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discharged axially through the exhaust duct to provide additional
ENGINE FUEL AND CONTROL SYSTEM
The engine fuel and control system pressurizes fuel routed to the engine
from the aircraft fuel system, meters fuel flow, and delivers atomized
fuel to the combustion section of the engine. The system also
supplies high-pressure motive-flow fuel to the aircraft fuel system for
jet pump operation. The major components of the system are the
thrust levers, the engine-driven fuel pump, the fuel control unit, the
digital electronic engine control (DEEC), surge bleed control, fuel
heater (optional) and the engine synchronizer.
PM-121
2-1
Change 1
Pilot's Manual
AB-AREA BLEED
N 1- LOW PRESSURE ROTOR (FAN) SPEED
N2- HIGH PRESSURE ROTOR (TURBINE) SPEED
P3 - COMPRESSOR DISCHARGE PRESSURE
PT2 - ENGINE INLETTOTAL PRESSURE
TT2-ENGINE INLET TOTAL TEMPERATURE
ITT- INTERSTAGE TURBINE TEMPERATURE
WF-FUEL FLOW
FUEL CONTROL LOGIC DIAGRAM
Figure 2-I
Learjet 31A
i earjet 31A
Pilot's Manual
THRUST LEVERS
Two thrust levers, located on the upper portion of the pedestal, are
operated in a conventional manner with the full forward position being
maximumpower. Stops at the IDLE position prevent inadvertent
reduction of the thrust levers to CUTOFF. The IDLE stops can be released
by lifting a finger lift on the outboard side of each thrust lever.
A "floating" control, consisting of ball bearings and races enclosed in a
flexible casing", connects each thrust lever to the corresponding engine's
hydro-mechanical fuel control. Aflight director go-around button
is installed in the left thrust lever handle . Alandinggear/cabin altitude
warning horn mute button is installed in the right thrust lever
handle. If thrust reversers are installed a thrust reverser control lever
is mounted piggyback fashion on each thrust lever. Refer to ENGINE
THRUST REVERSERS for a functional description of the thrust reverser
levers.
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 aircraft jet pumps. The pump consists of a low-pressure pump element,
high-pressure pump element, relief valve, filter, and motiveflow
provisions. On aircraft equipped with fuel heaters, the fuel pump
also incorporates an anti-icing valve which monitors fuel temperature
downstream of the filter element and mixes warm fuel from the fuel
heater with the low-pressure pump element discharge flow to prevent
filter element icing. In the event the fuel pressure drop across the
fuel filter reaches a preset level, due to clogging, the filter bypass
valve will open and fuel will bypass the filter. On aircraft not equipped
with fuel heaters, a red bypass indicator button will pop out. The fuel
bypass indicator button is visible through a small, circular, push-toopen
door on the engine nacelle.
FUEL CONTROL UNIT
The fuel control unit is mounted on the fuel pump and contains the
hydro-mechanical 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 highpressure
rotor. In addition, the mechanical governor provides manual
control when the digital electronic engine control (DEEC) is deactivated.
When activated, the DEEC controls fuel scheduling by means of a
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PM-121
2-3
Original
Pilot's Manual
Learjet 31A
DIGITAL ELECTRONIC ENGINE CONTROL (DEEC)
An digital electronic engine control (DEEC), located in the tailcone, is
provided for each engine. TheDEEC is basically an engine speed governor,
with provisions for fuel limits during acceleration and deceleration.
The N1 DEEC (Aircraft 31-183 and subsequent) governs based on
low-pressure rotor speed. The N2 DEEC (Aircraft 31-035 thru 31-182)
governs based on high-pressure rotor speed. Each DEEC performs governing,
limiting, and fuel scheduling functions forengine start and continuous
operation. Input parameters utilized by both DEECs are: inlet
pressure (PT2), inlet temperature (TT2), interstage turbine temperature
(rM, low-pressure rotor speed (Ni), high-pressure rotor speed (N2),
and power lever angle (PLA). The power lever angle is sensed by the
DEEC from a potentiometer in the fuel control unit. An electrically
heated mast in the inlet duct houses the inlet temperature sensor and
inlet pressure tap for the DEEC. Interstage turbine temperature, lowpressure
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