After being pressurized by the high pressure pump, the pressurized fuel
passes through:
• a “Manual Shut-off Valve”, directly connected to the ECL, which
permits to isolate the engine from the fuel supply (Valve is closed when
ECL is at OFF)
• a “Shut-Down Solenoid Valve”, directly controlled by the ENG MODE
switch on the Engine control panel (ECP) is energized closed during
the engine shut-down procedure. This valve must be open during
manual engine start procedure, ie ENG MODE switch must be at IDLE
or FLIGHT.
A Bypass Valve keeps a constant value of differential pressure across the
Fuel Metering Valve.
A by-pass flow line sends the exceeding fuel back to the low pressure pump.
The FMM includes a Fuel Metering Valve (FMV) that is controlled by the
Electronic Engine Control (EEC) or by the ECL to meter the fuel flow over
the full range of the engine operation.
Depending on the FMM mode of operation (AUTO or MANUAL), the Fuel
Metering Valve uses different methods to meter the fuel flow:
• in Automatic mode the EEC commands a torque motor
• in Manual mode the ECL moves mechanically.
The metered fuel flow is sent to the fuel nozzles through the FCOC.
NF LIMITER OPERATION
The Nf limiter is an indipendent subsystem of the EEC that receives an
indipendent NF input from the TQ sensor and compares it to a threshold
value of 111% NF.
In the event that an overspeed is detected, the NF limiter energizes the
overspeed solenoid valve in the FMM, which reduces the fuel flow to
minimum. When NF drops below 109%, the overspeed solenoid valve is
deenergized and the fuel flow recovers. This cycling will occur for as long as
the overspeed condition persists.
The overspeed protection is active both in AUTO and MANUAL modes.