Avionics News September 2015 - 18
WIRED FOR STANDBY
Continued from page 17
REDUNDANCY VERSUS REDUNDANCY
One of my examples installed a twin-screen system in his
Part 23 piston single, displacing the attitude indicator, the
directional gyro, the turn coordinator and the vertical speed
indicator. He kept the analog altimeter and airspeed indicator,
even knowing that his IFR airplane met FAA requirements
for redundancy. The PFD and MFD are, for all practical
purposes, identical; how they're wired and the software used
determines which box functions as which.
Both boxes boast integral backup batteries; the wiring
between them makes either box capable of providing all the
essential information of both should one display fail. This
works because both contain the appropriate sensors for their
jobs - attitude reference and air data.
Not so for another pilot, who installed in his experimental
aircraft a popular two-screen EFIS for the experimental
market - again, with one designated a PFD, the other as an
MFD. Failure of a screen still lets the remaining display
function in its reversionary mode and packs the display to
the max.
But other failures can deprive the pilot of important
functionality, such as attitude information.
For example, if the GPS feeding the PFD fails, it causes a
loss of accurate heading indication; loss of the MFD brings
loss of the autopilot control even if the reference information
remains from the attitude sensor.
True redundancy in this case requires an alternative
attitude instrument - or at least a second attitude reference
system that doesn't depend on GPS for part of its reference
abilities.
For many EFIS converts, costs are a barrier to going fully
digital for main and standby functions. Two of everything
digital is simply beyond their budget. Alternatives do exist.
ANALOG ALTERNATIVES
Replacing two or four of the primary analog six-pack can
bring the benefits of digital to the cockpit, but how to meet
the redundancy requirements in instrumentation still looms.
It's simple; keep the analog attitude and turn gyros.
Better still, go electric with the AI and drop a standby
alternator onto the pad the air pump once occupied. A new
regulator to work with the standby alternator, some guidance
on load shedding to match the standby alternator's limits,
and you're in business.
But even without the all-electric solution, the single
main-ship battery still suffices for standby power. Installing
18
avionics news
*
september
2015
an alternator controller with a warning-light output can add
comfort to the cockpit by ensuring the loss of output gets
noticed - quickly.
DIGITAL ALTERNATIVES
A pairing of electronic instruments capable of fully
filling in should one fail gets you the redundancy needed
- even with only the one main-ship battery for standby
power in the event of an alternator failure.
Unable to take this approach? Keep the two analog
gyros from above, and make them all-electric.
If a single display is the limit, redundant sensor
systems for both air and attitude references can help keep
the single screen fully functional and meet regulatory
requirements; but the plane still needs alternative attitude
and air-data references in the event that one screen fails
- a rarity, sure, but not unheard.
A single-screen upgrade can bring many advantages
to the pilot - among them synthetic vision system
capabilities and the benefits of WAAS-based GPS
approaches guided by the bars overlaid on the PFC.
Several avionics makers offer digital-panel companion
standby instruments, with all the attitude and air-data
sensors built in - and their own standby power source
integrated into the instrument.
The benefits of this approach over keeping backup
analog gyros comes in the similarity of the standby
instrument to the PFD that failed. Same look, same
air-data orientation and, if included, same navigation
input the PFD would normally show. All that packed into
instruments as small as 3.5 inches square - or 3.5 inches
by 7 inches and horizontally oriented.
Best of all, the smallest options fit all the data of six
flight instruments into a space as small as a single gyro
instrument. Experimental-aircraft operators enjoy an
even wider range of choices through suppliers catering to
their desires.
ELECTRICAL ALTERNATIVES: TAKE THE BUS -
IF THEY HAVE THE FARE
Digital-panel redundancy is only as good as the power
system driving the displays and sensors. And that's where
nothing beats electrical-system redundancy - redundancy
beyond the main-ship battery.
Think about it. The FAA requires the battery to last a given
time, depending on the type of operations. Adding to the
electrical load, using the same capacity battery assures that
the time battery power is viable will shrink proportionately
with the increased load, possibly taking that time period
Table of Contents for the Digital Edition of Avionics News September 2015
Avionics News September 2015 - Intro
Avionics News September 2015 - Cover1
Avionics News September 2015 - Cover2
Avionics News September 2015 - 1
Avionics News September 2015 - 2
Avionics News September 2015 - 3
Avionics News September 2015 - 4
Avionics News September 2015 - 5
Avionics News September 2015 - 6
Avionics News September 2015 - 7
Avionics News September 2015 - 8
Avionics News September 2015 - 9
Avionics News September 2015 - 10
Avionics News September 2015 - 11
Avionics News September 2015 - 12
Avionics News September 2015 - 13
Avionics News September 2015 - 14
Avionics News September 2015 - 15
Avionics News September 2015 - 16
Avionics News September 2015 - 17
Avionics News September 2015 - 18
Avionics News September 2015 - 19
Avionics News September 2015 - 20
Avionics News September 2015 - 21
Avionics News September 2015 - 22
Avionics News September 2015 - 23
Avionics News September 2015 - 24
Avionics News September 2015 - 25
Avionics News September 2015 - 26
Avionics News September 2015 - 27
Avionics News September 2015 - 28
Avionics News September 2015 - 29
Avionics News September 2015 - 30
Avionics News September 2015 - 31
Avionics News September 2015 - 32
Avionics News September 2015 - 33
Avionics News September 2015 - 34
Avionics News September 2015 - 35
Avionics News September 2015 - 36
Avionics News September 2015 - 37
Avionics News September 2015 - 38
Avionics News September 2015 - 39
Avionics News September 2015 - 40
Avionics News September 2015 - 41
Avionics News September 2015 - 42
Avionics News September 2015 - 43
Avionics News September 2015 - 44
Avionics News September 2015 - 45
Avionics News September 2015 - 46
Avionics News September 2015 - 47
Avionics News September 2015 - 48
Avionics News September 2015 - 49
Avionics News September 2015 - 50
Avionics News September 2015 - 51
Avionics News September 2015 - 52
Avionics News September 2015 - 53
Avionics News September 2015 - 54
Avionics News September 2015 - 55
Avionics News September 2015 - 56
Avionics News September 2015 - 57
Avionics News September 2015 - 58
Avionics News September 2015 - 59
Avionics News September 2015 - 60
Avionics News September 2015 - 61
Avionics News September 2015 - 62
Avionics News September 2015 - 63
Avionics News September 2015 - 64
Avionics News September 2015 - 65
Avionics News September 2015 - 66
Avionics News September 2015 - 67
Avionics News September 2015 - 68
Avionics News September 2015 - 69
Avionics News September 2015 - 70
Avionics News September 2015 - 71
Avionics News September 2015 - 72
Avionics News September 2015 - 73
Avionics News September 2015 - 74
Avionics News September 2015 - 75
Avionics News September 2015 - 76
Avionics News September 2015 - 77
Avionics News September 2015 - 78
Avionics News September 2015 - 79
Avionics News September 2015 - 80
Avionics News September 2015 - Cover3
Avionics News September 2015 - Cover4
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