Chapter 1 - Engine Starting and Ignition Systems

Question 1

What do gas turbine engines require to achieve their self-sustaining RPM, as compared to a piston engine?

Answer 1

A higher rate of turning

Question 2

What is self-sustaining RPM?

Answer 2

The engine speed from which the engine will accelerate to idle RPM, under its own power, smoothly and without overheating

Question 3

What is the balancing act of starting an engine?

Answer 3

Short a start time as possible to prevent stall, but without exceeding the turbine temperature limitation which could lead to ‘thermal shock’

Question 4

What is a ‘hung start’?

Answer 4

The combined torque of the starter and the turbine isn’t enough to keep the engine accelerating to idle RPM

Question 5

Why should an engine be shut down if a ‘hung start’ has resulted?

Answer 5

If fuel is burning but the engine fails to accelerate, a very high turbine temperature will be apparent

Question 6

What might cause a ‘hung start’?

Answer 6

HP shut-off valve has not been opened

Question 7

What three things does a start require?

Answer 7

• Rotation (for air)
• Fuel
• Ignition

Question 8

What is meant by the fact that Isopropyl Nitrate (IPN) is a monofuel?

Answer 8

Provides its own oxygen

Question 9

What is the health and safety hazard of IPN?

Answer 9

Easily passes into the bloodstream and can give headaches

Question 10

Describe a gas turbine starter?

Answer 10

Free turbine connected to the main shaft of the engine(s) to be started

Gas turbine is itself started by a small electric motor and is then used to start the main engine

Once the main engine is started, the gas turbine starter is shutdown

Question 11

What are the sources of air for the low pressure air starter unit?

Answer 11

• APU
• Ground supply
• An engine that is already running

Question 12

What are the three types of valves used in air starter systems?

Answer 12

• Non-return valves
• ON/OFF valves
• Pressure regulating valves

Question 13

What nominal value will the pressure regulating valve hold the pressure to?

Answer 13

28 ±2 psi

Question 14

Where will the igniter plugs in the combustion chambers ignite the fuel?

Answer 14

As it is sprayed from the burners in to the toroidal vortex

Question 15

What time must elapse before handling the various types of ignition system?

Answer 15

HT lead or ignite plug - at least 1 minute

Air gap plugs - at least 3 minutes

Question 16

What is the charge delivered by a HEIU?

Answer 16

Anywhere between 18,000 to 28,000 volts

Question 17

When will ignition be automatically switched on?

Answer 17

Continuously during a stall warning, and for 1 minute after the stall warning has cancelled

Question 18

When will ignition be manually switched on (not including starting)?

Answer 18

When landing (or taking-off) on very wet or slushy runways

Question 19

In what form is the energy dissipated in a HEIU?

Answer 19

High-voltage, high current discharge

Question 20

What is the purpose of the choke coil in a HEIU?

Answer 20

Flatten out the jagged bursts of charge and smooth them out to make for a longer duration spark at the plug

Question 21

What is the purpose of the safety resistor in the HEIU?

Answer 21

Protect the circuit in the event of it firing with no plug installed

Question 22

What is the purpose of the discharge resistor in the HEIU?

Answer 22

Discharge the contents of the reservoir (storage) capacitor to earth to make the unit safe to handle

Question 23

What is the purpose of the HT lead?

Answer 23

Protect other electrical and electronic services from interference from the very strong magnetic pulse which will occur every second or so

Question 24

Describe the discharge of a surface discharge plug?

Answer 24

High-energy flashover from the pellet to the body which projects the spark an inch or more into the combustion chamber

Question 25

What is the central electrode in an air gap type igniter plug, as in cathode or anode?

Answer 25

Cathode

Question 26

What is required when starting an engine from a running engine?

Answer 26

The RPM of the supplying engine must be raised to the specified point to give the correct air pressure and to prevent the supply engine from exceeding its turbine temperature limitations

Question 27

What are the types of engine starting?

Answer 27

• Isopropyl Nitrate
• Gas turbine
• Windmill
• Electric
• Low pressure air turbine

Question 28

Where might a sprag clutch be found?

Answer 28

Inside a low pressure air turbine starter

Question 29

What type of ignition system is used on all jet engines?

Answer 29

High-energy ignition with a dual system, with each ignition unit connected to its own igniter plug

Question 30

How does a typical DC ignition unit work?

Answer 30

An induction coil, operated by a trembler mechanism, passes an AC voltage into the secondary circuit

AC is half-wave rectified by a diode and the remaining energy is stored in a large capacitor

When the energy stored in the capacitor creates sufficient ionisation in a sealed glass chamber, the charge will cross the discharge gap, through a choke coil to the plug