B. The main engine control is hydromechanical and operates using
fuel-operated servo valves.
C. The MEC performs the following functions:
(1)
Controls engine speed by metering fuel to the engine fuel nozzles during all modes of operation. Fuel not used by the MEC is returned to the fuel pump downstream of its low pressure element. The MEC also uses fuel from the fuel pump as a hydraulic medium.
(2)
Automatically schedules fuel flow to maintain the thrust lever speed setting and establishes the maximum safe fuel flow limit under any operating condition. As conditions vary the limits vary according to predetermined acceleration and deceleration schedules. In order for the MEC to determine the schedules, certain parameters such as compressor discharge pressure (CDP), 9th-stage compressor bleed pressure (CBP), compressor inlet temperature (CIT) and core engine speed (N2) must be sensed. The MEC senses these parameters and also amplifies and computes them to establish acceleration and deceleration limits for fuel flow. The computed limits are compared with actual fuel flow and are imposed when the limits are approached.
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CFM56 ENGINES
737-300/400/500/ /MAINTENANCE MANUAL ////////////////////////
SENSOR TEMPERATURE T2
SEE A
SEE C
1 ENGINES WITH HPTCC TIMER
MEC
THRUST POSITION MAX REVERSE LEVER SHUT-OFF FUEL BOX CONTROL FUEL SEE B A
FWD 130` 45` IDLE TIMER HPTCC
75`
LEVER POWER PORT DISCHARGE FUEL MAX FWD POSITION THRUST
B SOLENOID IDLE RESET FWD Main Engine Control Location Figure 1 C 1 TRANSMITTER FUEL FLOW
EFFECTIVITYùùùùùùùùùùùùùùùùùùùùùùùùùùùùùù. ú
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ALL ú ú C02 Page 2 ú Jun 25/87
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
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CFM56 ENGINES
737-300/400/500/ /MAINTENANCE MANUAL ////////////////////////
(3)
Schedules variable stator vane (VSV) position and directs fuel pump high pressure fuel (PF) to the VSV actuators to position the vanes as the schedule changes. The signal which determines the position of the VSV actuators is the result of the control computing N2 and CIT specifically for this purpose.
(4)
Schedules variable bleed valve (VBV) position and directs fuel pump high pressure fuel (PF) to the VBV hydraulic actuator to position the valves as a function of the VSV schedule. A signal to open the VBV's is imposed when the reverser is actuated, or a rapid change in VSV actuation to close occurs, to supersede the normal VBV scheduled position.
(5)
Schedules the position of turbine clearance air valves by the output of signal pressures (TC1 and TC2) which are a function of engine speed (N2).
(6)
ENGINES WITH THE HPTCC TIMER; Initiates turbine clearance control timer sequencing by supplying TC3 signal pressure to the timer when core engine speed reaches a specific rpm during initial acceleration to takeoff power.
(7)
Provides positive fuel shutoff operation independent of power lever position.
(8)
Defines and regulates its own operating pressures.
D. MEC Adjustments
(1)
The MEC provides for adjustment of the following items:
(a)
Specific gravity of fuel
(b)
Ground idle speed
(c)
Flight idle speed
(d)
VSV feedback cable length
(e)
VBV feedback cable length
