Avionics News September 2015 - 17

main-ship battery - most aircraft's default standby power
source - quickly fades under the loads of VHF radios, GPS
navigators, transponders, autopilots, intercoms and lights.
But today's wonder panel, the digital flight deck, adds
depth to the crisis.
In the typical analog panel, the turn indicator is the only
gyro flight instrument powered by electricity; in a smaller
segment of the fleet, the directional
gyro and/or attitude indicator may
also run off the electrical system
- instead of these gyros spinning
on the power of air flow plumbed
through by a pressure or suction
system.
The loss of communication and
navigation radios isn't alone a
crisis situation, bad as it may seem.
Aircraft engines typically need no
electricity to keep running.
But lose attitude information?
For whatever cause, that's potential
life-and-death when flying inside
the eggshell of IMC.
And all-digital panels - today's
dominating glass panels - depend
completely on electron flow to
maintain air and attitude sensors
and the indicators on which pilots
depend to survive flight where no
visible horizon exists.
Losing electrical power to digital
panels threatens everything - flight
instruments, primary flight displays
and multifunction displays join all
those same systems at risk in analog panels.
Maintaining a backup alternative is critical in any IFR
panel - more so when the panel gyros move from a splitpower approach to any one single source of power. These
days, that's increasingly electricity.
Regardless, standby power is a requirement for both
regulatory and survival reasons.

directional gyro) - with a combination glass system. But
they also retain the AI and, sometimes, the DG, both airdriven, as backup for the glass.
Those pilots retain the depth of redundancy of
their old all-analog panels, even if they still lack any
alternative to the main-ship battery for electrical-system
backup or, for that matter, alternative air source to power
the analog gyros.
Still, lacking an alternative
electrical-power source,
everything electrically powered
remains at risk from a system
failure.
It's for times like the above
IMC-bound pilot's electricalsystem failure that standby
electrical options beyond the
main-ship battery make sense
beyond regulatory requirements.
Many pilots became backup-power
believers after just such a failure
flying behind analog panels.
A few of our acquaintances
carped about their avionics shop
for noting the Federal Aviation
Administration requirements for
alternative attitude indication and
power for all-glass cockpits.
Three of them lost their
memories of those complaints
the first time their standby power
or standby instruments came
into play and helped them safely
conclude a flight; an alternator
failure in IMC made another one a believer; a screen
failure of a PFD convinced the second of the regulatory
value of retaining his analog attitude indicator before
adding a digital, battery-powered standby PFD when the
shop installed his two-screen system.
The third? He took his cues from the experiences of
the other two and installed a pair of complementary
glass instruments capable of backing up the other - each
equipped with its own standby battery, for good measure.
The goal is to avoid flying with a system vulnerable to
a single point of failure taking down the electrical system
and/or the panel components.

Digital-panel
redundancy is only
as good as the power
system driving the
displays and sensors.
And that's where
nothing beats electricalsystem redundancy -
redundancy beyond the
main-ship battery.

STAYING HALF-AND-HALF: PART DIGITAL, PART
ANALOG, SOME ELECTRIC, SOME PNEUMATIC
A common approach for many pilots and aircraft
owners involves making their stacks split-technology
panels by replacing a couple of gyro instruments -
typically the attitude indicator and gyro compass (or

Continued on following page
avionics news

september

2015

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