User Guide

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In the navigation mode, annunciated “NAV,” the autopilot
will follow the course set in the navigation indicator - whether the
information is coming from a VOR receiver or the GPS. In the
172, the heading “bug” must be set to the same value as the
desired course; in the larger airplanes, this value is set by a sec-
ond knob on the HSI.
Finally, there are a couple of approach modes. The basic
mode, annunciated “APR,” functions the same way as “NAV,” but
is more sensitive to allow greater precision as the airplane nears a
runway. A modified mode, annunciated either “REV” (for
“reverse”) or “BC” (for “back course”), is used only on a few spe-
cial non-precision approaches that use the “wrong side” of an ILS
localizer to approach the opposite end of the runway normally
used for an ILS. This mode has the same sensitivity as “APR,” but
reverses its responses to needle displacement since the airplane is
flying “the wrong way” on the approach.
All of these are “lateral modes,” in which the autopilot
guides the airplane from side to side. The airplanes larger than the
172 have autopilots that also incorporate “vertical modes.” The
simplest, already mentioned, simply holds whatever pitch attitude
existed when the autopilot was first engaged. Altitude hold,
annunciated “ALT,” will hold the aircraft at a specific altitude
above sea level - the altitude at which the “ALT” feature was
engaged on the more basic units, while the more sophisticated
autopilots will allow you to preset the desired altitude on an exter-
nal dial, and can control the airplane to climb or descend, then
level off automatically at the desired altitude.
Last, but sometimes far from least at the end of a long day in
nasty weather, these autopilots, when in “APR” mode on an ILS,
can capture and follow the ILS glideslope on final approach. Fly
up to the final approach fix at the right altitude and with the air-
plane configured for the descent, and when the “APR CAPTURE”
and “GS CAPTURE” lights come on, just extend the gear, reduce
power if necessary, and wait for the runway to show up in the
windshield; even in the Malibu Mirage or Navajo Chieftain, the
autopilot should be able to get you down as close as 200 feet
above the ground, and within half a mile of the runway threshold,
before you have to take over and land visually. In the Hawker, the
“magic brain” can take you all the way to touchdown.
The 172’s simple autopilot is a “single-axis” system. This
means it can steer the airplane from side to side (using the
ailerons) and even track navigational radios, but control of alti-
tude, climbs, or descents, is always left to the human pilot. The
more sophisticated systems in the Malibu Mirage and the Navajo
Chieftain can control altitude as well, and even execute creditable
ILS approaches, while the top-line systems in the Beech King Air
and especially the Hawker can literally fly the airplane from take-
off to touchdown, regardless of weather.
All of these systems have “lockout” logic that won’t let you
engage them unless they’ve satisfactorily passed their preflight test
on the ground. And while they differ widely in capability, their
various modes (if available) have the same nomenclature regard-
less of which airplane you’re flying.
The “basic autopilot” mode will hold the airplane’s wings
level (and, in every airplane larger than the 172, will also attempt
to hold the pitch attitude present when the autopilot was
engaged). Even this seemingly simple function can be remarkably
valuable, especially if you’re (a) on instruments and (b) busy - say,
trying to look at a map or tune a radio at the same time. The FAA
feels so strongly about autopilots that they won’t even allow a sin-
gle pilot to carry passengers for hire in instrument flight conditions
unless the airplane has a functioning autopilot - and during his or
her semiannual check flights, that pilot has to show the FAA that
he or she knows how to use the autopilot in all its modes.
In the 172, the autopilot also provides a valuable backup.
Although its more sophisticated modes depend on the directional
gyro, it can still provide its basic wing-leveling function if the gyro
or its vacuum pumps fail.
In the heading mode, annunciated “HDG,” the autopilot
will hold an actual heading, preselected by setting the “bug” on
the directional gyro to the desired value. On the larger airplanes,
the simple directional gyro is replaced by a multifunction instru-
ment called a Horizontal Situation Indicator (HSI); it’s described
in the next chapter.
Flight Instruction
Flight Instruction