Autopilot Pitch Trim; Trim-In-Motion Indication - BOMBARDIER LEARJET 40 Pilot's Manual

AUTOPILOT PITCH TRIM

When the autopilot is engaged, it can drive the horizontal stabilizer
trim to alleviate elevator servo loading. The autopilot pitch trim func-
tion is contained in the #2 IC autopilot processor. When elevator servo
current exceeds a predetermined threshold for a given period of time,
this is considered to be a steady state error and trim will run. As the
trim runs, the horizontal stabilizer is re-positioned and the air load on
the elevator primary servo is reduced. When this load falls below the
threshold level, trim stops running.
Whenever the autopilot is engaged, #1 IC trim functions, which in-
cludes config/Mach trim, drop off-line. The autopilot commands pitch
trim based on elevator servo current demand and airspeed. Autopilot
pitch trim engagement is controlled by the autopilot engage logic. An
autopilot engage signal is provided to the horizontal trim actuator. If
the autopilot is disengaged as a result of a monitor trip, the aural tone
alert will sound until the MSW switch is pushed. A red AP will also be
displayed on the PFDs and flash for five seconds and then go steady.
The #2 IC monitors for uncommanded trim, trim direction, and incor-
rect trim rate. If the actuator detects one of the above faults, a CAS is
displayed.
The following CAS illumination is specific to the autopilot pitch trim:
CAS
AP ELEV MISTRIM

TRIM-IN-MOTION INDICATION

A trim-in-motion potentiometer is installed on the secondary trim ac-
tuator. When the autopilot energizes the secondary trim actuator for
more than 2 to 3 seconds, a series of audible clacker sounds is transmit-
ted through the audio system. A built-in time delay allows trim opera-
tion for approximately 2 to 3 seconds before the clacker sounds which
prevents a nuisance alarm on the clacker. For longer periods of contin-
uous trim, the clacker will alert the crew. Unusual long periods of auto-
pilot trimming may indicate trim runaway. There is no trim-in-motion
clacker for any trim operation other than autopilot trim.
PM-132A
Color
Amber Autopilot elevator servo holding excessive
torque.
Pilot's Manual
Description
5-19