cooling effect of the air is limited. Approximately 75 percent  of the incoming airflow is used to cool the engine. TOT is used throughout this discussion for simplicity  purposes.  [Figure 1-94] The gas turbine engine mounted on most helicopters is  made up of a compressor, combustion chamber, turbine,  and accessory gearbox assembly. The compressor draws  filtered air into the plenum chamber and compresses it.  Common type filters are centrifugal swirl tubes where debris  is ejected outward and blown overboard prior to entering  the compressor, or engine barrier filters (EBF), a paper  element type filter, encased in a frame with a screen/grill  over the inlet, and usually coated with an oil. This design  significantly reduces the ingestion of foreign object debris  (FOD). The compressed air is directed to the combustion  section through discharge tubes where atomized fuel is  injected into it. The fuel/air mixture is ignited and allowed  to expand. This combustion gas is then forced through a  series of turbine wheels causing them to turn. These turbine  wheels provide power to both the engine compressor and the  accessory gearbox. Depending on model and manufacturer,  the rpm range can vary from a range low of 20,000 to a range  high of 51,600. The transmission system transfers power from the engine to  the main rotor, tail rotor, and other accessories during normal  flight conditions. The main components of the transmission  system are the main rotor transmission, tail rotor drive  system, clutch, and freewheeling unit. The freewheeling unit,  or autorotative clutch, allows the main rotor transmission to  drive the tail rotor drive shaft during autorotation. Helicopter  transmissions are normally lubricated and cooled with their  own oil supply. A sight gauge is provided to check the oil  level. Some transmissions have chip detectors located in the  sump. These detectors are wired to warning lights located  on the pilot’s instrument panel that illuminate in the event  of an internal problem. Some chip detectors on modern  helicopters have a “burn off” capability and attempt to correct  the situation without pilot action. If the problem cannot be  corrected on its own, the pilot must refer to the emergency  procedures for that particular helicopter.  Power is provided to the main rotor and tail rotor systems  through the freewheeling unit which is attached to the  accessory gearbox power output gear shaft. The combustion  gas is finally expelled through an exhaust outlet. The  temperature of gas is measured at different locations and is  referenced differently by each manufacturer. Some common  terms are: inter-turbine temperature (ITT), exhaust gas  temperature (EGT), or turbine outlet temperature (TOT).  The rotor system is the rotating part of a helicopter which  generates lift. The rotor consists of a mast, hub, and rotor  blades. The mast is a cylindrical metal shaft that extends  upwards from and is driven, and sometimes supported, by  the transmission. At the top of the mast is the attachment  point for the rotor blades called the hub. The rotor blades are  then attached to the hub by any number of different methods.  Main rotor systems are classified according to how the main  1-43 Figure 1-94.  Many helicopters use a turboshaft engine to drive the main transmission and rotor systems. The main difference between  a turboshaft and a turbojet engine is that most of the energy produced by the expanding gases is used to drive a turbine rather than  producing thrust through the expulsion of exhaust gases. Output Shaft Air inlet Compression Section Turbine Section Combustion Section Gearbox Section Inlet air Compressor discharge air Combustion gases Exhaust gases Combustion liner Exhaust air outlet Compressor rotor Fuel nozzle Igniter plug N1 Rotor N2 Rotor Stator Gear Main Rotor System Transmission