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The Fuel system

The Fuel system consists of a two stage pump, a filter a hydromechanical fuel control unit (FCU), fuel manifolds, a flow divider and fuel spray nozzles located in the annular combustion chamber.


Garrett TPE331 engine fuel system

The engine driven fuel pump is a two stage pump, the first stage being a low pressure pump that receives the fuel from the fuel tanks at boost pump pressure.

The first stage of the pump then increases this pressure and directs the fuel through a bypass filter to the second stage high pressure side of the fuel pump where the fuel pressure is increased for efficient operation of the engine fuel system.


Engine driven fuel pump components

The engines hydromechanical fuel control unit (FCU) functions to meter the fuel for all phases of engine operation.

The fuel that is distibuted to the spray nozzles at the combustion chamber is delivered at calculated amounts based on the amount and pressure of air being discharged from the engine compressors (P3 air pressure).


The hydromechanical fuel control unit (FCU)

The FCU is a technical piece of equipment and includes the:

  • P3 Bellows,
  • Metering valve,
  • Bypass valve,
  • Electronic fuel shutoff valve,
  • Manual shutoff valve,
  • Fuel enrichment valve, and the
  • Over speed governor.

The FCU operates in two modes, Automatic and Manual. The automatic mode is the normal computer controlled mode of operation and is known as ‘Normal Mode’. The manual mode of operation is the computer off mode and is known as ‘Manual Mode’.

During Manual mode, the governing P3 air pressure value is a mechanical function of the power lever position which is controlled by the pilot. This is an electrical function during Normal mode operations. The resulting P3 air pressure will position the fuel metering valve to allow the correct ratio of fuel to flow through the system to the fuel spray nozzles in the combustion chamber. The bypass valve in the FCU will allow any excess fuel not required for the fuel spray nozzles to return through the pump via the inlet side of the second stage high pressure fuel pump.


Bypass valve allowing excess fuel to return via the high pressure pump

During the start cycle the electronic and manual fuel shutoff valves are closed below 10% RPM. In Normal mode, the electronic fuel shutoff valve is opened by the fuel computer. In Manual mode, the manual fuel shutoff valve is opened by the pilot selecting the 10% switch at 10% RPM.

To shut down each engine, the electronic and manual fuel shutoff valves will close when the appropriate engine STOP buttons are pushed. The manual fuel shutoff valve is connected in series with the electronic fuel shutoff. When either switch is closed the engine will shutdown.

Start Manual switches used for engine start during manual mode Engine stop buttons

The manual fuel shutoff valve also acts as an emergency fuel shutoff and is mechanically operated by the respective condition lever which is mounted on the centre pedestal. The condition lever when moved to the EMER SHUTOFF position will also move the propeller feathering valve to the feather position after moving the emergency fuel shutoff valve to the closed position.


Left hand Condition lever shown in EMER SHUTOFF position

This interconnection between the manual fuel shutoff valve and the propeller feathering valve ensures that the fuel is shutoff before the propeller begins to feather. Moving the condition lever out of EMER SHUTOFF opens the manual fuel shutoff valve and allows the feather valve spring to return the feather valve spring to its normal position. Engine oil pressure will be required to actually unfeather the propeller in flight.


Mechanically controlled feather valve

 
Fuel System (p.1)


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