Avionics News October 2019 - 23

The pilot of the household paused, momentarily
stumped because he considered autopilots as one of those
signs that an aviator has "arrived," if you will.
Then the nonflying spouse let the pilot spouse off
the hook.
While the flying spouse stumbled to form an answer, the
nonflying spouse tossed a well-known aviation magazine on
the lap of the flying spouse.
"Why didn't you tell me about the safety benefits of an
autopilot, and why didn't you say you need help sometimes?"
she asked.
The two previously flew together for nearly 2,000 hours,
and all those hours flew without an automatic flight control
system installed and operating in their complex piston single.
The nonflying spouse regularly held the yoke as the flying
spouse took pen in hand after the controller chimed in with
the dreaded phrase, "November something-something, stand
by to copy an amended clearance."
The nonflying spouse then commented on the rough air
to ask how the flying spouse handled such situations when
flying alone, which was most of the time.
"Very carefully," the flying spouse responded, "very
carefully."
You can probably guess how this discussion concluded:
With the pilot spouse diving into research to identify the best
flight control-system solution for their bird and their budget.
"We can get it installed at annual," the nonflying spouse
offered, "and we'll have it to help when we fly home for
the holidays."
What pilot would possibly decline that offer?
That was years ago when the dominant single-axis systems
worked well tracking a heading or following a VOR signal.
Two-axis models added altitude hold, while other so-called
wing levelers did only what the name implies: Hold a wingslevel attitude independent of any navigation input.
Such systems remain on the market, joined in recent years
by flight control systems far more capable and sophisticated
than anything short of a business-turbine panel - back then.
Today, the choices available would have seemed farfetched for light piston aircraft - flight control systems
capable of all the above functions, plus following a GPS
course, following the flight plan in a GPS navigator, even

navigating complex instrument approaches, including
descents to decision height on short final.
These functions once stood as the exclusive purview of
highly sophisticated flight-management systems used in
turbine-powered aircraft, from light general aviation up
through and including business jets and airliners.
Best of all, many of these systems are available for far less
than our turbine-flying friends' systems, with many within
reach of owners of even the most modest aircraft.
And the selections are significant.
Some basics
For decades, the dominant autopilot sensing technology
for light GA aircraft used a rate-based system driven by the
electric turn coordinator to sense attitude changes. Prior to
those systems, pneumatic-driven systems used air pressure to
move the appropriate control surface in the correct direction.
The other sensing technology employed in autopilots
is the attitude-based system, which senses roll and pitch
changes from the attitude indicator.
The rate-based approach enjoys some advantages and
disadvantages, like most any aircraft system.
On the safety side, electric turn coordinator rate
gyro works independently of the attitude gyro, which
predominantly gets power from the aircraft vacuum system
used to drive the attitude gyro.
For the attitude-based system, the failure of either the
attitude gyro itself or the vacuum pump powering the attitude
indicator spells the loss of the flight control function. With
the same failure - an attitude indicator failure of whatever
cause - the turn coordinator and the rate-based autopilot
suffer no ill effects.
The rate-based autopilot continues to drive the flight
control system unaffected by the failure of the attitude
indicator or air pump.
Additionally, rate gyros won't tumble due to flight in
unusual attitudes. That's why flight instructors teach their
students to recover from unusual attitudes or loss of control
using the turn gyro - either the turn and
bank 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
Continued on following page
avionics news

*

october

2019

23



Avionics News October 2019

Table of Contents for the Digital Edition of Avionics News October 2019

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Avionics News October 2019 - No label
Avionics News October 2019 - Cover1
Avionics News October 2019 - Cover2
Avionics News October 2019 - 1
Avionics News October 2019 - 2
Avionics News October 2019 - 3
Avionics News October 2019 - 4
Avionics News October 2019 - 5
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Avionics News October 2019 - Cover3
Avionics News October 2019 - Cover4
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