(c)  
Repositioning the stabilizer to nose-up direction will rotate a set of cams reacting against droop springs. This spring action modifies the Q-spring forces to reduce computed pressure as the stabilizer trims the airplane. A spring attached to the input arm will move the cam to airspeed-control-only position if the stabilizer-to-computer rod fails.

 (d)  
A differential pressure switch in the feel computer evaluates both output computed pressures. If a difference in pressure of 25% is sensed, the comparator supplies a signal for an amber light circuit on the P-5 panel.

A/P Elevator Actuator Operation

 (a)  
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.

Stabilizer Trim Operation

 (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.

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 ú ú 07 Page 80A ú Mar 12/00
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
737-300/400/500MAINTENANCE MANUAL
 (d)  Trim Authority:
 Main Electric Trim Flaps retracted 2.5-12.5 units Flaps extended 0.25-12.5 units
 Autopilot Trim 0.25-14.0 units Manual Trim 0-17.0 units
 (e)  
The green band range of the stabilizer trim indicator shows permissible takeoff trim range. An intermittent horn sounds if takeoff is attempted with the stabilizer trim NOT in the green band range.

 (f)  
Main electric and autopilot trim motors may be disengaged by individual cutout switches located on the control stand.

 (g)  
A control column operated stabilizer trim cutout switch will stop operation of the electric trim when the control column movement opposes trim direction. A trim switch override is located on the control stand and bypasses the control column operated stabilizer trim cutout switch, when the guard is raised and the switch positioned to OVERRIDE. This allows electric trim to be used regardless of control column position.

 (h)  
The speed trim system provides positive speed control during low speed/high thrust conditions with flaps extended. The flight control computer(s) provides servo command signals to position the stabilizer to maintain the correct airspeed, with normal elevator control forces.

 P. Pitch Channel Computer Operation (Fig. 31)
 (1)  
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.

 (2)  
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.

 (3)  
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 commands. Analog outputs are provided to the elevator actuator, the stabilizer trim servo, and the mach trim actuator. A digital output is sent to the EFIS symbol generator. This output has F/D commands and mode discretes which appear on the EADI.
　
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