General Elevators Stabiliser Rudder Ailerons Spoilers Flaps & Slats EICAS Messages

Flight Controls - Stabiliser

Stabiliser Trim Control

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

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

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

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

There are three modes of stabiliser trim control:

electric
alternate
automatic

Electric Trim

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

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

Alternate Trim

As Installed

Alternate Stabiliser Trim Levers
Alternate trim control is provided by the stabiliser trim levers on the control stand. Pushing both levers simultaneously mechanically commands trim changes. The signals override any other conflicting trim inputs.
Alternate Stabiliser Trim Switches
Alternate trim control is provided by the alternate stabiliser trim switches on the control stand. Pushing both switches simultaneously electrically commands trim changes and provides an increased range of stabiliser travel. The signals neutralise any other conflicting trim inputs.

Stabiliser Trim Limits

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

Automatic Trim

The stabiliser 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 stabiliser as airspeed changes. Electric, backup (Alternate), or autopilot trimming inhibits the Mach speed trim system.

Automatic stabiliser 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 stabiliser. 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 stabiliser command that opposes the change in speed. This stabiliser 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 stabiliser, 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 stabiliser at half the rate available to the pilot, its operation is barely perceptible to the pilot. Also, the stabiliser movement commanded by this system is usually small because it turns off each time the pilot trims the stabiliser 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.

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

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

Stabiliser Cut-Out Switches

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

Pilot Override

The control column can be used to interrupt stabiliser trim commands by means of a Control Column operated trim cutoff switch. This feature allows the pilot to quickly stop uncommanded trim changes. The stabiliser 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 stabiliser brakes fails to release during trimming by the Electric trim switches; the stabiliser trim rate is one-half of the normal control wheel stabiliser 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 stabiliser 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/Stabiliser Trim Module

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

Rudder