Avionics News December 2021 - 45

the output of the generating source, be it alternator or generator.
In the event of a generating-source failure, that battery
becomes the only source for power to the panel, the lights and,
for many retractable aircraft, the landing gear. Retractable-gear
aircraft must have an alternative to the main extension/retraction
system, typically pilot-powered.
But an electrical-system failure can neutralize the main system
moving the gear, and facing the need to use the manual-extension
can exacerbate an already tense situation. Throw in instrument meteorological
conditions and the need to fly an instrument-approach
procedure while working a landing-gear problem can domino on
the pilot, distract the pilot at all the wrong times, and contribute to a
number of undesirable outcomes.
Dealing with a recalcitrant landing-gear system is the ultimate
in tension-raising events in a cockpit in the soup. In most situations,
a redundant electrical system may help the pilot avoid such a
crisis, and many others: loss of lighting, loss of communication and
navigation systems is another problem avoidable with a redundant
electrical system.
Acknowledging that the battery is the first line of redundancy,
from here on, when we speak of electrical-system redundancy,
we're talking about redundant generating sources and redundant
electrical connections - as in a secondary or emergency electrical
buss.
From the perspective of the pilot-in-command, few events elevate
adrenaline levels higher than losing part or all of your panel instruments
while flying in instrument meteorological conditions. Absent
a visible horizon, the most-challenging failure involves losing two
air-driven gyroscopic instruments - the attitude indicator and the
directional gyroscope compass.
Losing the AI deprives the pilot of direct indication of pitch and
roll. In most aircraft with a standard six-pack of flight instruments,
suction loss also deprives the pilot of the gyro compass - a morestable
heading indicator compared to the dancing and spinning of a
whiskey compass card.
While instrument pilots train for such losses, of course, practicing
" partial panel " flying is necessary to survive the failure. When a
failed suction pump deprives the attitude indicator of power to keep
the gyro spinning - just when the pilot most needs it - partial panel
flying provides a partial solution.
Partial panel forces pilots to control the aircraft attitude and direction
by extrapolation and interpretation of the remaining gyro(s)
and air-data instruments. Equally unpleasant: The loss of electricalsystem
power. Losing electrical power in IMC means flying without
the turn indicator. In IMC, losing the turn gyro makes more challenging
coordinating and controlling turns.
Many instrument pilots opt for a standby suction system to
provide alternative power to the air-driven gyros. But the issue of
standby air power holds less relevance with each passing year.
The reason? The advent of lower-cost glass instruments attracting
ever more small-aircraft owners looking for alternative instrumentation
- specifically instrumentation not subject to the known
frailties of the dry air suction pump or in any way dependent on
forced air flow.
The trend favors glass systems with no connection to air
pumps and plumbing; pitot and static connections are their only
connection to the air at all. These days, most new aircraft, from
light sport up to business jets, come with a glass cockpit.
For the fleet aged 20 and older, analog still prevails. But options
exist to change that dependence on the analog six-pack.
Nonetheless, converting to digital instruments from analog,
while understandable, doesn't completely eliminate the prospect
of partial-panel flying from an electrical-system failure.
Instead, going all-electrical in the panel simply shifts the airplane
from one with two inherent, redundant failure points to one
with a single point of failure: the electrical system. In a conventional
panel, electrical-system loss costs only the turn gyro among
the flight instruments - though eventually the pilot will also lose
radios and navigation gear and lights and landing gear (in some
cases) and every other electrically powered item in the aircraft.
Fortunately, electrical power is one aircraft system with
multiple paths to gaining that edge, or hedge if you will, over the
prospect of losing all electrical power. Many of those same pilots
who opted for a standby air-power system in their mixed-power
panels also elected to add more depth to the redundancy of their
electrical system by installing some form of an alternative power
source.
Many options exist to enhance electrical-system redundancy
already in the aircraft - and many of the options we portray here
work equally well in all-electric and mixed-power panels.
Any serious instrument pilot would be well-advised to consider
one of these electron-moving options for system redundancy.
With suction and air-driven gyros continuing to exit generalaviation
panels in favor of electrically powered instruments, this
article examines options for alternative electrical-power sources.
Going all-electric means the Federal Aviation Administration
will want redundancy in power and instruments - altitude,
airspeed and attitude, for which the avionics community provides
many options. But secondary, standby or emergency power options
are a must - and an upgrade the pilot will find most useful
in the event of an electrical-system failure.
Many retrofit options exist using approved hardware. At the
same time, a savvy electrical technician can work with an owner
or operator to design a standby or dual-buss system.
While this may require a Form 337 approval from a local
Flight Standards District Office, the resulting system should
exactly match the aircraft's needs and use.
There may never come a day when the pilot faces such an
in-flight crisis, but they do happen. And when one happens to the
pilot flying behind an all-glass-cockpit panel, the cost and hassle
of creating a standby power source will be considered a welcome
investment by everyone touched by the crisis. q
avionics news * december 2021 45
Avionics News December 2021

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