(a) On liftoff, the HPTCC timer solenoids are energized to prevent the timer from being started if 95% N2 was not achieved during takeoff.
(5)
ENGINES WITHOUT THE HPTCC TIMER; The turbine clearance control valve, through MEC fuel pressure signals, regulates the flow of 5th and 9th stage compressor bleed air to the high pressure turbine (HPT) shroud to maintain optimum high pressure rotor tip clearance.
(6)
The engine idle control system uses an input from the ground sensing relay to give the necessary idle speed. The LOW IDLE light shows there is not sufficient engine RPM. The air sensing relay prevent LOW IDLE light illumination when the airplane is on the ground.
B. Main Engine Control
(1) Main Engine Control Operation can be divided into 3 functional systems: servo control system; fuel metering system; and engine VSV, VBV and HPT clearance control system.
(a) Servo Control System (Fig. 7).
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FROM FUEL SHUTOFF VALVE (PUMP UNLOADING COMMAND)
TO DIFF PRESS PILOT VALVE (UNLOADING COMMAND)
(FUEL PUMP HPELEMENT INLET)
PRESSURES
P FUEL BYPASS
BP HIGH PRESSURE HEATED FUEL
FP CASE PRESSURE, CONTROL INTERMEDIATE SERVO PRESSURE (P +100 PSIG)
CR B G-M56-MM-0075(1)-02-BP SERVO PRESSURE, CONTROL HIGH SERVO PRESSURE (P +200 PSIG)
C B
MEC Servo Pressure Control System Block Diagram
Figure 7
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1) The servo control system is supplied heated fuel, PF, from the servo fuel heater heat exchanger to do the functions that follow: a) Servo Pressure Regulation
1. Fuel metering and schedule computing are
_ accomplished through use of servo valves and pistons. Regulated servo pressures (PC and PCR) used by the hydraulic circuits in the MEC are kept at a constant value above the bypass pressure (PB) by the servo pressure regulating valves.
b) Pump Pressure Relief (Fig. 7 and 8).
1. The fuel pump is relieved of excessive pressure if
_ the fuel shutoff valve is abruptly closed by opening the bypass valve via a command from the pump unloading valve. Servo pressure, PC, is supplied to the differential pressure pilot valve instead of PF by the pump unloading valve when the pump unloading valve is actuated by linkage connecting it to the shutoff valve. The fuel supply pressure, PS, is greater than the servo pressure, PC, and the bypass valve is opened to a bypass position.
c) MEC Case Pressure Relief (Fig. 7).
1. Another relief valve is provided between the MEC
_ intermediate servo pressure regulating valve, PCR, which controls the case pressure of the MEC, and the fuel bypass valve to prevent excessive pressure in the control case.
(b) Fuel Metering System (Fig. 8)
1) Fuel is supplied to the MEC at fuel pump discharge pressure, PS. The pump supplies more fuel than is necessary for all engine operating condition. Required fuel flow, WF, is determined by the metering valve orifice area with excess fuel being directed through the bypass valve back to the fuel pump high pressure (HP) inlet. The fuel metering system establishes and keeps the necessary fuel flow, WF, through the functions that follow:
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