartridge starter is ignited.

Answer 6

An electrically fired detonator initiates the burning of the cartridge charge.

Question 7

What is the primary advantage of air starters?

Answer 7

They are light, simple, and economical to operate.

Question 8

What pressure range is typically used for air starters?

Answer 8

30–50 pounds per square inch (psi)

Question 9

Fill in the blank: The starter turbine is rotated by air taken from an _______.

Answer 9

[external ground supply, auxiliary power unit, or cross-feed from a running engine]

Question 10

What does the sprag clutch assembly do in an air starter?

Answer 10

Engages automatically as the rotor starts to turn and disengages when the drive coupling turns faster.

Question 11

What is the function of the pressure regulating and shutoff valve?

Answer 11

It regulates air pressure and controls the air supply to the starter.

Question 12

What are the two types of electric starting systems used on gas turbine engines?

Answer 12

• Direct cranking electrical systems
• Starter-generator systems

Question 13

True or False: The direct cranking electric starter must disengage after the engine has started.

Answer 13

True

Question 14

What is the advantage of a starter-generator system?

Answer 14

It combines the functions of both starter and generator, reducing weight and components.

Question 15

What are the four field windings in a starter-generator?

Answer 15

• Series field (C field)
• Shunt field
• Compensating field
• Interpole or commutating winding

Question 16

What is the typical voltage and peak amperes required for starting with a starter-generator?

Answer 16

24 volts and 1500 peak amperes

Question 17

What is the role of the undercurrent controller in a starter-generator?

Answer 17

To ensure positive action of the starter and keep it operating until the engine reaches sufficient speed.

Question 18

What must be checked if the starter does not operate?

Answer 18

• Air supply
• Electrical continuity in cutout switch
• Starter drive coupling

Question 19

How can low air supply affect the starter?

Answer 19

It may prevent the starter from accelerating to normal cutoff speed.

Question 20

What does the presence of metallic particles on the magnetic drain plug indicate?

Answer 20

Small fuzzy particles indicate normal wear; larger particles indicate internal difficulty.

Question 21

What is the role of the shunt field in a conventional voltage control circuit for a generator?

Answer 21

It is connected to provide voltage control.

Question 22

What do compensating and commutating or interpole windings provide?

Answer 22

Almost sparkless commutation from no load to full load.

Question 23

What does the undercurrent controller do in a starter-generator?

Answer 23

Controls the starter-generator when used as a starter and assures positive action of the starter.

Question 24

What is required to start an engine equipped with an undercurrent relay?

Answer 24

Close the engine master switch.

Question 25

What action does energizing the throttle relay perform?

Answer 25

Starts the fuel pumps.

Question 26

What three relays close when the battery and start switch is turned on?

Answer 26

* Motor relay * Ignition relay * Battery cut-out relay

Question 27

Why is opening the battery circuit necessary during the starting process?

Answer 27

To prevent damage to the battery from the heavy drain of the starter motor.

Question 28

What happens as the motor builds up speed in a starter-generator circuit?

Answer 28

The current draw of the motor begins to decrease.

Question 29

What is the action taken by the undercurrent relay when current decreases to less than 200 amps?

Answer 29

It opens the circuit from the positive bus to the relay coils.

Question 30

What occurs when the undercurrent relay de-energizes?

Answer 30

Stops the start operation sequence.

Question 31

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?

Answer 31

The stop switch.

Question 32

What is a probable cause for the engine not rotating during a start attempt?

Answer 32

Low supply voltage to the starter.

Question 33

What should be checked if the power switch is suspected to be defective?

Answer 33

Check the switch for continuity.

Question 34

What should be done if the ignition switch in the throttle quadrant is defective?

Answer 34

Replace the switch.

Question 35

What should be checked if the starter relay is suspected to be defective?

Answer 35

Check for 48 volts DC across the starter relay coil.

Question 36

What indicates a defective starter?

Answer 36

Improper voltage at the starter with the start circuit energized.

Question 37

What should you do if the starter drive shaft is sheared?

Answer 37

Replace the engine.

Question 38

What is a probable cause if the engine starts but does not accelerate to idle?

Answer 38

Insufficient starter voltage.

Question 39

What should be done if the engine fails to start when the throttle is placed in idle?

Answer 39

Check the ignition system for proper operation.

Question 40

What is the primary purpose of the engine turbine ignition system?

Answer 40

To ignite the fuel in the combustion chamber during engine starting and to provide continuous ignition under certain conditions. ## Footnote Continuous ignition is used during take-off, landing, and emergency situations.

Question 41

How does the turbine engine ignition system operate during the starting sequence?

Answer 41

It operates for a short period to ignite the fuel and is switched off once combustion is initiated.

Question 42

What is continuous ignition and when is it used?

Answer 42

Continuous ignition is a lower voltage mode used to relight fuel and prevent engine flameout during conditions like take-off, landing, or abnormal situations.

Question 43

What type of ignition system do most turbine engines use?

Answer 43

A high-energy, capacitor-type ignition system that is air-cooled by fan airflow.

Question 44

What components are typically included in a turbine engine ignition system?

Answer 44

* Two identical independent ignition units * Two exciter units * Two transformers * Two intermediate ignition leads * Two high tension leads

Question 45

What is the role of the generator in the ignition system?

Answer 45

The generator produces power any time the engine is turning, supplying the ignition system with energy.

Question 46

What is the maximum charge that the storage capacitor can assume in a typical ignition system?

Answer 46

Approximately 4 joules.

Question 47

What is the significance of the high voltage supplied to the igniter spark gap?

Answer 47

It provides a high degree of reliability for ignition under varying conditions of altitude and temperature.

Question 48

What is a key feature of the exciter unit in a capacitor-type ignition system?

Answer 48

It is hermetically sealed to protect components from adverse conditions and prevent flashover at altitude.

Question 49

Describe the function of the igniter plug in a turbine engine ignition system.

Answer 49

It ignites the fuel and must withstand much higher energy currents than a conventional spark plug.

Question 50

What distinguishes an annular gap igniter plug from a conventional spark plug?

Answer 50

Its electrode gap is larger, allowing it to arc more easily due to lower operating pressures.

Question 51

What maintenance tasks are essential for the turbine engine ignition system?

Answer 51

* Inspection * Testing * Troubleshooting * Removal and installation

Question 52

What is a common issue if there is no igniter spark when the system is ON?

Answer 52

Possible issues include ignition relay problems, transformer unit failure, or issues with the high tension lead.

Question 53

What should be inspected on the ignition lead terminals?

Answer 53

They should be free of arcing, carbon tracking, and cracks.

Question 54

What is the recommended wait time after shutdown before handling high voltage components?

Answer 54

At least 20 minutes.

Question 55

What are the signs of a faulty igniter plug?

Answer 55

* Cracked insulator * Damaged semiconductor * Hot electrode erosion

Question 56

What should be done before disconnecting the ignition lead from the igniter plug?

Answer 56

Disconnect the low-voltage primary lead from the ignition exciter unit and wait at least one minute.

Question 57

What is the danger associated with the turbine ignition system?

Answer 57

It uses very high voltages that can cause serious injury or death if safety precautions are not followed.

Question 58

Fill in the blank: Continuous ignition is used to relight the fuel and keep the engine from _______.

Answer 58

stopping

Question 59

True or False: The igniter can be checked by listening for a clicking noise as the engine begins to turn.

Answer 59

True

Question 60

What should you always refer to when performing maintenance on the starting system?

Answer 60

The relevant engine maintenance manual for the specific engine type.

Question 61

What must be done to circuit breaker switches before starting work on the starting system?

Answer 61

They must be open/off and tagged to indicate not to operate.

Question 62

What hazard zones must be observed when testing the starting system?

Answer 62

Engine intake and exhaust hazard zones.

Question 63

What type of gloves should be used when handling hot parts?

Answer 63

The correct hand gloves (PPE).

Question 64

What is essential to follow when working on engine ignition systems?

Answer 64

All manufacturer's instructions.

Question 65

What can residual high voltage and current in the ignition exciter cause?

Answer 65

Injury or death if released.

Question 66

What is the most likely way for an accidental discharge to occur?

Answer 66

By touching the igniter.

Question 67

What is the standard procedure for performing igniter maintenance?

Answer 67

Disconnection of the igniter lead coupling nuts at the exciter end first.

Question 68

What type of tools must be used when working on the ignition system?

Answer 68

Insulated tools.

Question 69

What should not be touched with bare hands during ignition system maintenance?

Answer 69

Coupling nuts or connectors.

Question 70

What can touching the ignition units without taking safety precautions result in?

Answer 70

Serious injury or death.

Question 71

What should be ensured before touching the ignition units?

Answer 71

That there is no electricity left in the ignition units.

Question 72

How long should you wait after opening the applicable circuit breakers before touching the ignition units?

Answer 72

At least three minutes.