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TROUBLE  PROBABLE CAUSE  ISOLATION PROCEDURE  REMEDY 
Trim drifts from setting during flight (Cont) Excessive yaw when switching from A and B hydraulics to standby or from standby to A or B hydraulics in flight Experiencing high rudder pedal forces OR Rudder pedals kick back or yaw damper backdrives rudder pedals during flight OR Erratic yaw damper operation or rudder oscillates with yaw damper engaged OR Erratic rudder pedal steering on ground with yaw damper engaged  High system friction and poor centering Standby actuator out of rig Binding within the standby actuator Possible binding or restriction of the input linkage of the Main PCU.  Check rudder centering per 27-21-0, Rudder and Rudder Trim Control System -Adjustment/Test. If rudder does not return to neutral, system friction may be excessive. Refer to faulty rudder centering trouble shooting. Check rigging per 27-21-0, Rudder and Rudder Trim Control System -Adjustment/Test. Shutoff all hydraulics. Disconnect input control rod from the standby input lever. Measure the force required to move the lever throughout its range of motion. This force, when applied tangentially (900) to the input crank arm and at approximately the bolt/clevis centerline, shall not exceed one pound. Shutoff all hydraulics. Disconnect input pushrod and verify no binding or restriction of linkages of the Main PCU as the input crank is rotated throughout its range of travel (AMM 27-21-0/201). If force required to move PCU input crank is greater than 40 lbs, PCU is defective.  Adjust as required. Replace the standby actuator. Verify PCU input crank can move freely. Replace worn or defective parts. 

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2.  AIRPLANES POST-SB 27-1252; STBY RUD ON Light Problems -Fault Isolation
A.  Description:
(1)  
The STBY RUD ON light is on the P5-3 panel.

(2)  
There are three different ways that the STBY RUD ON light will be illuminated:

(a)  
the flight crew selects STBY RUD on, using system A or B flight control switch on the P5-3 panel

(b)  
auto standby logic detects low pressure on either the A or B hydraulic systems, flaps are not up and either in-air or wheel speed greater than 60 knots.

(c)  
force fight monitor (FFM) detects force fight in the main rudder PCU.

B.  Possible Causes:
(1) 
 Wiring problem

(2)  
Force Fight Monitor problem

(3)  
Auto Standby Circuitry

(4)  
P5-3 Flight Control Module

(5) 
 Rudder Main PCU.

C.  Initial Evaluation
WARNING:  MAKE SURE THAT PERSONS AND EQUIPMENT ARE CLEAR OF ALL CONTROL SURFACES BEFORE YOU SUPPLY HYDRAULIC POWER.  AILERONS, RUDDER, ELEVATORS, FLAPS, SPOILERS, SLATS, AND THRUST REVERSERS CAN MOVE QUICKLY WHEN YOU SUPPLY HYDRAULIC POWER.  THIS CAN CAUSE INJURY TO PERSONS AND DAMAGE TO EQUIPMENT.
(1)  Pressurize the rudder hydraulic system A, B, and Standby (AMM 27-21-0/201).
(a)  
Set the FLT CONTROL A and B switches on the P5-3 panel to the ON positions.

(b)  
Push the left rudder pedal forward until it reachs the stop, hold for 10 seconds, then return the left rudder pedal to the neutral position.

(c)  
Push the right rudder pedal forward until it reachs the stop, hold for 10 seconds, then return the right rudder pedal to the neutral position.

(d)  
If the STBY RUD ON light is illuminated, do the Fault Isolation Procedure D.(2) below.

(e)  
If the STBY RUD ON light is not illuminated, do the Fault Isolation Procedure D.(1) below.

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27-21-0 Page 110  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  May 01/03 

D.  Fault Isolation Procedure
(1) If the STBY RUD ON light is not illuminated in the Initial Condition check, do these steps:
　
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