(7)
In the alternate-electric-stabilizer trim mode, there are mechanical stops on the upper and lower gimbals to limit ballscrew movement. The stabilizer leading edge up limit is approximately +3.85 degrees or 24.86 - 25.46 inches (631.4-646.6 mm) of the dimension B on ballscrew actuator or 0.20 +/- 0.25 units of trim on position indicator. The stabilizer leading edge down limit is approximately -11.36 degrees or 0.80 - 1.40 inches (20-35 mm) of the dimension B or 15.30 +/- 0.25 units of trim.
(8)
For all stabilizer trim modes, the stabilizer trim limits are same as the alternate trim mode, except when you trim the stabilizer leading edge up with the flaps in the fully retracted position. In this case, the dimension B is 20.43 - 21.03 inches (519-534 mm) or approximately 3 units of trim.
F.
Stabilizer Trim Drive Hydraulic Motor
(1)
Two identical hydraulic motors power the stabilizer trim drive. The motors are constant-displacement, bent-axis, piston-type units. The direction of motor rotation is controlled by fluid through the unit. By reversing fluid flow, the direction of motor rotation can be reversed. The motors may be operated continuously, intermittently, or stalled without damage at rated pressures and in a system having proper overload relief. There are no controls or instruments on the hydraulic motor. Operation is controlled entirely by the STCM's.
G.
Stabilizer Trim Secondary Brake
(1)
The secondary brake is a completely enclosed sealed unit. The brake consists of a housing, shaft subassembly, brake disc pack, pressure plate and piston, and three concentric brake pressure springs. The brake is normally engaged, under which condition the brake springs are compressing the stator and rotor discs together. On introducing hydraulic pressure into the brake pressure port, the piston is forced back, compressing the pressure springs and pulling the pressure plate away from the disc pack. The brake release springs cause a separation of each layer of rotor and stator discs as the pressure plate is retracted by the piston. On removal of the axial force of the brake pressure springs, the rotor discs and shaft are free to rotate.
H.
Stabilizer Trim Limit Switch and Position Transmitter Modules (LSTM's)
(1)
AIRPLANES WITH STAB TRIM LEVERS ON THE CONTROL STAND; the stabilizer trim control system uses three LSTM's (AMM 27-48-00). The LSTM's are connected by cables to the leading edge of the stabilizer torque box. Each LSTM contains two position transmitters (one synchro and one rotary variable differential transducer (RVDT)), three travel limit switches, and a greenband switch. The synchro transmitter sends position data to the control stand stabilizer position indicators. The RVDT sends position data to the stabilizer position module (SPM). The travel limit switches in the left and right LSTM's limit the electrical trim range in every mode except for manual lever operation. The center LSTM greenband switch provides signals to the warning electronics unit (WEU) for takeoff warning.
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
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757
MAINTENANCE MANUAL
(2)
AIRPLANES WITH ALTERNATE STAB TRIM SWITCHES ON THE CONTROL STAND; the stabilizer trim control system uses three LSTM's (AMM 27-48-00). The LSTM's are connected by cables to the leading edge of the stabilizer torque box. Each LSTM contains two position transmitters (one synchro and one rotary variable differential transducer (RVDT)), five travel limit switches, and a greenband switch. The synchro transmitter sends position data to the control stand stabilizer position indicators. The RVDT sends position data to the stabilizer position module (SPM). The travel limit switches in the LSTMs limit the electrical trim range. The center LSTM greenband switch provides signals to the warning electronics unit (WEU) for takeoff warning.
I.
Stabilizer Trim Cut-off Switches (Fig. 5)
(1)
The column operated stabilizer trim cutoff system interrupts electrical trim commands when the control columns are moved in opposition to stabilizer motion. There are four pairs of switches located on the structure beneath the control columns and actuated by cams attached to the control columns. Four switches are operated by control column movement in the elevator-up direction, and four by movement in the elevator-down direction.
(2)
The cutoff switches interrupt stabilizer trim motion in opposite direction to elevator commands if the control columns are displaced a few degrees forward or a few degrees aft of neutral in a direction opposed to the stabilizer trim. Four switches control the left SAM operation and the other four control the right SAM. The switches are wired such that if the columns are operated asymmetrically, as would occur after an abnormal system condition, both trim modules will remain operative in both directions. This allows full rate trim capability after a jam. The STAB TRIM control levers have control authority over the stabilizer trim cutoff switches.
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