500  Horizontal Stabilizer Trim Control System Circuit 
Aug 15/68  Figure 2 (Sheet 2)  27-41-0 
Page 7 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

 Horizontal Stabilizer  500 
27-41-0Page 8   Figure 3 BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  May 15/68 

5.  Stabilizer Trim Brake System
A.  A stabilizer trim brake system (figure 4) arrests out-of-trim stabilizer motion when the control column is moved in the direction opposing the stabilizer motion. The stabilizer trim brake is a self-contained mechanical unit, located in the forward nose compartment of the airplane under the aisle control stand. The unit is provided with mounting provisions to be attached independently, and consists of the following rotating shafts: elevator input, stabilizer input, and override input.
B.  The unit operates in such a way that whenever a rotation in one direction is introduced by the elevator control system, the elevator input shaft will prevent the stabilizer input shaft from rotating in the same direction but will allow free rotation in the opposite direction as elevator input shaft. Should a runaway stabilizer cause an AIRPLANE NOSEDOWN or AIRPLANE NOSEUP pitch, moving the elevator control column backward or forward, respectively, will stop the stabilizer from further travel in the out-of-trim direction and allow operation in the opposite direction. The override input shaft is used for releasing the brake mechanism in the unit. The shaft has 2 positions, OFF and OVERRIDE and is spring loaded to OFF. When the override shaft is turned to OVERRIDE position, the brake mechanism wil1 be released regardless of elevator input shaft position. When released from OVERRIDE position the shaft will automatically return to OFF position and restore the brake mechanism to the position called for by the elevator input shaft.
6.  Stabilizer Jackscrew and Gearbox Assembly
A.  The stabilizer jackscrew and gearbox assembly positions the stabilizer by converting electric actuator, autopilot actuator or cable drum rotary motion to linear motion. (See figures 1 and 5.) The stabilizer jackscrew and gearbox assembly consists of a ball nut and jackscrew, a gimbal assembly, a cable drum, and the gearbox consisting of gearing and brakes. The gearbox is connected to a bulkhead in the fuselage by a lower gimbal which allows fore and aft angular motion as the stabilizer is positioned. A upper gimbal connects the ball nut to the stabilizer front spar fitting. This gimbal prevents any binding of the screw while the ball nut positions the stabilizer. A safety rod is installed in the jackscrew shaft to support the stabilizer in the event of jackscrew failure.
500 
Nov 15/66  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  27-41-0 Page 9 

G10718 G10792
Stabilizer Trim Brake  500 
27-41-0Page 10   Figure 4 BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  Nov 15/66 

500  Stabilizer Jackscrew and Gearbox Assembly 
Nov 15/66  Figure 5  27-41-0 
Page 11 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

B.  The primary brake system in the stabilizer trim gearbox prevents any aerodynamic loads on the stabilizer from rotating the jackscrew when the control system is not being operated. When the jackscrew shaft is driven by the jackscrew gearbox in a direction which increases the airloads on the stabilizer, a brake plate rotates through a ratchet producing a clicking sound in the assembly.
C.  The auxiliary brake system i s provided in case of primary brake system failure. When the jackscrew is turned by the jackscrew gearbox, the auxiliary brake system is released through a gear driven by the gearbox. If the primary brake system fails, the jackscrew shaft rotation applies the auxiliary brake to prevent any additional shaft rotation.
7.  Electric Actuator
A.  The main electric actuator provides power to actuate the stabilizer during normal operation (Fig. 1). The unit consists of a three-phase induction motor, two electromagnetic clutches and the output shaft to drive the jackscrew and gearbox assembly. The trim motor is energized by 115 volt, three-phase, 400-Hz ac power and is capable of emergency operation on two-phase supply. The electromagnetic clutches control the direction of rotation of the trim motor output shaft and are energized by 28 volts dc.
8.  Autopilot Actuator
A.  The autopilot actuator provides an electrical means for actuating the stabilizer. Control is provided by the autopilot. The autopilot actuator consists of a three-phase reversible two speed induction servomotor, an electromagnetic clutch with a gear reduction, and an output shaft to drive the jackscrew gearbox. The servomotor is energized by 115 volt, three-phase, 400-Hz ac power and is capable o f emergency operation by a two-phase supply. Reversing is accomplished by phase reversing through the use of a direction relay. Separate servomotor windings provide a speed change ratio of six to one. Selection of the servomotor speed is by a speed change relay. The high speed mode is selected when an input signal is received from the flap switches and the low speed mode is selected with no flap switch input. The flap switches are actuated when flaps are at 2 degrees or more. The electromagnetic clutch and the control relays are energized by 28-volt dc power.
　
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