Turboshaft engines have a high power-to-weight ratio and are widely used in helicopters. Figure 6-16 shows a typical turboshaft engine. This engine is an axial-flow turboshaft engine incorporating the free turbine principle. It is comprised of a compressor, combustor, gas generator turbine, and power turbine. The engine is equipped with a control system that modulates fuel flow to maintain constant power turbine output speed for a given speed selector setting in the governed range. This system maintains the selected speed by automatically changing the fuel flow to increase or decrease gas generator speed. The pilot determines the speed by positioning the power lever. The control system provides automatic protec- tion against compressor stall, turbine overtemperature, overspeed of either turbine assembly, and combustion flameout. An emergency throttle system is provided for use in case of fuel control failure. A starter, mounted at the nose of the engine, drives the gas generator rotor and engine accessories for engine starting. The engine is installed with its nose facing forward and supported by engine mounts bolted to the aircraft fuselage. Air is supplied to the engine through the inlet air duct, located inside the right-hand side door of the center nacelle. An alternate air door is attached to the duct by a hinge. Air is supplied through the alternate air door when an insufficient amount of air comes into the engine through the main air duct. The engine is installed so that with the nacelle removed, all accessories and components can be easily reached and maintained. Turbofan Engines There are also many different models of this type of engine. The S-3, AV-8, and F/A-18 are examples of aircraft that use this engine. The turbofan engine (fig. 6-17) is similar to the turboprop,  except  a  fan  replaces  the  turboprop propeller. One basic operational difference between the two engines is the airflow. The fan is inside a cowling, and as a result the airflow through the fan is unaffected by the aircraft's speed. These factors eliminate loss of operational efficiency at high speeds, which limits the maximum airspeed of propeller-driven (turboprop) aircraft. The turbofan engine has a duct-enclosed fan mounted at the front or rear of the engine. The fan runs at the same speed as the compressor, or it may be mechanically geared down. An independent turbine located to the rear of the compressor drive turbine may also drive the fan. The fan draws in more air than the compressor of a turbojet engine because of the larger area of the inlet. Because the larger amount of air is compressed and accelerated by the fan, the air completely bypasses the burner and turbine sections of the engine and exits through the fan exit ducts. Since the air is not heated by burning fuel to obtain thrust, the turbofan engine has lower fuel consumption. To develop thrust, the turbofan engine accelerates a large amount of air at a relatively low velocity, which improves its propulsion efficiency. Compared to the turbojet, the turbofan engine has a low engine noise level. The low noise level results from the lower gas velocity as it exits the engine tailpipe. One reason for the decreased velocity is an additional turbine stage in the engine. This additional turbine stage extracts power from the exhaust gases to drive the fan. The aircraft powered by a turbofan engine has a shorter takeoff distance and produces more thrust during climb than a turbojet of approximately the same size. This extra thrust allows the turbofan aircraft to take off at a much higher gross weight. Gas Turbine Engine Component Controls, Systems, And Sections In addition to the five major components discussed as part of the turbojet engine, there are numerous controls, systems, and sections that are common to all four types of gas turbine engines. Among the more important of these are the fuel control, lubrication system, ignition system, and accessory section. FUEL CONTROL.—Depending upon the type of engine and the performance expected of it, fuel controls may vary in complexity. They may range from simple valves to automatic computing controls containing hundreds of intricate, highly machined parts. The pilot of a gas turbine powered aircraft does not directly control the engine. The pilot's relation to the power plant corresponds to that of the bridge officer on a ship. The bridge officer obtains engine response by relaying orders to an engineer below deck, who, in turn, actually moves the throttle of the engine. Modern fuel controls are divided into two basic groups, hydromechanical and electronic. The controls sense some or all of the following engine operating variables: 1. Pilot's demands (throttle position) 6-11