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 28
A
737-300/400/500MAINTENANCE MANUAL
(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.
(c)
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.
(d)
When the autopilot is engaged the autopilot actuators control the elevators. During automatic modes of operation the FCC provides pitch command signals to the transfer valve on the autopilot actuator. The A/P actuator rotates the aft quadrant to provide an input signal to the PCU. Operation of the PCU is the same as in pilot operation.
(2) Stabilizer Trim (Fig. 28)
(a)
The horizontal stabilizer trim is operated by either a main electric trim motor, an autopilot trim motor, or by manual cable control. Pilot stabilizer trim switches actuate the main electric trim motor when the airplane is flown manually and trip the autopilot engage switch if autopilot is engaged.
(b)
The main electric trim has two speed modes: high speed with flaps extended, and low speed with flaps retracted.
(c)
Manual control of the stabilizer is through cables connected to the same mechanism, allowing the pilot to manually position the stabilizer by the trim wheels. The trim wheels follow automatically when electric stabilizer trim is actuated, and the stabilizer trim indicator shows the trim unit setting.
O. Pitch Channel Computer Operation (Fig. 29)
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A 737-300/400/500MAINTENANCE MANUAL
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(1)
(2)
(3)
(4)
A
737-300/400/500MAINTENANCE MANUAL
The Flight Control Computer (FCC) pitch channel provides automatic or manual flight directed control of the airplane in the pitch axis. The system uses both digital and analog inputs for mode control and analog outputs for automatic elevator/stabilizer control or commands to the Flight Director (FD) pitch command bar.The digital inputs used by the FCC pitch channel are from the inertial Reference Unit (IRU), Flight Management Computer (FMC), Digital Air Data Computer (DADC), and the DFCS Mode Control Panel (MCP). These inputs include mode select logic, flight guidance parameters, command signals and gain control. These are processed by ARINC 429 receivers within the FCC for internal use.The analog inputs to the FCC originate from the pitch force transducers, alpha vane sensor (angle of airflow), flap position sensor, VHF NAV receiver, MCP, and radio altimeter. In addition, inputs from the elevator position sensor, and neutral shift sensor provide command feedback. An input from the stabilizer position synchro provides stabilizer information for stab trim command. Analog outputs are provided to the elevator actuator, the ADI, the stabilizer trim servo, the mach trim actuator and the flight mode annunciator.The FCC is divided up into various sections to handle the various requirements. These are the pitch outer loop, and pitch inner loop.
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