Fuel Flow Totalizing Systems
motor-driven logic and signal-conditioning
channels. Using a stepping signal to drive control
Figure 6-54. view B, shows the indicator of
logic, the motor driver controls positive and
a fuel flow totalizing system. The pointer of this
negative 8-Hz ac signals between phases of both
instrument usually shows the combined rate of
fuel rate-of-flow transmitter motors.
fuel flow into two or more engines. Also, if only
one engine is operating, the pointer gives a true
The signal-conditioning channel for each
indication. A continuous reading of the pounds
engine system receives pulses from the coils of its
of fuel remaining in the aircraft fuel cells appears
transmitter. A pulse shaper, for each channel,
in the small window. Before starting the engines,
converts the time between transmitter drum and
impeller coil pulses into a rectangular pulsewidth
you set the total amount of fuel in the aircraft
signal. An averaging filter processes this converted
on the pounds-fuel-remaining indicator by using
signal, which provides a low-ripple dc signal input
the reset knob on the front of the instrument. As
soon as the engines are running, the fuel flow
to the fuel rate-of-flow indicator. The size (0 to
pointer shows the rate of fuel consummation.
5 volts dc) of this signal is proportional to flow
rate. A test circuit permits testing the power
The fuel remaining indicator starts counting
supply. A tap-off motor, phases A and B from
toward zero, giving a continuous reading of fuel
remaining in the cells. Numbers rotate past the
the motor driver, routes an 8-hertz signal through
window like those of the mileage indicator of an
an external test switch to the signal-conditioning
automobile speedometer.
channels for processing. The results of the
processed signal are displayed on the indicator.
The entire fuel flow totalizing system consists
of two or more fuel flow transmitters, an
The fuel rate-of-flow indicator, a vertical scale
amplifier, and an indicator.
indicator, displays rate of fuel flow for each
engine on parallel scales. The scales are from 0
FUEL FLOW TRANSMITTERS. --The fuel
to 13. The scale reading, multiplied by 1,000,
flow transmitters are almost identical to those
show the rate (pounds per hour) at which the
already discussed in the single system. In the fuel
engine is consuming fuel. The upper left and right
flow totalizing system, the transmitters connect
OFF failure flags show a loss of power, or signal
electrically, so their combined signals go into the
to the indicator. The indicator has two separate
fuel flow amplifier as one.
channels, one for each engine. The channels
include a control transformer servo amplifier,
FUEL FLOW AMPLIFIER. --The fuel flow
servomotor, gears, and sprockets. The indicator
amplifier is an electronic device that supplies
channels receive 115 volts of ac for signal
power of the proper size and phasing to drive the
processing. An input of 0 to 5 volts dc from
indicator. The speed at which the indicator motor
the fuel rate-of-flow power supply produces
runs depends on the transmitter signal going into
an output. This output is from the control
the amplifier.
transformer rotor winding to the servo amplifier.
FUEL FLOW TOTALIZER INDICATOR. --
The servo amplifier modifies the signal to the
The fuel flow totalizer indicator contains a
proper impedance and power level to drive the
two-phase variable speed induction motor. This
channel servomotor. Shaft rotation controls
motor travels in one direction only; however, the
transformer output, and the rotation reduces
speed varies. As the rate of fuel consumption
transformer output voltage. When the output is
increases, more and more power goes to the
null, the motor, gear train, and sprockets come
indicator motor. This causes the speed of the
to rest at a rate equivalent to the input.
motor to increase proportionally to the rate
of fuel consumption. The motor turns a magnetic
The test selector switch on the MASTER
TEST panel tests the indicators. The self-test
drum-and-cup linkage (similar to the tachometer
indicator hysteresis cup), which causes pointer
circuit in the indicators disconnect the fuel rate-
deflection. The deflection is proportional to the
of-flow transmitter input circuits. It then connects
motor speed, and thus proportional to the rate
to an appropriate test signal within the two
indicator channels. The channel circuitry
of fuel consumption. At the same time, a linkage
processes the test signal and drives the indicator
having a friction clutch drives the pounds-fuel-
remaining indicator. The clutch is disengaged
tapes to indicate 4,200 to 4,400 pounds per hour.
