Engine-Driven Fuel Pump; Hydromechanical Fuel Control Unit - BOMBARDIER LEARJET 45 Pilot's Manual

Pilot's Manual

ENGINE-DRIVEN FUEL PUMP

The engine-driven fuel pump provides high-pressure fuel to the engine
fuel control system as well as motive-flow fuel for operation of the air-
craft jet pumps. The pump consists of a low-pressure pump element,
high-pressure pump element, high-pressure relief valve, filter, filter by-
pass valve, and motive-flow provisions.
The fuel pump is mounted to the accessory drive gearbox of the engine.
Fuel entering the first stage low-pressure element is pressurized to flow
through the fuel heater/oil cooler and filter. A second flow path for this
fuel is to the Auxiliary Motive Flow Pump (AMFP). The fuel from the
AMFP is used to operate the various jet pumps in the wing tanks. Fuel
that is supplied to the fuel heater/oil cooler and filter is passed on to
the pump high-pressure element. The high-pressure element provides
fuel at the fuel pressures required by the hydromechanical fuel control
unit. The high-pressure relief valve protects the fuel pump and hydro-
mechanical fuel control unit from extreme fuel pressure surges. A fuel
filter bypass valve begins to open at a pressure differential of 9 to 12 psi
(62 to 82 kPa) and allows flow of unfiltered fuel to the inlet of the high-
pressure pump.
The following CAS illuminations are specific to the fuel pumps:
CAS
FUEL PRESS LOW
FUEL FILTER

HYDROMECHANICAL FUEL CONTROL UNIT

The hydromechanical fuel control unit meters the required amount of
fuel to the engine combustor that corresponds to TLA, atmospheric and
engine operating conditions. The unit is mounted on the fuel pump and
contains the hydromechanical fuel metering section, thrust lever input
and position potentiometer, shutoff valve, and a mechanical governor.
The mechanical governor functions as an overspeed governor for the
high-pressure rotor. In addition, the mechanical governor provides
manual control when the DEEC is deactivated. When activated, the
DEEC controls fuel scheduling by means of a torque motor located
within the hydromechanical fuel control unit. The torque motor con-
trols the metering section of the hydromechanical fuel control unit.
2-4
Color
Red Fuel pressure is low at the associated (L or
R) engine's fuel pump inlet.
White The engine or wing fuel filter, on the associ-
ated (L or R) side, is becoming clogged.
Description
PM-126A