27-31-00
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737-300/400/500MAINTENANCE MANUAL
 C.  In the event of a failure of either hydraulic system A or system B, the remaining system is capable of sustaining normal elevator control. A mechanical input from the control columns will be transmitted through the control system to both power control units. The control unit receiving hydraulic pressure will respond when the main control valve is displaced. The depressurized power control unit will be driven by the operating control unit through the output torque tube linkage.
 D.  In the event of failure of both hydraulic system A and hydraulic system B, a manual reversion mode will allow full elevator control by mechanical means. A noticeable deadband exists when the elevator system is controlled manually. During manual operation, an input to the control system moves the elevator PCU input crank until the crank hits the stops and then the PCU is used as a link to drive the elevator. In the manual reversion mode, elevator movement is assisted by tabs and aerodynamic balance panels. As the elevator is displaced from neutral, a pressure differential is generated across the balance panels to reduce elevator control forces. The use of autopilot is not possible when the airplane is in manual mode.
 E.  A modular assembly consisting of six hydraulic isolation valves is mounted to structure in the empennage, aft of the pressure bulkhead. An isolation valve is installed in each of the system A and system B hydraulic lines to the feel computer, the elevator power unit, and the rudder power unit. The valves are manually operated and provide a means of isolating individual hydraulic components for ground leakage and flow tests.
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737-300/400/500MAINTENANCE MANUAL
ELEVATOR AND TAB CONTROL SYSTEM - TROUBLE SHOOTING
TROUBLE  PROBABLE CAUSE  ISOLATION PROCEDURE  REMEDY 
Insufficient elevator travel  Elevator control system out of rig Foreign objects elevator system components Forward quadrant stops incorrectly installed Excessive system feel forces interfering with  Check for proper rigging of elevator control system (Ref 27-31-00, A/T) Check for foreign objects in area of column, forward and aft quadrants, output torque tube, and centering unit. To determine if problem is in centering unit, disconnect centering unit input rod and check elevator travel With hydraulic power on, first check that column forces are not excessive. Check if quadrant is contacting stops before required travel is obtained Refer to trouble shooting item. Excessive force required to move control columns, power on.  Rig elevator control system as required Remove foreign objects or interference Correct improper installation 

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 ALL  ú ú 01 Page 101 ú Jun 25/84
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A 737-300/400/500MAINTENANCE MANUAL
TROUBLE  PROBABLE CAUSE  ISOLATION PROCEDURE  REMEDY 
Excessive or low  ____NOTE 
501 AMM 27-31-00/ checks per elevator force forces with on. (Determine columns. Power move control force required to  system or binding interference in probable cause is elevator travel, tain portions of is only in cer-If excessive force due to bad or worn parts. If 
due to worn parts. system friction system or high artificial feel cause is in the range, probable excessive force is through the entire travel 
Too much force  Worn forward  Disconnect elevator cables  Replace control 
through full  quadrant parts  from the aft input torque  column support 
range of control  tube. Make sure that the  bearings per 
column travel  control column can move  AMM 27-31-54/ 
with power on  freely forward and aft.  401. 
Too much force  Mechanism or  While moving the control 
through part of  cable catch  column forward and aft, 
　
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