Avionics News April 2016 - 20

ANALOG OR DIGITAL

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legacy aircraft, suffer with a reputation for failing at the worst
time - and from the fragile nature of some designs.
Pressure pumps, an air-power source found on a significant
percentage of aircraft, enjoy a better reputation and track
record - but are still subject to deterioration and failure.
More reliable?
Multiple sources of alternative air power exist.
For either suction or pressure system, a standby source of
air power answers power-redundancy needs.
The suction source can be as simple and dependable as
a suction tap into one of the intake runners on the aircraft
engine, as available under one STC'd system. Coupled with a
low-suction warning light and a cockpit control, this approach
enjoys the benefits of simplicity and reliability. As long as the
engine turns, it will create suction to spin the gyros.
Other alternatives install a pump/motor combination, with
the motor driven by the aircraft electrical system.
Either system, the stand-alone pump or the engine-suction
approach, require some form of warning light and control to
switch on the respective system.
Of course, regardless of which alternative air power they
choose, the standby air must be connected to the aircraft pneumatic plumbing to drive the needed instruments.
Squeezing more juice out of the aircraft
For the electrical components, standby electrical power
provides the answer. The benefits of standby electrical power
extend well beyond the flight instruments to keeping power
for radios, navigators and surveillance transponders.
Most aircraft provide a measure of electrical-system redundancy as standard, with the main-ship battery providing the
first line of defense against a loss of the generating system. But
when that battery drops below a certain level, electrically-powered aircraft components will begin to drop off line. And a failure of the main-power relay can take out the battery completely.
Or a full-blown second alternator is an option for some
aircraft, usually with a second power buss or, at the least, a
standby buss.
Backup electrical options include a second, standby alternator to provide limited, but steady, power; air-driven standby
alternators provide one option independent of engine power.
And various standby battery options also come into play
here, tied into a limited list of equipment or specific to certain
components. For example, some avionics systems can be tied
to an independent standby battery to keep alive those instruments in the event of an electrical-system failure.
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april

2016

Many experimental-aircraft oriented panel packages provide an option for a standby battery to keep them running in
the event the electrical system dies.
The challenge for this approach is deciding which equipment to tie into the standby battery and tying the battery into
the main-power source to keep it topped off.
Simplicity defined: Redundancy for a singlepower system by going all-electric
One approach was starting to gain traction when the glasspanel revolution landed, and it remains an option for any
aircraft owner interested in adding electrical flight instruments
and eschewing totally any air-driven gyro instruments.
Going all-electric in the standard six pack narrows
standby power needs by half and allows an aircraft owner
to eliminate the air-driven instruments from the stack and
eliminate the air pump.
Removing the air pump frees up a drive pad for a standby
alternator - but stops just short of going all-in on an all-glass
upgrade.
It's an option attractive to owners of aircraft old enough to
be facing a full panel upgrade priced higher than the value of
the aircraft - but with options for solid-state alternatives to the
three standard gyros.
According to Replacement of Vacuum Driven Attitude Indicators in 14 CFR, Part 23/CAR 3 Airplanes, and FAA policy
ACE-23-08, finalized Sept. 14, 2015, "There is data showing
that the mean time between failures of traditional vacuumdriven attitude indicators is only a few hundred hours."
A few hundred hours.
That same FAA policy paper also described data showing
that the mean time between failures of modern electronicallydriven attitude indicators may be substantially higher.
First-hand experience convinced one writer we know
who found belief in the frailty of suction pumps with a
failure in IMC.
Only a couple months later, a second failure in IMC, this
time of the aircraft's electrical-generating abilities, similarly convinced that pilot of the value of electrical-system
redundancy.
PS-ACE-23-08: FAA's broadened
options under Part 23.1311
Last September, the FAA published a modified policy PSACE-23-08, titled Replacement of Vacuum Driven Attitude
Indicators in 14 CFR, Part 23/CAR 3 Airplanes, and the new
policy, years in the making, opens up options to go all-electric

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Table of Contents for the Digital Edition of Avionics News April 2016