(e)  
The MCP digital data bus MCP-3 provides outputs to the FMC and to the autothrottle. This data originates in either FCC A or FCC B. The MCP selects which input to use for its digital output based on the Master Light logic. The only time the FCC B source is used is when only the B Master Light is on.

 O. Pitch Axis Flight Control Operation (Fig. 30)
 (1)  
Elevator Operation

 (a)  
The elevator provides primary pitch control of the airplane lateral axis. The elevators are operated by elevator power control units (PCU). In event of dual hydraulic failure, a manual reversion allows the elevators to be driven directly through a mechanical control system. Inputs to the elevator PCUs are from the control columns, mach trim, autopilot and the neutral shift mechanism.

 (b)  
Pilots inputs to the power control unit (PCU) are from the control column thru cables to aft quadrant and a rod to the PCU. The PCU moves the upper torque tube to control the elevator. As the PCU moves, the input crank is recentered stopping the movement of the PCU. A parallel input from the aft quadrant into the feel and centering unit affords feel to the pilot.

 (2)  
Neutral Shift Operation

 (a)  The neutral shift mechanism consists of rods connected to the stabilizer which rotate the feel and centering unit when the stabilizer is moved. Moving the feel and centering unit will rotate the aft quadrant and provide an input into the elevator power units to trim the elevator in conjunction with stabilizer trim. The mach trim actuator is part of the neutral shift rods and provides an input to the elevator power units to raise the elevator as mach increases above 0.615 mach.
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A737-300/400/500MAINTENANCE MANUAL
EFFECTIVITYùùùùùùùùùùùùùùùùùùùùùùùùùùùùùù.AIRPLANES WITH EFIS ú
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
STAB 
TRIM  STABILIZER  NEUTRAL SHIFT 
PILOT AUTO-ELEC MAIN  MECHANISM 
NORMAL 
LIMIT 
CONTROL  CENTERING 
CUTOUT  MAIN ELECT  A/P  CAM 
TRIM SERVO  STAB  ELEVATOR 
RIDE OVER- SERVO TRIM  CENTERING FEEL AND 
PITOT  UNIT 
NORM  STATIC 
STAB TRIM SWITCH COLUMN ACTUATED  TO RIGHT PCU & ELEV 
UPPER 
OVERRIDE  TORQUE 
TUBE 
CONTROL COLUMN  SYSTEM 
ACTIVATED  B HYD 
STAB TRIM  SYSTEM  PCU 
CUTOUT LOGIC  INPUT 
CRANK 
DIFFERENTIAL 
PRESSURE SWITCH 
STAB 
TRIM 
SW 
SYSTEM A 
HYD SYSTEM 
COLUMN CONTROL  FEEL COMPUTER ELEVATOR  Q DIAPHRAM 
DROOP 
FORCE BALANCE  SPRING 
STABILIZER  VALVE  RELIEF  STATIC
CONTROL FORWARD  WHEELS MANUAL TRIM  VALVE  PITOT 
QUADRANT 

MACH TRIM ACTUATOR CENTERING LINKAGE
DUAL FEEL
ACTUATOR B A/P ELEV ACTUATOR RETURN LEFT ELEVATOR
A A/P ELEV ACTUATOR
LEFT
ELEVATOR
ACTUATOR

LOWER
TORQUE
TUBE

AFT 
QUADRANT 
Pitch Control System Schematic 
Figure 30 

 (3)

(4)

(5)

A
737-300/400/500MAINTENANCE MANUAL
Elevator Feel Operation
 (a)  
The feel system contains two basic units, the feel and centering unit and a feel computer. The feel and centering unit consists of a cam, spring loaded follower, two hydraulic actuators and related linkage. The feel computer is a dual unit containing two independent pressure regulating devices, one for system "A" and one for system "B".

 (b)  
Pitot pressure from two pitot tubes located on the left and right side of the vertical stabilizer is applied to one side of a bellows. These forces react against the Q-spring which positions the force balance valve. This action regulates the output pressure to the feel actuators. THe two output pressures are not additive. The highest of the two computed pressures are reacted against, the remaining computed pressure is available but in a standby condition.
　
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