73-21-00
ALL ú ú N04 Page 23 ú Jun 18/00
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
////////////////////////A / PW4000 SERIES /747-400 / ENGINES/MAINTENANCE MANUAL ////////////////////////
c) If PT4.95 is not found, the EEC will change to N1 mode. d) If actual PB is not found, the EEC will synthesize PB from N2 and PT4.95. e) If actual N1 is not found, the EEC will synthesize N1 from N2. f) If the data for the EEC programming plug is not found, the EEC will change to N1 mode.
4) The engine will go to idle power if the following occurs: a) If the TRA input fails. b) If the thrust reverser LVDT shows that the thrust
reverser is deployed when forward thrust is commanded. 5) An engine will shut down if the following occurs:
a) If there is a dual channel EEC Alternator failure and the engine does not have a Supplemental Control Unit (The SCU will provide back-up power, if available).
b) If the EEC internal power supply has a dual channel failure.
6) If the EEC finds faults that put it in a no-dispatch configuration, ENG (1, 2, 3, or 4) CONTROL advisory message will show on the main EICAS as an advisory message and can can on the auxiliary EICAS as a status message.
7) If the EEC finds faults that put it in a short time limited dispatch configuration, ENG (1, 2, 3, or 4) EEC C1 status message can show on the auxiliary EICAS.
8) If the EEC finds faults that put it in a long time limited dispatch configuration, ENG-(1,2,3, or 4) EEC CATEGORY 2 FAULT (CH-A) or ENG-(1,2,3, or 4) EEC CATEGORY 2 FAULT (CH-B) fault messages are stored in the CMCS.
(2)
Fuel Metering Unit (FMU)
(a)
Fuel which goes into the FMU from the fuel pump goes through the servo filter. The servo filter is a wash-type filter in which the main fuel flow goes across the servo filter, parallel to the mesh. The main fuel flow across the servo filter does not let a collection of the unwanted material stay on the filter surface. For this reason, servicing is not necessary.
(b)
Fuel that goes through the servo filter supplies clean, pressurized servo flow (PF) to operate valves in the FMU and other hydraulically operated valves and actuators on the engine. PF also is sent to the PR regulator which supplies a constant pressure servo flow (PR) of 260 psi (1792 kPa) to the metering valve.
(c)
During engine start, when the FUEL CONTROL switch is put in the RUN position, the energy in the shutoff solenoid is removed and the start and run solenoid is energized. This puts the sequence valve in position to let the minimum pressure and shutoff valve open and let PF close the windmill bypass valve.
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73-21-00
ALL ú ú N06 Page 24 ú Oct 10/96
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
(d) After the servo filter, the main fuel flow goes through the metering valve. The metering valve is a pressure balanced floating valve with a variable opening. Operation of the metering valve includes the torque motor, flapper valve, and dual resolver in the FMU and the EEC. It is a closed loop system. 1) The EEC supplies a command signal to the torque motor in relation to the throttle lever position. The torque motor contains two coils, one for each channel of the EEC. Only one coil is used at a time. 2) The torque motor controls the flapper valve position which causes a modulating pressure (PM) when PR is bled to supply interstage pressure (PD). 3) The metering valve is balanced with PR at one end of the valve and PM, which is less than or the same as PR, at the other end. At idle, PM is the same as PR. During acceleration, PM decreases and lets the metering valve move, exposing a larger opening for fuel. 4) A feedback lever attached to the metering valve turns the dual resolver. The dual resolver closes the control loop when it sends an independent signal to each channel of the EEC which relates to the valve position.
5) If an N1 or N2 overspeed occurs, the EEC commands the overspeed cutback solenoid to open. With the overspeed cutback solenoid open, PR is bled to PM and makes them the same. This moves the metering valve to the idle position.
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