HORIZONTAL STABILIZER TRIM CONTROL SYSTEM -TROUBLE SHOOTING
1. Stabilizer Trim Control System -Trouble Shooting
TROUBLE  PROBABLE CAUSE  ISOLATION PROCEDURE  REMEDY 
Stabilizer trim system binding or inoperative Excessive force required for manual operation  Foreign objects in cable drum, sprockets, forward mechanism, or ball nut and jackscrew gearbox Loose wire connections or damaged motor on main electric actuator Loose wire connections or damaged electromagnetic clutches Damaged jackscrew gearbox assembly or bent jackscrew Defective safety relay or electric circuit Defective trim control relay or electric circuit Defective phase sequence relay or electric circuit Excessive chain tension  Check force required to operate stabilizer trim control wheel. Ref 27-41-0, Test Stabilizer Trim System Check for continuity and for 115 volts ac at main electric actuator. If 115 volt ac is present, main electric actuator is defective Check for continuity and for 28 volts dc at electromagnetic clutches. If 28 volt dc is present, main electric actuator is defective Check jackscrew gearbox and jackscrew for smooth operation. If excessive chatter is present, unit is defective Check safety relay circuit for continuity. If 28 volt dc is present, safety relay is defective Check trim control relay circuit for continuity. If 28 volt dc is present, trim control relay is defective Check for voltage at input and output side of phase sequence relay. If 28 volt dc is present, phase sequence relay is defective Check chain tension. (Ref 27-41-41)  Remove object and repair damage Tighten loose connections, repair circuit as required, or replace main electric actuatorTighten loose connections, repair circuit as required, or replace main electric actuatorReplace jackscrew gearbox and jackscrew unit Repair circuit as required or replace safety relayRepair circuit as required or replace trim control relayRepair circuit as required or replace phase sequence relay Adjust chain tension 

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TROUBLE  PROBABLE CAUSE  ISOLATION PROCEDURE  REMEDY 
Excessive force required for manual operation (Cont) Warning horn fails to sound Stabilizer moves in one direction only electrically and in both directions manually  Excessive cable tension Defective auxiliary brake Defective electric warning circuit Defective warning switch Defective electromagnetic clutches in main electric actuator Defective limit switch Defective electric circuit Defective safety relay  Check cable tension, misalignment, rubbing or chafing (Ref 27-41-41, R/I) Test auxiliary brake (Ref 27-41-0, A/T) Manually operate warning switch to actuate warning horn. If horn does not sound, circuit is defective Operate stabilizer outside of green range and position No. 1 or 2 thrust lever full forward. If warning switch does not actuate horn, warning switch is defective Check for voltage at main electric ac actuator electromagnetic clutch. If voltage is present, main electric actuator is defective Check applicable limit switch for continuity. If voltage is present, limit switch is defective Check for continuity at applicable limit switch and at electromagnetic clutch Check control switches and circuit for continuity through light and safety relay. If continuity checks out and light illuminates, safety relay is defective  Adjust cable tension. Repair damage or replace cable as required Repair or replace auxiliary brake as required Repair electric warning circuit as required Replace warning switch (Ref 31-26-24, R/I, for airplane nose up switch, and 24-41-101, R/I, for airplane nose down switch) Replace main electric actuatorReplace limit switchRepair applicable circuit as required Repair applicable circuit or replace safety relay 

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TROUBLE  PROBABLE CAUSE  ISOLATION PROCEDURE  REMEDY 
Stabilizer trim will not hold position in flight Light inoperative Stabilizer moves in one direction only Stabilizer inoperative in nose down direction only Stabilizer trim wheel coasts more than 2 turns  Defective jackscrew gearbox and jackscrew assembly Defective light bulb Defective light circuit Rusty and/or corroded jackscrew Defective trim control switch Defective electromagnetic clutches in main electric actuator Defective electromagnetic clutches in autopilot actuator  Check auxiliary brake. If brake is working properly, jackscrew gearbox and jackscrew assembly are defective Press to test light bulb Press to test light bulb. If bulb illuminates, circuit is defective Check visually Disconnect trim control switch assembly. (See 27-41-111, Remove stabilizer Trim Control Switch) Check for continuity. If there is no continuity, switch assembly is defective Check for continuity and for 28 volts dc at electromagnetic clutches. If 28 volt dc is present, main electric actuator is defective With autopilot engaged, check for continuity and for 28 volts dc at autopilot electromagnetic clutches. If 28 volts dc is present autopilot actuator is defective  Replace jackscrew gearbox and jackscrew assembly Replace light bulb Repair light circuit as required Lubricate jackscrew. (Ref Chapter 12, Servicing) Replace trim control switch assembly Replace main electric actuatorReplace autopilot actuator 
　
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