(f)
The metered fuel flow which goes out of the metering valve goes through the minimum pressure and shutoff valve, out of the FMU, to the fuel distribution valve.
(g)
During engine shutdown, when the FUEL CONTROL switch is put in the CUTOFF position, the energy in the start and run solenoid is removed and the shutoff solenoid is energized. This puts the sequence valve in position to let the minimum pressure and shutoff valve close when P2 decreases to less than 300 psi (2068 kPa). At the same time, the sequence valve releases PF at the windmill bypass valve which lets it open.
(h)
The windmill bypass valve prevents fuel pressure increase when the engine is not in operation but still turns. This occurs during engine start when the engine is being motored up to speed before the FUEL CONTROL switch is moved to RUN, after engine shutdown when the engine is spooling down, and when the engine is windmilling. During these conditions, there is positive fuel pressure and PF. Fuel flows through the metering valve and windmill bypass valve back to the fuel pump.
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73-21-00
ALL ú ú N06 Page 25 ú Oct 10/96
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
////////////////////////A / PW4000 SERIES /747-400 / ENGINES/MAINTENANCE MANUAL ////////////////////////
(3)
EEC Alternator (N2 Transducer)
(a)
While the N2 rotor turns, the EEC alternator rotor turns and supplies an alternating current at 30 volts ac to each channel of the EEC independently. In the EEC, the current is rectified and the voltage is regulated.
(b)
At approximately 10% N2, the EEC alternator will supply
sufficient power for the EEC to come on-line.
(c)
The EEC senses the input power frequency from the EEC alternator to find N2 rotor speed. A different winding supplies a varying voltage signal of 6.0-77.0 volts ac to the components that follow: 1) N2 speed card for engine in operation logic and starter
cutout. 2) EIU's for backup N2 indication and N2 indication less than 10%. 3) AVM signal conditioner to correlate vibration to engine rotor.
(d)
N2 rotor speed is received by the EIU's from the EEC on a
digital data bus. This data is used for N2 indication on
EICAS.
(4)
EEC Speed Transducer (N1)
(a)
While each tooth on the N1 rotor shaft goes by the transducer tip, a voltage is induced in the pickup coils. The voltage signal is sent to the EEC at a frequency in proportion to the rotor speed.
(5)
EEC Inlet Total Pressure/Temperature (PT2/TT2) Probe
(a)
The EEC finds measures the sensing elements resistance to find the total inlet temperature. When the probe anti-ice heating element is energized, the EEC biases the sensing element resistance to compensate for heat induced error.
(b)
Inlet total pressure is sent directly to the P2 transducer in the channel A half of the EEC. The P2 transducer changes the pressure signal to an electronic signal to be used by the two channels of the EEC.
(6)
EEC Thermocouple Probes
(a)
The EEC oil temperature, fuel temperature, and TT3 thermocouple probes have two chromel/alumel thermocouples. The thermocouples supplies independent temperature signals to each channel of the EEC.
(7)
Minimum/Approach Idle Control System (Fig. 4)
(a)
The EEC can command the engine to operate at one of two idle speeds; minimum or approach idle. With the thrust lever at idle position, the engine will operate at minimum idle but not in the conditions that follow: 1) The airplane is in the air and the flaps are in a landing
configuration. 2) The airplane is in the air and NACELLE ANTI-ICE switch is ON.
(b)
The idle control system includes the components that follow for each engine: 1) EEC/TMC Discretes Card
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