Speed Trim System
The speed trim system (STS) is a speed stability augmentation system designed to improve
flight characteristics during operations with a low gross weight, aft center of gravity and high
thrust when the autopilot is not engaged. The purpose of the STS is to return the airplane
to a trimmed speed by commanding the stabilizer in a direction opposite the speed change.
The STS monitors inputs of stabilizer position, thrust lever position, airspeed and vertical
speed and then trims the stabilizer using the autopilot stabilizer trim. As the airplane speed
increases or decreases from the trimmed speed, the stabilizer is commanded in the
direction to return the airplane to the trimmed speed. This increases control column forces
to force the airplane to return to the trimmed speed. As the airplane returns to the trimmed
speed, the STS commanded stabilizer movement is removed. STS operates most frequently
during takeoffs, climb and go-arounds.
Conditions for speed trim operation are as follows:
- Airspeed between 100 KIAS and Mach 0.5
- 10 seconds after takeoff
- 5 seconds following release of trim switches
- Autopilot not engaged
- Sensing of trim requirement
Mach Trim System
A Mach trim system provides speed stability at the higher Mach numbers. Mach trim is
automatically accomplished above Mach .615 by adjusting the elevators with respect to the
stabilizer as speed increases. The flight control computers use Mach information from the
ADIRU to compute a Mach trim actuator position. The Mach trim actuator repositions the
elevator feel and centering unit which adjusts the control column neutral position.
