Instrumentation

Autopilot
26
Such safeguards are implemented by limiting the torque applied to the servomotors, and also
by allowing them either to slip, or to be completely disengaged, in the event that present torque
limits are exceeded. The methods adopted usually depend on either mechanical, electrical or
electromechanical principles.
Engagement Criteria
Autopilot Interlocks
Before coupling an autopilot with the aircraft’s control system the integrity of the Autopilot
Inner Loop must be established to ensure that it may safely take control of the aircraft.To
monitor the performance of the inner loop components a system of interlocks is provided which
close to allow autopilot engagement and hold it engaged if the correct valid signals have been
received. The function of the interlocks can be represented by a number of relays in series (see
Figure 26.7), although in modern aircraft the actual switching is more likely to be accomplished
by solid state logic switching. Failure of a circuit monitored by a relay will cause the autopilot to
disengage accompanied by the associated aural and visual warning indications. Operation of
the disengage switch will have the same effect.
V DC
V DC
Figure 26.7 Autopilot interlocks
Conditions of Engagement
Before the autopilot can be engaged, certain conditions must be met. These conditions vary
with aircraft type. For the 737-400 these conditions are as follows.
Each autopilot can be engaged by pressing a separate CMD or CWS engage switch. A/P
engagement in CMD or CWS is inhibited unless both of the following pilot-controlled conditions
are met:
• No force is being applied to the control wheel.
• The Stabilizer Trim Autopilot Cut-out Switch is at NORMAL.
Once the above conditions are satisfied and no failures exist, either A/P can be engaged in
CMD or CWS by pressing the respective engage switch. Control pressure applied after an A/P
is engaged in CMD overrides the A/P into CWS pitch and/or roll. The light remains illuminated
in the CMD engage switch.
26 Autopilot
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