S 285-146
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757 MAINTENANCE MANUAL
S 985-147
(25) Push the autopilot wheel disconnect switch twice.
I. Put the Airplane back to its initial condition.
S 865-148
(1)
Remove pressure from the right and center hydraulic systems (Ref 29-11-00).
S 865-149
(2)
Remove electrical power if it is necessary (Ref 24-22-00).
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MACH TRIM/SPEED STABILITY - DESCRIPTION AND OPERATION
_____________________________________________________
1. General (Fig. 1)_______
A. The Mach trim/speed stability mode of the stabilizer trim system provides incremental horizontal stabilizer position in response to changes of Mach and airspeed. The mode is only engaged when airborne (after a 20 second delay) and the manual and autotrim functions are not engaged.
B. This chapter contains Mach trim/speed stability operation. Refer to 22-22-00/001 for a general description of the Automatic Stabilizer Trim System. Refer to 27-41-00/001 for the component details and trim system operation of the Horizontal Stabilizer Trim Control System.
2. Operation_________
A. Functional Description
(1) Mach Trim Speed Stability Block Diagram (Fig. 2)
(a)
The Stabilizer Trim and Elevator Asymmetry Limit Module (SAM) selects the Mach/speed function as the active trim command source when: 1) The flight control computer (autotrim) function is not
used. 2) The control wheel mounted manual electric trim switches are not activated.
(b)
When an FCC or Mach/speed function is the trim command source, only one trim channel (left or right) is operated. The controlling SAM is randomly selected at power up. This provides half-rate trim (0.1 to 0.25 degrees/sec). During Mach/speed operation, the stabilizer position is related to airspeed or Mach. The SAM determines the desired stabilizer position command that operates the Stabilizer Trim Control Module (STCM) and ball-screw actuator. The SAM uses the following inputs to determine ARM and CONTROL output commands to the STCM: 1) Digital airspeed and Mach data from the Air Data Computers
(ADCs).
2) Flap position (up or down) from the Flap/Slat Electronic Units (FSEUs) and Flap/Slat Accessory Module (FSAM) (if installed).
3) Stabilizer position from the Rotary Variable Differential Transformer Transducers (RVDTs) via the Stabilizer Position Modules (SPMs).
(c) If the Mach/speed function is operational, flap position determines which control law (Mach trim or speed stability) is used. When the flaps are extended, speed stability is in control. When the flaps are retracted, Mach trim is in control.
(2) Mach Trim/Speed Stability Control Law Schedules (Fig. 3)
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MAINTENANCE MANUAL
MACH SPEED STABILITY SYSTEM ONLY
Mach Trim/Speed Stability System Figure 1
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