Flight Controls - Stabilizer Show Schematic only

Stabilizer Trim Control

Stabilizer. Center Hydraulic System Right Hydraulic System default

The stabilizer is powered by the Center and Right hydraulic systems. Stabilizer position commands are sent to the stabilizer trim control modules, which control hydraulic power to the stabilizer through a motor and brake system. There are two modules, one for each stabilizer hydraulic source.

Note : The 757 does not have a "Pitch Enhancement System".

Stab Trim IndicatorThe rate of trim varies and is controlled by the trim control modules. When airspeed increases, the trim rate decreases.

Stabilizer position is displayed on two stabilizer position indicators located on the control stand. Green bands indicate the normal trim settings for takeoff.

There are three modes of stabilizer trim control:

Electric Trim

Dual electric pitch trim switches located on the control wheel must be pushed simultaneously to command trim changes.

To set stabilizer trim less than 3 units with the flaps up, requires use of the Alternate Trim system.

Alternate Trim

  • Alternate Stabilizer Trim Levers

    Stab Trim Levers.

    Alternate trim control is provided by the stabilizer trim levers on the control stand. Pushing both levers simultaneously mechanically commands trim changes. The signals override any other conflicting trim inputs.

  • Alternate Stabilizer Trim Switches

    Stab Trim Switches

    Alternate trim control is provided by the alternate stabilizer trim switches on the control stand. Pushing both switches simultaneously electrically commands trim changes and provides an increased range of stabilizer travel. The signals neutralize any other conflicting trim inputs.

Stabilizer Trim Limits

AMM In the alternate-electric-stabilizer trim mode, there are mechanical stops on the upper and lower gimbals to limit ball-screw movement. The stabilizer leading edge up limit is approximately 0.20 units of trim. The stabilizer leading edge down limit is approximately 15.30 units of trim.

Automatic Trim

The stabilizer is controlled automatically by the autopilot or by a Mach Speed Trim system when the autopilots are not engaged. The Mach speed trim system improves speed stability by trimming the stabilizer as airspeed changes. Electric, backup (Alternate), or autopilot trimming inhibits the Mach speed trim system.

Automatic stabilizer trim uses only one trim control module and trims at one-half the electric or alternate trim rate.

Mach/Speed Trim System

Boeing The Mach/Speed Trim System functions only when there are no other commands to the stabilizer. The actual speed of the airplane at the time the system begins to operate is then the index speed. Any deviation from the Index speed will generate a stabilizer command that opposes the change in speed. This stabilizer movement will tend to return the airplane to the Index speed and has the affect of providing the required stick force per knot of airspeed change. If there are other trim commands to the stabilizer, either pilot trim input or autopilot trim Input, the Mach/Speed Trim System will cease to operate and will not reengage until such commands cease.

Because the Mach/Speed Trim System moves the stabilizer at half the rate available to the pilot, its operation is barely perceptible to the pilot. Also, the stabilizer movement commanded by this system is usually small because it turns off each time the pilot trims the stabilizer and when the autopilot is engaged. The Mach/Speed Trim System is also inhibited on the ground and for twenty seconds after liftoff.

An inoperative Mach/Speed Trim System will not perceptibly affect aircraft control or handling characteristics.

Non-Normal Operation

If a single autopilot is engaged, electric trimming causes the autopilot to disengage. If multiple autopilots are engaged, the electric trim switches are inhibited. Alternate trimming does not cause autopilot disengagement.

Unsched Stab Trim Warning. The UNSCHED STAB TRIM illuminates and the EICAS caution message UNSCHD STAB TRIM displays when uncommanded stabilizer motion is detected or, with Autopilot or Mach Speed Trim system engaged, the Stabilizer is moving opposite to the direction signalled.

The light and message also occur if alternate trim is used with an autopilot engaged.

Stabilizer Cut-Out Switches

The center and right stabilizer cut-out switches control hydraulic power to the respective stabilizer trim control module. Placing both switches in the CUT OUT position removes all hydraulic power from the stabilizer trim control modules.

Pilot Override

The control column can be used to interrupt stabilizer trim commands by means of a Control Column operated trim cutoff switch. This feature allows the pilot to quickly stop uncommanded trim changes. The stabilizer trim commands (both electric and automatic) are interrupted if the control column is displaced in the opposing direction.

Stab Trim Warning.

The STAB TRIM light illuminates and the EICAS advisory message STAB TRIM displays when one of the two stabilizer brakes fails to release during trimming by the Electric trim switches; the stabilizer trim rate is one-half of the normal control wheel stabilizer trim switch rate.

If the malfunction is unique to the electric trim control, full trim rate is available by using alternate trim. If both brakes remain engaged, no stabilizer trim is available.

Mach Speed Trim Warning.

The MACH SPD TRIM light illuminates and the EICAS advisory message MACH/SPD TRIM displays when the Mach speed trim system is inoperative.

Yaw Damper/Stabilizer Trim Module

On the 757-300, the Yaw damper/Stabilizer trim Module (YSM) combines the functions of the Rudder Ratio Changer module, Stabilizer Control Module and the Yaw Damper.