AEA Pilot's Guide 2020-2021 - 66

FLIGHT CONTROL SYSTEMS
Continued from page 65

or turn coordinator instrument - to first bring the wings to
level when recovering from an unusual attitude or upset.
Speaking of attitude indicator and vacuum pump
failures, rate gyros enjoy higher reliability than attitude
indicators. Attitude gyros may suffer with progressive
performance degradation over a period of time, typically
due to worn bearings supporting the spinning-mass gyro
wheel. Bearing wear in an attitude gyro causes precession
seen in the response performance of the autopilot. Rate
gyros continue to function with worn bearings to a high
level of performance until the spin motor fails completely.
Then there are the performance issues.
Since rate gyros can't "tumble," they operate
consistently at any attitude, nor are they damaged or
worn excessively by unusual attitudes. In addition, since a
consistent turn rate requires a lower bank angle at lower
airspeeds, rate autopilots often provide better aircraft turn
control at low airspeed.
Still, many operators prefer an attitude-based flight
control system, citing smoother performance, moreaccurate course and localizer intercepts and altitude
changes.
Time was when the choice was primarily budget driven
as much as pilot preference. The rate-based systems
typically offered a budgeting advantage over the attitudebased systems.
Many of these considerations became moot over the
past decade as avionics makers moved toward using
solid-state sensors to detect attitude changes and digital
air-data sensors for altitude control. Sure, some of the
spinning-mass gyro-driven models remain available and
function well.
But don't look for any new flight control systems using
those old-fashioned gyros for sensing. And that's where we
pick up our exam of today's autopilot options.
First, we examine the changes in sensing technology
and how those changes apply to various functions
available in today's modern flight control systems.
This article focuses most on the newest-technology
autopilots, with a brief review of long-existent models.
From simplest to sophisticated: Wing levelers,
single and two-axis systems
When it comes to automatic flight control systems,
nothing simpler exists than a stand-alone wing-leveler

system. By whatever source it references, the wing
leveler acts to hold level those wings. No heading or
navigation input.
Thus, to keep on course, the pilot must occasionally
correct the aircraft heading; the wing leveler knows only to
keep those wings level; drift varies with the winds aloft.
Give the wing leveler the ability to follow a navigation
input and that wing leveler evolves into a single-axis
autopilot. The ability to track the heading on a gyroscopic
compass came first; then came the ability to track a radio
signal such as a VOR transmitter. Today, the modern GPS
navigator joins those VHF navigation signals and gyrocompass headings in helping steer the aircraft.
Throw in the ability to hold an altitude and the two-axis
autopilot brings true automated, dynamic flight control
capabilities to the cockpit.
The higher the capabilities of the autopilot, the more
likely it is that it will sport a yaw damper to help counter a
tendency of some aircraft to fishtail a bit, the nose hunting
port to starboard and back. These are still considered twoaxis autopilots, even with the third surface of the rudder
trim working to counter the yawing.
Capabilities beyond the axes
Today's modern, digitally driven GPS navigators help
make sophisticated autopilots a powerful ally, effectively
becoming a de facto flight management system, or FMS.
In sophisticated business-turbine and commercial
aircraft, the FMS is effectively the brains of the plane,
with a database of navigation waypoints, VHF navigation
frequencies, a VHF navigator and WAAS GPS navigator.
All in a single box, typically found in the pedestal between
the chairs of the captain and first officer.
As digital autopilots grew in capabilities and GPS
navigation became more prominent, pilots began tasking
their autopilots to work hand-in-glove with the GPS
navigator in the FMS.
Add the specifics of instrument approaches to the FMS
database so that the autopilot can follow the approach
plate and the so-equipped general aviation aircraft enjoys
the operational sophistication level of business jets,
turboprops and commercial airliners.
Here are a few examples.
AVIDYNE
This company's digital autopilot, the DFC90, adds the
precision of an attitude-based flight control system and
Avidyne's innovative, safety-enhancing flight Envelope

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AEA Pilot's Guide 2020-2021

Table of Contents for the Digital Edition of AEA Pilot's Guide 2020-2021

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