DESCRIBE the types of accessories used on aircraft
- Accessories for gas turbine engines can be divided into 2 categories:
- Those driven by Bleed Air (Air Driven Accessories)
- Those driven Mechanically (Mechanically Driven Accessories)
DESCRIBE how accessories are driven
- Air Driven Accessories
- Compressor discharge air at high pressure and temperature is bled from the engine through ports or valves at intervals along the compressor case and at the end of the diffuser.
- It is ducted as a source of power for operating air conditioning units, cockpit pressurization and engine anti ice.
- Dual axial compressor engines usually have 3 separate bleed air systems:
- High pressure, low pressure and interstage bleed air.
- The high and low pressure systems are used to drive aircraft and engine components or accessories, while the interstage bleed valves are required to ensure compressor stability.
- Mechanically Driven AccessoriesThe other method of driving accessories is by a mechanical, geared drive taken directly from the main shaft connecting the turbine to the compressor.
- This method is used for tachometers, hydraulic pumps, generators, alternators and other accessories mounted near or connected directly to the engine.
DEFINE interstage bleed air
Ensure compressor stability. It is ducted overboard to prevent compressor stall during certain periods of low thrust operations. It is not used to drive accessories since it is not available at high thrust settings. It also lacks steady volume or pressure.
DESCRIBE the starting sequence for a gas turbine engine
- As soon as the starter has accelerated the compressor sufficiently to establish airflow through the engine, the ignition is activated and then the fuel is added.
- The starter must continue to accelerate the engine after light off and even after the engine reaches self accelerating speed.
- The higher the compressor RPM before the starter is secured, the shorter will be the total time required for the engine to attain idle RPM.
DESCRIBE abnormal starts of a gas turbine engine
- Hot StartExceeding the maximum allowable temperature for the turbine section during start.
- Hung StartSituation where the temperature within the turbine section continues to rise and the compressor RPM stabilizes below normal.
- False Start
- Occurs when compressor rpm stabilizes below normal and the turbine temperature remains within limits.
- Wet Start
- Situation in which the fuel air mixture does not light off initially, but has the capability to eventually ignite.
- Ignition problem and most dangerous type of abnormal start.
DESCRIBE a DC electric starter
- Most common type used on small engines.
- Mechanically connected to the compressor and is mounted on either the engine accessory gear box or the front frame of the engine.
- A battery, auxiliary power unit, or external electrical source may be used to supply power to the starter motor.
- T-34 and T-37 has a starter generator mounted on the engine accessory drive section and is able to function either as a starter or a generator.
DESCRIBE an air turbine starter
- Most common type used on large turbine engines.
- A small, geared, air turbine motor is attached t the engine. Air is directed to the air turbine, which accelerates the compressor. This air is supplied by a ground cart or an internal power unit (APU).
- On multi engine aircraft, after one engine is on line, bleed air from that engine is used to start the remaining engines.
DESCRIBE a basic aircraft ignition system
A high energy capacitor type ignition system is normally used for gas turbine engines. This provides both high voltage and an exceptionally hot spark, which gives an excellent chance of igniting the fuel air mixture at reasonably high altitudes.
DESCRIBE the two types of ignitors
- Annular Gap
- Protrudes slightly into the combustion chamber liner to provide an effective spark.
- Constrained Gap
- Does not follow the face of the plug. Instead, it tends t jump in an arc which carries it beyond the face of the chamber liner.
- The Constrained Gap plug operates at a cooler temperature than that of the annular gap plug.