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and speed control system, has no pressurizing valve
to divide the flow from the fuel pump into main and
primary flows.
ELECTRONIC ENGINE CONTROL
77. As stated in para. 8, some engines utilize a
system of electronic control to monitor engine
performance and make necessary control inputs to
maintain certain engine parameters within predetermined
limits. The main areas of control are engine
shaft speeds and exhaust gas temperature (E.G.T.)
which are continuously monitored during engine
operation. Some types of electronic control function
as a limiter only, that is, should engine shaft speed or
E.G.T. approach the limits of safe operation, then an
input is made to the fuel flow regulator (F.F.R.) to
reduce the fuel flow thus maintaining shaft speed or
E.G.T. at a safe level. Supervisory control systems
may contain a limiter function but, basically, by using
aircraft generated data, the system enables a more
appropriate thrust setting to be selected quickly and
Fuel system
111
accurately by the pilot. The control system then
makes small control adjustments to maintain engine
thrust consistent with that pre-set by the pilot,
regardless of changing atmospheric conditions. Full
authority digital engine control (FAD.E.G.) takes over
virtually all of the steady state and transient control
intelligence and replaces most of the hydromechanical
and pneumatic elements of the fuel system. The
fuel system is thus reduced to a pump and control
valve, an independent shut-off cock and a minimum
of additional features necessary to keep the engine
safe in the event of extensive electronic failure.
78. Full authority fuel control (F.A.F.C.) provides full
electronic control of the engine fuel system in the
same way as F.A.D.E.C., but has none of the
transient control intelligence capability used to
control the compressor airflow system as the existing
engine control system is used for these.
Speed and temperature control amplifiers
79. The speed and temperature control amplifier
receives signals from thermocouples measuring
E.G.T. and from speed probes sensing L.P. and in
some cases, L.P. shaft speeds (N1 and N2). The
amplifier basically comprises speed and temperature
channels which monitor the signals sensed. If either
N1, N2 or E.G.T. exceed pre-set datums, the
amplifier output stage is triggered to connect an
electrical supply to a solenoid valve (para. 47) or a
variable restrictor (para. 73) which override the F.F.R.
and cause a reduction in fuel flow. The limiter will
only relinquish control back to the F.F.R. if the input
conditions are altered (altitude, speed, ambient
temperature or throttle lever position). The limiter
system is designed to protect against parameters
exceeding their design values under normal
operation and basic fuel system failures.
Engine supervisory control
80. The engine supervisory control (E.S.C.) system
performs a supervisory function by trimming the fuel
flow scheduled by the fuel flow governor (F.F.G.) to
match the actual engine power with a calculated
engine power for a given throttle angle. The E.S.C.
provides supervisory and limiting functions by means
of a single control output signal to a torque motor in
the F.F.G. In order to perform its supervisory function
the E.S.C. monitors inputs of throttle angle, engine
bleed state, engine pressure ratio (E.P.R.) and air
data computer information (altitude, Mach number
and temperatures). From this data the supervisory
channel predicts the value of N1 required to achieve
the command E.P.R. calculated for the throttle angle
set by the pilot. Simultaneously a comparison is
made between the command E.P.R. and the actual
E.P.R. and the difference is compared with a
programmed datum.
81. During acceleration the comparitor connects the
predicted value of N1 to the limiter channel until the
difference between the command and actual E.P.R.
is approximately 0.03 E.P.R. At this point the
predicted L.P. shaft speed is disconnected and the
E.P.R. difference signal is connected to the limiter
channel.
82. The final output from the supervisory channel,
in the form of an error signal, is supplied to a ’lowest
wins’ circuit along with the error signals from the
limiter channel. While the three error signals remain
positive (N1 and E.G.T. below datum level and actual
E.P.R. below command E.P.R.) no output is signalled
to the torque motor. If, however, the output stage of
the E.S.C. predicts that E.G.T. will exceed datum or
that N1 will either exceed its datum or the predicted
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