Avionics News June 2022 - 39

Pitch Attitude Command
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system, or an air data heading reference
system. A turn and bank indicator
cannot be used, since it does not sense
movement around the lateral axis of the
aircraft. In addition, pilots owning airplanes
with gyros planning to upgrade
from a single-axis to a dual-axis system
must be aware their gyro may only
provide a single-axis reference. These
pilots will need to purchase a new instrument
to provide both pitch and roll
attitude information to the autopilot.
In other respects, the reference
principles are the same. The reference
could be AC, DC or digital.
As in part I, arbitrary DC reference
levels will be discussed here for
simplicity and clarity.
Altitude
Reference
Navigation
Coupler
Summing
Junction
Amplifier
Main Pitch
Reference
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Gyro Trim Adjust
Trim Servo
FIGURE 2: A full-featured autopilot will be able to hold an altitude, take commands from the
pilot, and follow a vertical path. In addition, the system will operate electric pitch trim.
Computer/amplifiers
Like the roll-only systems, the early autopilots were mostly
amplifiers. The newer autopilots have significant computing
power. The processes are similar. In fact, many early systems had
circuit boards that were identical to their roll counterparts, which
made troubleshooting easy. The technician could switch pitch and
roll boards and test to see if the symptoms changed axes.
Servos
The servo principles for pitch are identical to the principles
described in part I; however, since pitch systems include the control
of the trim system, two servos are required. Like the roll axis,
the servo could be electric, hydraulic, or in rare cases, pneumatic.
The pitch servo will be connected mechanically to the elevator or
stabilator. A second servo, called the trim servo, will be connected
to the elevator trim tab. Like roll servos, these servos can be found
in a variety of locations within the airframe. Most likely, they will
be located in the tail, aft of the passenger or baggage compartment,
but technicians should not be surprised to find them under
the floor or even behind the instrument panel.
Trim control
There are three methods of automatically controlling the servo
for pitch trim. The first way is by monitoring the output of the
computer amplifier and then, after a delay, begin trimming in the
same direction. This method anticipates the need for trim. The
next two ways monitor whether or not trim is necessary. Method
two involves a cable tension sensing device, which the control
cables of the aircraft pass through. When there is a need for trim,
one cable will be tight, and the other cable will be loose. The tight
cable will activate a microswitch, which tells the system trim is
necessary and in which direction to move. The third method is
built into the pitch servo and senses the effort put forth by the
motor within the pitch servo. When the torque reaches a predetermined
value, a signal is sent, which will activate the trim servo in
the proper direction.
In most installations, the pilot also has the ability to operate the
trim servo. Autopilots are designed to disconnect whenever the
pilot activates the trim system.
Pitch control
Figure 1 depicts the pitch axis of an autopilot with the aircraft
in level flight. Since the airplane is level, no reference voltage is
produced, so there is nothing to be amplified. In this case, neither
servo will move. Note the figure does not depict the trim servo.
Trim servo drive will be discussed later.
The bottom portion of Figure 1 depicts an aircraft in a nose-up
attitude. In this case, the reference will produce an error voltage,
which the computer amplifier will amplify to a voltage sufficient
to start the servo moving. The elevator will deflect, and the aircraft
will react by rotating in a nose-down direction until the error
voltage disappears.
Figure 2 shows the processes of a complete pitch system.
Added to this diagram is the summing junction, which like in roll,
adds the various error signals together before sending them to the
amplifier. As in the roll-only systems, this is simplified; a modern
autopilot will have push buttons or even touchscreens to make the
connections.
Like the roll system, there will be an adjustment to remove
Continued on following page
avionics news * june 2022 39
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Elevator
Control cable
Servo
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Command from
navigation system

Avionics News June 2022

Table of Contents for the Digital Edition of Avionics News June 2022