Avionics News March 2018 - 40

UPRIGHT MIGRATION
Continued from page 39

calculates available lift and speed margins in the background,
and issues visual and aural warnings when the aircraft is
nearing the edge of its normal flight envelope.
Acronym Confusion
While sharing the same acronym, as a capability,
the phase of flight defines vertical navigation. What
separates en route VNAV from approach VNAV is pilot
control, Stone explained. "When entering a flight plan
in the GTN, pilots can assign altitudes at, or so many
miles before or after a given fix, and the system will
calculate the vertical path so the aircraft will arrive at
the appropriate offset."
Giving an example of how the system reduces pilot
workload, he suggested "a descend via" ATC clearance.
The system "meets those constraints without having to
do the mental math and adjust the speed and descent
rates," he said.
Over-speed and under-speed protection is one of the
autopilot's capabilities, but it is independent of VNAV.
"The autopilot monitors the aircraft speed and angle of
attack, if available, and it increases or decreases pitch
to prevent an over- or under-speed situation," Stone
explained.
"If the aircraft is heading down a VNAV path and it starts
to over-speed, the autopilot relaxes the pitch to prevent the
over-speed and alerts the pilot with an annunciation and
aural alert," he continued. "The pilot then adjusts power as
necessary to recapture the VNAV path."
The same scenario plays out when the autopilot is
not engaged. Its electronic stability control technology
monitors the aircraft speed and attitude and gently, at
first, nudges the controls (through the same autopilot
servos) toward stable flight when it exceeds the specified
pitch, roll and speed parameters. If the pilot does not (or
cannot) take the hint, the autopilot will engage the flight
director and bring the aircraft back to straight and level.
When flying an instrument approach procedure, the
coupled autopilot system is flying "a mapped vertical
path that is, if you will, preprogrammed" by the specifics
of the approaches stored in the database, Stone said.
"In the approach world," he continued, "LNAV
provides just lateral guidance, LNAV/VNAV defined
both lateral and vertical guidance, and LPV delivers
localizer precision with vertical guidance, like the
40

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march

2018

terrestrial ILS. Each of them has different minima, a
minimum descent altitude or in the case of the LPV, a
decision height."
If sufficient data is available, such as "touchdown
zone elevation, missed approach point altitude, those
types of things," Stone said, the system can add vertical
guidance to a lateral navigation approach, making it
LNAV+V. With the necessary data, this advisory vertical
guidance also works without the input from WAAS/
SBAS (space-based augmentation systems) service.
Guided by the flight director's command bars, "this
gives pilots a nice stabilized approach," he continued.
But the vertical guidance is advisory only, he warned.
"Pilots must still comply with any step-down fixes
given in the LNAV approach and its minimum descent
altitude." In other words, vertical advice does not give
pilots a decision height or altitude.
The system also gives pilots vertical advice on visual
approaches. It is based on a published glide path angle
or 3-degree glideslope from the runway's threshold, and
it considers terrain and obstacle clearance.
When following a flight plan, the GTN also offers a
shortcut to optionally load and activate a visual approach
when it is within 5 miles of the destination airport if
the pilot has not already selected an approach. Again,
the result is a stabilized approach, guided by the flight
director command bars.
A TOGA (take off/go-around) button or switch is an
option to the GFC 500 system, said Stone, "but it might
be of limited value to the pilot of a Cessna 172." When
activated, it engages the flight director, which displays
the pitch up attitude appropriate to the aircraft, and "the
pilot pushes the power upright after he or she pushes the
TOGA button."
The function is essentially the same whether taking
off or going around, Stone continued, who emphasized
that technicians must configure it to a specific aircraft
so that the flight director's command bars depict the
optimized pitch attitude for the best rate of climb. "After
pushing TOGA and power, the pilot can either hand-fly
the airplane following the flight director's guidance or
engage the autopilot."
With "so many stand-alone systems - FMS, terrain
awareness - now integrated into larger systems," there's
no telling where this amalgamation of capabilities
will end up. Pausing for a moment, Stone said with
a chuckle, "One of these days, it will all be on our
smartphones." q



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