Avionics News July 2015 - 57

That opens the potential for avionics repair shops to help aircraft owners, both of certificated and experimental aircraft, over
the hump with standby electrical power that fulfills the Federal
Aviation Regulations and assures the pilot of get-down time
should the failure occur in IMC.

an alternative should those electrical replacements fail - but
still no alternative but the main-ship battery should the alternator fail ... or worse ... a failed relay that completely cuts power
from the battery.
For an all-electronic panel, a standby power source is not
only good common sense, it's a regulatory issue if the operator
wants to fly other than VFR.
The solution? One of many options for standby electrical power, some of them suited to a mixed panel, others best
employed when a panel becomes all electric.

Split personality: Three gyros, two power
sources, one big interpretation challenge
For decades, manufacturers followed a split path to their Part
23 solution for standby instruments for IFR flight; typically, air
flow drives the attitude and directional gyros while electricity
spins the turn indicator. The loss of any of the three puts the
The FAA guidance on GA standby power
pilot into a "partial-panel" condition.
As one would suspect this far into the electronic-cockpit
Some owners seek to eliminate their partial-panel concerns
revolution, standards and best practices exist, and that guidance
by installing electrically powered versions of all three gyro
starts with the egg - hatched from the advent of EFIS panels.
instruments.
As they have evolved, electronic
Sure, it rids the airplane of those
displays look nothing like most of the
worrisome air pumps - particularly
round dials they've replaced; they also
FOR A GROWING
the dry-air pumps common to gyroact and perform differently.
PERCENTAGE OF AIRCRAFT
spinning suction systems. Now everyAnd how they should be installed
thing - that is, everything - depends on
and powered falls under the realm of
HOMEBUILDERS, HIGHLY
electrical power.
the FAA's Advisory Circular 23.1311FUNCTIONAL GLASS
Even on the once-standard analog
1C, "Installation of Electronic Display
PANELS ARE NOT ONLY
panel of the past, an electrical-power
in Part 23 Airplanes."
VIABLE, BUT OFTEN
loss cut into the panel's communica
communicaThe FAA offers a good list of the
COMPETITIVE WITH
tions and navigation abilities - while
alternatives available to comply with
FILLING A STACK WITH
putting only a slight kink in attitude
the AC while leaving it to owners to
ANALOG INSTRUMENTS -
and orientation sensing by also taking
decide which option best suits their
ELECTRICAL OR AIR-DRIVEN
out the turn gyro. And to some pilots,
needs.
- AND HANGING AN AIR
the electrical system is only slightly
Savvy pilots quickly recognized
PUMP ON THE ENGINE.
more reliable than the suction system
that splitting their panel between old- though the electronics are typically
and-analog and new-and-digital meant
highly reliable.
retaining some of the old hardware - or
Electrical-power losses stem from several sources: Failure of
updating those same items to run on electricity instead of air.
an alternator drive belt; loss of the alternator itself - or a reguEven with electrically powered attitude and heading gyros
lator; or a failed battery relay.
for standby, the battery remained the only alternative power
When generating power stops, meeting the aircraft's electrisource for the electrically inclined hardware. And that's barely
cal needs falls to the main-ship battery. And even main-ship
acceptable under AC 23.1311-1C - acceptable only if there's a
batteries can fail when most needed.
way to monitor the condition of the ship's single battery.
A common solution to a suction loss employs a backup
And going all digital still raises the bar on the need for alterair-moving source; multiple options exist. One popular suctionnative electrical power. AC 23.1311-1C addresses the needs
system alternative employs an electrically powered air pump;
and solutions.
tapping into an engine's intake manifold for suction is another
In recent years, planemakers equipping their products with
option for standby air power.
all-glass panels moved from electrical analog miniature instruIn a mostly analog airplane, a little bit of redundancy of both
ments to fulfill standby power needs and on digital alternatives
air and electrical power seems prudent. But as aircraft owners
to the dominant trio of 2-inch instruments. Mooney, Cessna,
increasingly opt for digital panels, the two instruments mostCirrus, and Beechcraft and Piper equip their glass-paneled
often replaced have been the ones powered by air.
products with dual alternators and dual batteries. Diamond outThose who have not pursued an all-digital panel often leave
fit the Avidyne- and G1000-fitted aircraft with standby batteries
themselves with the suction pump and those two air-driven
Continued on following page
gyros as standby for an electrical power loss. Then they have
AVIONICS NEWS

JULY

2015

Table of Contents for the Digital Edition of Avionics News July 2015