THEORY OF OPERATION
a. Fuel System.
The fuel system (fig. 1011) stores and supplies fuel for engines 1 and 2, and the APU.
Internal fuel cells store 375 gallons (2437 lbs.) and the auxiliary fuel tanks store 920 gallons (5978 lbs.). The fuel
system provides a means of controlling refueling/defueling operations while monitoring fuel quantity. It also allows
the crew to transfer fuel from the forward fuel cell to the aft, or the aft fuel cell to the forward to maintain center of
gravity or to remove fuel from the auxiliary fuel tanks. Fuel cannot be transferred from internal fuel cells to the
auxiliary fuel tanks.
(1) The fuel system provides the crew a means of selecting engine fuel source through the crossfeed
system. A crew member may select the forward or aft fuel cell as a fuel source for engine 1, engine 2, or both.
This allows a fuel source option in the event of fuel cell damage or engine failure.
(2) The fuel system incorporates the NIU which depletes oxygen content in internal fuel cells to a level that
will not support combustion. During normal operation, the NIU diverts and regulates pressurized air to
approximately 25 psi at 94% nitrogen into the aft fuel cell. The aft fuel cell pressurizes the forward fuel cell.
(3) The fuel system can be refueled/defueled through a pressure fuel manifold which houses a SPA
(CONUS fitting) and a CCA (NATO fitting). The SPA has a 100 gallons per minute (gpm) flow capability, while the
CCA has a 60 gpm flow capability.
(4) Gravity fuel ports, located on each internal fuel cell and each auxiliary fuel tank, provide an alternative
way of refueling/defueling the helicopter in forward operating areas with portable fuel supplies.
(5) The fuel system consists of the fuel storage subsystem, fuel crossfeed/boost subsystem, fuel quantity
indication subsystem, fuel transfer subsystem, fuel warning subsystem, nitrogen inerting subsystem, gravity
refueling/defueling subsystem, pressure refueling/defueling subsystem, and an auxiliary fuel subsystem.
b. Fuel Storage System.
The purpose of the internal fuel cells are to provide fuel storage for engine and APU
starting and operation.
(2) System Operation (fig. 1012).
(a) The aft fuel cell is gravity fueled through a gravity filler port, or pressure fueled via the
refuel/defuel/transfer breakaway valve. The air vent/pressure relief valve vents air pressure out of the fuel cell
during pressure fueling. The fuel quantity transmitter outputs analog data for use with fuel indicators. The pilot
valve, pilot valve tube and fuel shutoff valve provide automatic fuel shutoff when the fuel cell is full. In case of
automatic fuel shutoff failure, the air vent/pressure relief valve vents fuel out of the fuel cell. The boost pump is
driven by pressurized air from the PAS. The boost pump pressurizes the fuel in the aft fuel cell for use during
engine starting and high altitude operations.
(b) During boost pump operation, boost pump suction draws fuel through an inlet screen on the bottom
of the 4-way check valve. Fuel is pressurized and pumped through the 4-way check valve to supply pressurized
fuel to engine 1, engine 2 and the APU.
(c) During normal fuel system operation, the fuel cell is pressurized by the NIU. Pressure from the aft
fuel cell pressurizes the forward fuel cell. The fuel/defuel check valve provides a seal to maintain fuel cell pressure
forcing the fuel through the 4-way fuel supply check valve to the APU and engine 2.
(d) The sump drain valve is used to take fuel samples or to drain the fuel cell. When the push-to-drain
control is pushed, the sump drain valve is opened. When the push-to-drain control is released, the sump drain
valve is closed.