G10948
524 Stabilizer Trim Limit Switches
Nov 15/76 Figure 6 BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 27-41-0 Page 13
B. All switches are operated by the same cam. The cam is mounted by a support tube to the horizontal stabilizer center section jackscrew attach fitting. The cam moves with the stabilizer to actuate the limit switches to the open position at the desired stabilizer travel limits. Opening of the switches removes power to the respective actuator to terminate stabilizer travel.
C. Refer to 31-26-0 D&O and Fig. 6 for takeoff warning limit switches.
9. Stabilizer Trim Relays
A. Stabilizer trim relays include the phase sequence relay, trim control relay and trim safety relay (Fig. 2). With power applied, the phase sequence relay contacts close and the safety relay holds the contacts in closed position allowing the trim control relay to control switching power directly to the motor for normal operation.
B. The stabilizer trim relays are mounted in the J5 junction box located immediately to the right of the nose gear wheel well in the forward lower equipment compartment and can be accessed through the aft right nose wheel well wall panel (panel 3105).
10. Operation
A. Normal control and operation of stabilizer trim is accomplished with electrical power. Control power for the relays, the electromagnetic clutches and the indicator light is 28 volts dc. The actuating power for the stabilizer trim motor is 115 volt, three-phase ac (Fig. 2). When the three-phase power source has the correct phase sequence, assuring correct direction of motor rotation, the phase sequence relay contacts will be closed. The clutch and relay control circuits are ready for normal electrical operation only when the main electric trim cutout switch is at the NORMAL position. With power supplied and the trim cutout switch at the NORMAL position, the stabilizer safety relay coil will be supplied 28-volt dc power to hold its contacts closed. This will allow the trim control relay to control switching power directly to the motor for normal operation. The actuation of either trim control switch will switch control power to energize the trim control relay and the appropriate electromagnetic clutch. This action connects three phase power to the motor, which rotates in one direction only, and engages the clutch which controls the direction of actuator drive. The stabilizer trim on warning light is also turned on when the trim control relay energizes, indicating electrical trim actuation. With the electrical trim motor actuating the jackscrew, the trim wheels will be turning and the trim indicators positioned through the manual system as it is driven from the cable drum on the jackscrew gearbox. The trim actuator will continue to drive the jackscrew in the direction selected until the trim control switch is released, the limit switch is actuated, or the cutout switch is positioned to CUTOUT. Limit switch actuation interrupts power to the clutch which disengages the motor. The motor will not be de-energized by limit switch actuation. The trim wheel rotation will stop and trim position indicator will show that the trim action has stopped at the extreme end of travel. Release of trim control switch will cause the control relay to drop out, interrupting motor power and extinguishing the stabilizer-trim-on warning light. Switching trim cutout switch to the CUTOUT position disconnects al1 normal control circuits, shorts all relay and clutch solenoids to ground, and de-energizes the safety relay to open the trim motor power circuit.
524
27-41-0 Page 14 BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. Feb 20/91
B. In event of simultaneous actuation of the trim control switches for opposite directions of trim, the switching will cause both electromagnetic clutches to engage and result in motor stall which may damage motor due to overheating.
C. Prior to takeoff, with the throttles advanced, and with the stabilizer not within the green range, the takeoff warning system will cause a warning horn to sound, indicating unsafe takeoff condition.
D. Manual control is accomplished by rotating either trim wheel on the control stand. The trim wheels are splined to a common shaft which has a sprocket. Rotation of the trim wheels transmits motion to the forward trim mechanism sprocket by a chain assembly. The forward trim mechanism sprocket drives the cable drum which is cable connected to the jackscrew gearbox cable drum. The jackscrew gearbox cable drum drives the jackscrew to position the stabilizer. The jackscrew gearbox cable drum and cable assembly provides a dual purpose system. During normal electrical operation it drives the forward mechanism to provide stabilizer position indication, and during manual operation it positions the stabilizer. If the electrical system is jammed, initial actuation of the trim wheel operates a clutch which disconnects both motors. The stabilizer trim brake system under the control stand arrests out-of-trim stabilizer motion. The system is actuated by elevator control column deflection which introduces the braking force.
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