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CFM56 ENGINES
737-300/400/500/ /MAINTENANCE MANUAL ////////////////////////
(c) Engine Cycle Control System Operation (Fig. 9)
 1)  Engine VSV, VBV and high pressure turbine clearance control (HPTCC) valve mechanical components are adjusted by the MEC to optimize engine performance. The first 4 stages of the high pressure compressor (HPC) variable stator vanes (VSVs) and variable bleed valves (VBVs) at the inlet to the HPC are adjusted to provide optimum performance of the compressor. In addition, the control regulates the clearance between the high pressure turbine blades and shroud of the high pressure turbine (HPT). This is done with the HPTCC valve which controls cooling air extracted from the compressor and pipes it on to the HPT shroud. This optimizes performance of the turbine. These MEC operations are done as follows: a) VSV Positioning
 1.  The VSV pilot valve is actuated by a signal from
_ the 3D cam until the stator vane feedback signal (VSV feedback cable) nulls the actuation through comparative linkage in the MEC. Two engine parameters, CIT and N2 core engine speed are measured to position the 3D cam and thus determine the VSV schedule. The VSV pilot valve directs a resulting high pressure hydraulic signal to one side of the pistons in the 2 VSV actuators attached on the core engine compressor. The actuators mechanically set the VSV's.
 b)  VBV Positioning
 1.  The VBV control system schedules the VBV's as a
_ function of VSV position or as modified by the VBV feedback reset system. The VSV feedback linkage is coupled to a VBV scheduling cam which actuates the VBV control valve.
 c)  VBV Actuation
 1.  The VBV control valve directs a high pressure
_ signal (PF) to a hydraulic actuator which drives the VBV's open or closed. The servo loop is closed by means of the VBV feedback cable which senses the VBVs position at the master door installed at the 9 o'clock position.
 d)  VBV Feedback Reset
 1.  The VBV reset valve or reverser signal valve
_ actuate the VBV reset piston to reset the VBV feedback to position the VBV's in a more open position during rapid deceleration or reverser operation. This is done to prevent possible engine stall conditions.
 e)  Turbine Clearance Signal Valves
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CFM56 ENGINES
737-300/400/500/ /MAINTENANCE MANUAL ////////////////////////

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CFM56 ENGINES
737-300/400/500/ /MAINTENANCE MANUAL ////////////////////////
1.  
ENGINES WITHOUT THE HPTCC TIMER;

_ The MEC contains 2 signal valves (TC1 and TC2) which are set by the turbine clearance control valve (TCCV) scheduling cam as directed by the tach servo valve. At selected speeds these valves will provide outputs of either fuel bypass pressure, PB, or servo pressure, PC, which will control the actuation of the HPT clearance control valve air bleed valves to a schedule of either open or closed positions of the valves to provide stage 5 or stage 9, or a combination of stage 5 and stage 9 HPC bleed air needed to optimize the HPT clearances.

 2.  
ENGINES WITH THE HPTCC TIMER;

_ The MEC contains 3 signal valves (TC1, TC2 and TC3) which are set by the turbine clearance control valve (TCCV) scheduling cam as directed by the tach servo valve. At selected speeds, TC1 and TC2 provide outputs of either fuel bypass pressure, PB, or servo pressure, PC, which controls actuation of the HPTCC valve air bleed valves to either open or closed positions and provide stage 5, stage 9 or a combination of stage 5 and stage 9 HPC bleed air needed to optimize the HPT clearances. The HPTCCV timer, on signal from the MEC, controls the operation of the HPTCCV during the first 182 seconds (nominal) of engine operation more than 13,800 rpm (95% N2) on takeoff roll. On liftoff, the HPTCCV timer solenoids are de-energized to prevent the timer from being activated if 95% N2 was not achieved during takeoff.
　
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