Fuel quantity Fuel tank FUEL SHOT OFF LOW FUEL LEVEL WARNING LIGHT MIX PULL LEAN Carburetor Throttle Shut-off valve Fuel strainer Primer Primer nozzle at cylinder Figure 4-22.  A typical gravity feed fuel system, in a helicopter with  a reciprocating engine, contains the components shown here.  4-13 Fuel Supply System Fuel Systems Centrifugal Clutch Some aircraft utilize a clutch for starting. This allows the  engine to be started without requiring power to turn the  transmission. One advantage this concept has is that without  a load on the engine starting may be accomplished with  minimal throttle application. However, caution should also  be used during starting, since rapid or large throttle inputs  may cause overspeeds.  tanks are usually mounted to the airframe as close as possible  to the CG. This way, as fuel is burned off, there is a negligible  effect on the CG. A drain valve located on the bottom of the  fuel tank allows the pilot to drain water and sediment that may  have collected in the tank. A fuel vent prevents the formation  of a vacuum in the tank, and an overflow drain allows fuel to  expand without rupturing the tank.  The fuel travels from the fuel tank through a shut-off valve,  which provides a means to completely stop fuel flow to the  engine in the event of an emergency or fire. The shut-off  valve remains in the open position for all normal operations. Once the engine is running, tension on the belts is gradually  increased. When the rotor and engine tachometer needles are  superimposed, the rotor and the engine are synchronized, and  the clutch is then fully engaged. Advantages of this system  include vibration isolation, simple maintenance. When the  clutch is not engaged, engines are very easy to overspeed,  resulting in costly inspections and maintenance. Power, or  throttle control, is very important in this phase of engine  operation. Most non-gravity feed fuel systems contain both an electric  pump and a mechanical engine-driven pump. The electrical  pump is used to maintain positive fuel pressure to the  engine pump and may also serve as a backup in the event of  mechanical pump failure. The electrical pump is controlled  by a switch in the cockpit. The engine driven pump is the  primary pump that supplies fuel to the engine and operates  any time the engine is running. A fuel filter removes moisture  and other sediment from the fuel before it reaches the engine.  These contaminants are usually heavier than fuel and settle to  the bottom of the fuel filter sump where they can be drained  out by the pilot. A centrifugal clutch is made up of an inner assembly and  an outer drum. The inner assembly, which is connected to  the engine driveshaft, consists of shoes lined with material  similar to automotive brake linings. At low engine speeds,  springs hold the shoes in, so there is no contact with the outer  drum, which is attached to the transmission input shaft. As  engine speed increases, centrifugal force causes the clutch  shoes to move outward and begin sliding against the outer  drum. The transmission input shaft begins to rotate, causing  the rotor to turn slowly at first, but increasing as the friction  increases between the clutch shoes and transmission drum.  As rotor speed increases, the rotor tachometer needle shows  an increase by moving toward the engine tachometer needle.  When the two needles are superimposed (in the case of a  coaxial-type gage), the engine and the rotor are synchronized,  indicating the clutch is fully engaged and there is no further  slippage of the clutch shoes. The turbine engine engages the clutch through centrifugal  force, as stated above. Unless a rotor brake is used to  separate the automatic engagement of the main driveshaft and  subsequently the main rotor, the drive shaft turns at the same  time as the engine and the inner drum of the freewheeling unit  engages gradually to turn the main rotor system. The fuel system in a helicopter is made up of two components:  supply and control. The supply system consists of a fuel tank or tanks, fuel quantity  gauges, a shut-off valve, fuel filter, a fuel line to the engine,  and possibly a primer and fuel pumps.  [Figure 4-22]  The fuel 