Fuel System

 

The left and right wing fuel tanks each consist of two sections: one inboard and one outboard of the engine nacelle. Each wing tank supplies fuel to its respective engine via a hopper tank located in the inboard section of the tank. The hopper tank is supplied with fuel from the main tank through flapper check valves.  

The hopper tank, which holds 200 pounds, provides a positive supply of fuel to the engine feed system through flapper check valves in the wall of the tank. A negative g suction feed inlet cannister in the hopper surrounds the inlet check valve to ensure a fuel supply during momentary negative g conditions. Each outboard section contains two fuel level sensors, an overfill float switch, and a filler cap on the top surface of the wing. 


WING TANKS

The wing tanks are actually two sections connected by a vent hose and an interconnect hose that permits gravity flow from the outboard section to the inboard section. Total capacity is 5,690 pounds usable fuel. A flapper check valve at the inboard end of the interconnect hose prevents reverse flow during wing-low flight. Each inboard section contains a hopper tank, standby boost pump, fueling valve, low-level float switch, magnetic dipstick and drain valves . Fuel can gravity flow between wing tanks when the CONN VALVE is open to correct an imbalance. Fuel quantity indicators are calibrated to show usable fuel remaining.

VENTING SYSTEM

Each wing tank has its own independent vent system. The inboard section is vented to the outboard section which, is vented via a vent line port on the underside of the wing.  A baffle in the outboard section prevents fuel from sloshing out through the vent. The vent system maintains a slight positive pressure on the fuel during flight. It also provides an outlet if the fuel tank is overfilled.

ENGINE FEED SYSTEM

There are two separate, independent engine feed systems to provide fuel to the engines. Each system consists of an engine-driven boost pump, standby boost pump, two differential pressure switches, suction feed inlet check valve, fire shutoff valve, and a fuel filter. Engine-Driven Boost Pump This pump, creating suction to draw fuel from its respective tank, is the normal source of fuel supply for the engine. A pressure switch, sensing differential pressure, automatically activates the electrically driven standby boost pump if the engine-driven pump fails. 

  • Main Fuel Pump 
The main fuel pump is driven by the engine accessory gearbox and gets fuel from the inboard tank to the HP pump inlet. Main pump failure results in automatic activation of the related standby pump located inside the respective fuel hopper when pressure differential across the engine-driven pumps is 5 psi.
  •  HP Fuel Pump 

The high pressure fuel pump develops the high pressure necessary to spray the fuel into the burner can. The pump is located inside the Hydro Mechanical Unit/HMU. The HP fuel pump is driven by the engine accessory gearbox. The HP pump inlet is pressurized by the main pump to prevent fuel cavitation within the HP pump. Failure of the HP pump will result in an immediate flameout.

  • Standby Pump

Inside the hopper tank, it acts as a back up to the related main pump. Each standby pump is powered electrically by its respective battery bus  They are controlled via the L and R STBY PUMP switches on the overhead fuel panel in channel A or via pressure switches that sense engine-driven boost pump pressure. The switches have positions labeled "AUTO," "OVRD," and "OFF." Normal operation is in the guarded "AUTO" mode position. In OVRD, the pumps will operate continuously. In the OFF position pump operation is inhibited.   

 

During engine start, the standby boost pump is actuated to provide fuel pressure to the engine. With the STBY PUMP switch in AUTO and the engine condition lever advanced to START, a switch sensing pressure differential at the engine-driven boost pump completes a circuit to start the standby boost pump, which supplies pressure to turn on the STBY PUMP ON light. The standby boost pump, drawing fuel from the hopper, provides fuel through the engine-driven boost pump, fuel heater, and filter to the engine. As the engine starts, the pressure switch senses main fuel pump pressure and the MAIN PUMP light goes out. The STBY PUMP ON light goes out, indicating the standby boost pump has been deenergized. The engine-driven boost pump is now providing flow for engine operation, drawing fuel through the inlet check valve in the negative-g cannister within the hopper.

FUEL SHUTOFF

 A fire shutoff valve is located in each fuel feed line on the front spar inboard of the engine. The valves are controlled by the L and R ENG fire handles on the overhead panel and operated via 28-VDC electrical power from their respective left and right battery buses. 


INTERCONNECT VALVE

The interconnect valve is a 28-VDC motor operated valve in the line connecting left and right fuel tanks. The valve is controlled by the CONN VALVE switch with positions labeled "OPEN" and "CLOSED." In each wing fuel gravity flows from the outboard section to the inboard section through the interconnect hose equipped with a flapper valve to prevent reverse flow. An electrically operated interconnect valve in the line connecting left and right tanks can be opened with the CONN VLV switch on the overhead panel. With the valve open, gravity flow balances the fuel load in each wing. The maximum allowable imbalance is 200 pounds. Fuel flows from the wing tank into the hopper tank through flapper check valves. The amber CONN VLV OPEN light above the switch remains illuminated until the valve is fully closed.

 CROSS FEEDING 

The crossfeed valve is opened by selecting the X-FEED switch on the overhead fuel panel to ON. Fuel can then be transfered from the opposite side fuel tank directly to the engine, not the opposite side tank. The related STBY PUMP must also be selected OVRD for the tank in which fuel quantity is to be reduced.

FUEL FILTER

A fuel filter is installed in each fuel feed line downstream of the related engine-driven boost pump. The L or R FUEL FILTER light will illuminate with a differential pressure of 9 psi across the filter, warning of impending filter clogging. The filter has an internal bypass capability if pressure differential exceeds 18 psi.

 

Notes

Max useable fuel is 5,690 lbs./850 gal , 2,845 lbs/425 gal per side

Fuel temperature is monitored in the right side fuel line prior to the fuel shutoff valve.

The sensors for fuel LOW TEMP are located downstream of each respective prop gearbox oil/fuel heat exchanger. Hot oil from the PGB heats the fuel line to prevent ice crystals. 

The fuel low level light illuminates when the quantity is below 300lbs +/- 70 lbs or 30-45 minutes of fuel remaining. 

The fuel quantity test pegs the indicator at 1,000 lbs +/- 50 lbs

The fuel flow test pegs the indicator at 760 lbs +/- 35

The X-FEED ON and CONN VLV OPEN on the fuel panel are not connected to the CWP.

The X-FEED ON and CONN VLV OPEN  lights are an indication of valve position (not switch).

The fuel tank vent is also used as an overflow system.

The green STBY PRESS light on the fuel panel indicates positive pressure is sensed in the line, not pump operation. Pressure from the selected fuel pump will also trigger a green light on the receiving side when pressure is sensed.

To send fuel to the opposite engine, select the STBY PUMP on the side you want to reduce fuel then open the X-FEED. Fuel is fed to the engine, not transferred to the opposite fuel tank. 

The interconnect line between wing tanks can be open on the ground to help equalize any fuel imbalance.