Chp 3 ( Starting System) Flashcards
What do all gas turbine engines require for starting?
All gas turbine engines require starter systems which can accelerate the engine from rest to a speed for stable operation.
What are the basic components of a starting system?
The basic components are: 1. Starter Motor 2. Ignition Circuit 3. Fuel/Air Supply 4. Control System.
What is the function of the Starter Motor?
The Starter Motor is capable of turning the engine to self-sustaining speed and can do so without ignition or fuel.
What does the Ignition Circuit do?
The Ignition Circuit provides a high energy ignition system to establish combustion and operates independently.
What is the role of the Fuel/Air Supply?
The Fuel/Air Supply introduces a fuel/air mixture into the engine for combustion.
What is the purpose of the Control System?
The Control System monitors the start sequence and protects speed and temperature limits during the initial unstable stage of operation.
What is Self-Sustaining Speed?
Self-Sustaining Speed is the lowest RPM from where the turbine will accelerate the engine without assistance from the starter.
What types of starters are there?
Types of starters vary with engine aircraft requirements and may include electric, gas turbine (GTS), air, or hydraulic.
How does an Electric Starter Motor function?
Most helicopter engines use a starter generator for initial engine turning, mounted on the accessory gearbox, turning the compressor until self-sustaining speed is reached.
What happens after the engine reaches self-sustaining speed?
Once self-sustaining speed is reached, the drive is from the rotating assemblies through the radial drive to the accessory gearbox, where it drives the starter generator as a generator.
Why is turning the engine necessary?
Turning the engine is necessary to create conditions within the combustion chamber for ignition to take place.
What are the advantages of a Starter Generator?
Less weight, Low Cost, No Disengagement mechanism (No Dog Clutch).
What is the normal operating time for a starter to prevent overheating?
Approximately 30 seconds.
What is the required cooling period between each 20 seconds start cycle?
Normally 1 minute.
How long should the starter cool after 3 attempts?
Approximately 15 minutes.
What three things must be present for ignition to take place?
Compressed Air, Atomised fuel, Ignition-spark.
What is the function of the Igniter Plug?
It is positioned to ignite the mixture in the combustion chamber.
What is a Surface Discharge Plug?
A discharge plug with no air gap, where the discharge is initiated by electrical leakage.
How does a heavily carbonised igniter plug perform compared to a clean plug?
It gives a better spark by causing greater initial leakage.
What is an Air Gap Plug?
The air gap type plug is similar in operation and design to a piston engine ‘spark plug’. It has a large air gap between the electrode and body for the spark to cross.
What voltage is required for an Air Gap Plug to function?
It requires a potential difference of 25,000 volts to ionise the gap before a spark will occur.
What is the function of a Torch Igniter?
The Torch Igniter provides a finely atomised, independent fuel spray and a High Energy, Surface Discharge Plug to ignite the fuel.
How is fuel and electricity controlled in a Torch Igniter?
Fuel and electricity to the Torch Igniter is controlled by the Starter Switch, and on release of the switch, the fuel flow is cut off by a Solenoid Valve.
What happens to the Torch Igniter after the Starter Switch is released?
The Torch Igniter is purged by air to prevent carbon forming.
What provides compressed air in the starting cycle of a torch igniter system?
The compressors turning under the power of the starter motor.
How is atomised fuel delivered into the combustion chamber?
By the injectors. The fuel may come from a separate fuel system or as part of the main fuel control unit.
What generates the electric spark in the torch igniter system?
A high energy ignition unit delivers the spark into the combustion chamber adjacent to the fuel injectors.
What happens when the fuel ignites in the combustion chamber?
The burning fuel plus the starter motor increases the RPM of the engine.
What occurs at a certain RPM during the starting cycle?
The heat energy is sufficient for the engine to accelerate, and it becomes ‘Self-Sustaining’.
What can be switched off once the engine is self-sustaining?
The starter motor along with the ignition systems.
What are High-Energy Ignition Units (HEIU’s)?
HEIU’s are used in most engines and are designed to deliver a high voltage, high current discharge at the igniter plug from a low voltage AC or DC supply.
How many igniters does each engine usually have?
Each engine usually has two separate igniters, positioned in opposite sides of the flame tube.
What is the lethal warning associated with HEIU’s?
The energy stored in the HEIU is potentially dangerous even though the capacitor is discharged.
What should be done when the electrical supply is disconnected?
Disconnect the low-tension lead and wait at least one minute to allow the capacitor to discharge.
What are the uses of the ignition system?
The ignition system can be used during start up, testing, re-light, icing conditions, turbulence, and take off in heavy rain.
What is the first routine maintenance point for starting systems?
Starter/generator. Security of attachment of clamp (V-clamp and worm) drive clips. Inspect for high frequency vibration damage.
What should be checked regarding cables in maintenance?
Security of attachment of cables. The cables and leads should be examined for serviceability (i.e. damage, contamination from fuel, oil, etc.)
What is a critical issue with damaged ignition leads?
If the HT leads become damaged, then the HT Energy could track, which could prove to be fatal.
What problems can occur with igniters/injectors?
Injectors are prone to blockage from carbon build up. Igniter plugs are susceptible to erosion due to high voltage discharge. Either problem will result in a slow wet start or no start at all.
How is the igniter tested?
A press to test button on the pilots’ console is usually tested before each start (cracking noise).
