Avionics News August 2018 - 17

ity and air evolved into the familiar six pack of three air-data
eter, an airspeed indicator and an attitude indicator, a packinstruments, driven by the pitot-static system, and three
age often crafted out of repositioning existing instruments
spinning-mass gyroscopic instruments - two of which used a
after installing a glass PFD.
vacuum (or pressure) pump, and one powered by the aircraft's
The benefits include costs - the airspeed indicator, altimeter
electrical system.
and attitude indicator already exist; it's simply a matter of moving
This split power arrangement automatically provides a
them out of the way of the new PFD while keeping them within
level of power redundancy, albeit in a somewhat cumbersome,
the pilot's line of sight.
complicated, partial-panel form of redundancy.
Simple, though the analog gyros - the attitude indicator
The standard standby capabilities depend on the pilot's abiland gyro compass - still retained the shortcomings the digital
ity to interpolate what inoperable instruments would indicate,
PFD eliminate because most aircraft still depend on the fragile
if functioning, from the indications of the remaining operable
vacuum pump.
instruments. Hence the name, "partial panel."
This solution began to give way to more reliable alternatives
Ask any instrument-rated pilot about the amount of training
the instant the first digital standby packs appeared - packages
time they spent flying under a visionmore closely resembling the main digital
restricting hood or glasses, practicing
instruments they back up.
"partial-panel" flying. The instructor covMain instruments today - the modern
ered one or more of the six and made the
incarnation
of the old analog six pack - fit
WHILE SOME RESEARCH
instrument student maneuver the aircraft,
within
the
confines
of the primary flight
SUGGESTS THAT PILOTS
staying within limits, on command, using
display.
The
digital-electronic
versions of
FLYING BEHIND GLASS
only the remaining instruments to fill in the
that old analog six pack still include an
PANELS SUFFER SLIGHTLY
blanks created by covered instruments.
altimeter, an airspeed indicator, a vertical
FEWER ACCIDENTS
While these factors still apply for
speed indicator, attitude indicator, gyroTHAN THOSE PILOTING
operators flying traditional analog sixscopic compass and turn coordinator with
TRADITIONAL SIX-PACK
pack, steam-gauge stacks, that population
its slip/skid ball.
PANELS, SOME RESEARCH
continues to decline with operators taking
Some PFDs offer the option of displayALSO SUGGESTS THAT
advantage of the many options available to
ing the primary instruments in their old
THE IMPROVEMENT STEMS
convert to glass-panel technologies.
steam-gauge format - six round dials,
MORE
FROM
THE
LACK
OF
While some research suggests that pilots
for each of the three air-data instruments,
AIR-POWERED GYROS AND
flying behind glass panels suffer slightly
three more for the gyro instruments.
THE ASSOCIATED PUMP.
fewer accidents than those piloting tradiBut in modern PFDs, vertical tapes
NO PUMP, NO AIR-DRIVEN
tional six-pack panels, some research also
replace the round dials of the air-data
GYRO FAILURES.
suggests that the improvement stems more
instruments - airspeed, vertical speed,
from the lack of air-powered gyros and the
altitude - while the gyro instruments
associated pump. No pump, no air-driven
more closely reflect their originals with
gyro failures.
round-instrument displays.
While not completely true - glass primary flight displays have
Today's typical PFD even includes digital replacements of
been known to fail - a body of belief has developed that standby
navigation indicators pilots use to follow guidance from both
electrical power and modern standby instrument packs helped
VHF and GPS navigators.
drive the improvements.
As all-electrical systems, however, today's all-glass instruConsider the options available for meeting Federal Aviation
ments depend on the uninterrupted flow of electrons through the
Administration requirements for standby instruments to back up
aircraft's electrical system.
digital PFDs.
Gone is the inherent power redundancy of the old days.
No more split-power redundancy for the six pack on the PFD.
Backup instruments of all stripes
But the FAA wisely retained the requirement for the same three
With the recent-years transition to digital glass cockpit panels
stand-alone, standby instruments required in the event of the PFD
from the traditional three-power-sources analog panels, new
display failing: attitude indicator, airspeed indicator and altimeter.
redundancy options emerged - and are required.
The purpose of standby instruments noted in this AC ensures
FAA Advisory Circular No. 23.1311-1C provides the guidance
that adequate air-data and flight attitude displays are available to
for installations of electronic displays in Part 23 aircraft.
the pilot during all phases of flight, particularly during system
In the past two decades, we've seen standby instruments
failures. The AC even specifies that individual indicators should
evolve.
Continued on following page
First, we saw an air-powered-and-analog trio of an altimavionics news

august

2018

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