Flight Training - August 2014 - 13

HOW IT WORKS

BY IAN J. TWOMBLY

Autopilots
Look ma, no hands!
WHAT SEEMS LIKE a complicated device is
actually quite simple when you compare
its operation to your own body. An autopilot is essentially a mechanical version
of you, the pilot. Your eyes see a bank or
pitch change. They send a signal to your
brain, which processes the change then
sends another signal to your muscle,
which makes a control input. That
muscle receives feedback, and the whole
process forms a loop.

Autopilots take their cues from either
the attitude indicator (so-called attitudebased) or turn coordinator (so-called
rate-based). Sensors in the instruments
send a signal to a processor that determines the necessary pitch or roll input. It
then sends an electrical signal to a servo,
which is nothing more than a motor that
actuates the necessary control.
To avoid overcontrolling, a sensor in
the servo sends another signal back to

the processor, completing a feedback
loop that ensures the controls don't go
too far afield in an effort to correct a
pitch or bank change.
As autopilots get more sophisticated,
they can control heading, intercept and
track a course, climb or descend at a
specific rate, capture altitudes out of a
climb or descent, and even anticipate
turns with GPS.

WHICH SYSTEM IS BETTER?
Pros and cons of each autopilot
Manufacturers tend to stick to either rate-based
or attitude-based methods, and there are pros and
cons to each. An attitude-based system allows for
a greater variety of bank angles, and some say a
more responsive design. But the vacuum pump and
attitude indicator are more prone to failures.

AXIS OF CONTROL

STEVE KARP

One, two, or more

An autopilot is often described by how many different controls it can manipulate. Here's what each means, in general terms.
Single axis-One control, usually roll
Dual axis-Two controls, usually pitch and roll
Three axis-Roll, pitch, and yaw, controlled through a damper
Four axis-In helicopters, roll, pitch, yaw, and collective. This is somewhat analogous to an airplane's three-axis system with
autothrottles.

AUGUST 2014 FLIGHT TRAINING

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Flight Training - August 2014

Table of Contents for the Digital Edition of Flight Training - August 2014

Contents
Flight Training - August 2014 - Cover1
Flight Training - August 2014 - Cover2
Flight Training - August 2014 - Contents
Flight Training - August 2014 - 2
Flight Training - August 2014 - 3
Flight Training - August 2014 - 4
Flight Training - August 2014 - 5
Flight Training - August 2014 - 6
Flight Training - August 2014 - 7
Flight Training - August 2014 - 8
Flight Training - August 2014 - 9
Flight Training - August 2014 - 10
Flight Training - August 2014 - 11
Flight Training - August 2014 - 12
Flight Training - August 2014 - 13
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Flight Training - August 2014 - 16
Flight Training - August 2014 - 17
Flight Training - August 2014 - 18
Flight Training - August 2014 - 19
Flight Training - August 2014 - 20
Flight Training - August 2014 - 21
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Flight Training - August 2014 - 25
Flight Training - August 2014 - 26
Flight Training - August 2014 - 27
Flight Training - August 2014 - 28
Flight Training - August 2014 - 29
Flight Training - August 2014 - 30
Flight Training - August 2014 - 31
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Flight Training - August 2014 - 33
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Flight Training - August 2014 - 35
Flight Training - August 2014 - 36
Flight Training - August 2014 - 37
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Flight Training - August 2014 - 40
Flight Training - August 2014 - 41
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Flight Training - August 2014 - Cover3
Flight Training - August 2014 - Cover4
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