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The aircraft fuel filter annunciator circuits are wired through the
squat switch and maycause the FUEL FILTER light to illuminate only
if the aircraft is on the ground. The engine fuel filter annunciator circuits
are not wired through the squat switch and may cause the FUEL
FILTER light to illuminate either in flight or on the ground. Refer to
FUEL SYSTEM GLARESHIELDLIGHTS.
FUEL FLOW INDICATOR
The FUEL FLOW indicator is a circular scale, dual-reading instrument
and is located on the center instrument panel. The indicator utilizes
two pointers (one labeled "L", and the other labeled "W') to indicate
fuel flow. The indicator scale reads from 0 to 2000 pounds per
hour in 50 pound-per-hour increments. A fuel-flow transmitter for
each engine measures fuel-flow by means of a rotor-turbine installed
in the engine fuel supply line between the fuel control unit and the
fuel manifold . As fuel flows through the turbine a pick-up coil in the
transmitter emits pulses which are applied to a monitor. The monitor
converts the pulses into a continuous DC current which is applied to
the indicator. The monitor also applies a pulsating DC signal to the
fuel counter on the fuel control panel. Each pulse represents one
pound of fuel. The fuel flow indicating system operates on 28 VDC
supplied through a 10-arnp current limiter.
FAN SPEED (NI) INDICATORS
A Fan Speed (N1) indicator for each engine is installed on the center
instrument panel. Each indicator utilizes both afour-place digital display
and a circular scale with pointer to indicate Fan Speed (Ni). The
circular scale is marked from 0% to 110% in 5% RPM increments. The
digital display units, tens, and hundreds numerals are white numerals
on a black background and the decimal digit is a black numeral on
a white background. N1 speed is measured by a monopole transducer
at the aft end of the low-pressure rotor. A speed gear is attached to
the low-pressure shaft rotating around a stationary transducer. As the
speed gear turns within the transducer, its teeth cause the magnetic
flux path in the air gap to be constantly changing. The dual output
transducer monopole produces two separate and identical signals
due to the changing magnetic field. The frequency of the output signal
represents the rotating speed of the rotating N1 group. One out-
PM-121
2-11
Original
Pilot's Manual
Learjet 31A
put signal is applied to the Fan Speed (N1) indicator and the other to
the DEEC. An OFF flag on the indicator will be in view whenever
electrical power is not available to the instrument. Electrical power
for the indicators is 28 VDC supplied through the 2-amp L and R N1
circuit breakers on the pilot's and copilots circuit breaker panels respectively.
The Ni indicators are powered by EMER BAT 1 during
EMER BUS mode and electrical power system failures. Refer to Airplane
Flight Manual for instrument limit markings.
TURBINE SPEED (N2) INDICATORS
A Turbine Speed (N2) indicator for each engine is installed on the
center instrument panel. Each indicator utilizes both a four-place digital
display and a circular scale with pointer to indicate Turbine Speed
(N2). The circular scale is marked from 0% to 110% in 5% RPM increments.
The digital display units, tens, and hundreds numerals are
white numerals on a black background and the decimal digit is a
black numeral on a white background. N2 speed is measured by a
monopole transducer adjacent to the output gear of the transfer gearbox.
The dual output transducer produces two separate and identical
signals. The frequency of the output signal represents the speed of the
rotating N2 group. One output is routed to the DEEC and the other is
applied to the Turbine Speed (N2) indicator. An OFFflag on the indicator
will be in view whenever electrical power is not available to the
instrument. Electrical power for the indicators is 28 VDC supplied
through the 2-amp L and R N2 circuit breakers on the pilot's and copilot's
circuit breaker panels respectively. Refer to Airplane Flight
Manual for limit markings.
TURBINE TEMPERATURE (ITT) INDICATORS
A Turbine Temperature (ITT) indicator for each engine is installed on
the center instrument panel. Each indicator utilizes both a three-place
digital display and a circular scale with pointer to indicate Turbine
Temperature (ITT). The scale is marked from 100°C to 1000°C in 500C
increments. The digital display numerals are white numerals on a
black background. Interstage turbine temperature for each engine is
sensed by Chromel-Alumel parallel wired thermocouples positioned
between the high- and low-pressure turbine sections. The signal from
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