Avionics News January 2016 - 19
FOR THE PART 23
PART 27 ROTARYWING SEGMENTS, L-3
STANDBY UNIT YET,
More complex, higher-performance aircraft might sport
standby systems for electricity and air, and a relatively small
percentage might sport electrically driven alternatives to the AI
From the start, the realization existed that preparing for an
instrument failure of a primary flight display entailed continued
practice with old analog instruments. Operators generally kept
the AI to preserve redundancy as avionics and instrument companies developed sundry alternatives to a small AI, a small ASI,
and a small altimeter.
Meanwhile, the mantra of staying competent at flying the
partial panel instrument dominated redundancy thinking. The
practiced pilot, in theory and training, could discern what the
missing instrument(s) would show from interpreting one or two
of the three gyro instruments and the air-data dials' behavior.
Thanks to the power split between air and electricity, the odds
of losing more than two were considered small. Thousands of
airplanes still fly with this panel architecture.
But few new airframes enter the fleet of GA aircraft sporting
spinning-mass gyros for the three flight instruments.
Digital screens dominate today, with the primary flight display combining the full six-pack complement into one display.
The AI presentation dominates; the turn-indicator is there,
along with the slip/skid ball, albeit redesigned; ditto for the
DG, with variations in how the compass rose presents: the topdown 360-degree dial, as before, an arc, or a three-dimensional
360-degree graphic presented from a side-on perspective.
The air data, in place of the three different dials, generally
present as vertical scales: airspeed indicator on the left side;
altitude on the opposite side of the AI; and flanking the altimeter
on the right, a tape for the vertical-speed indicator.
Going back to trying to fly by interpreting three small analog
instruments and a whiskey compass doesn't mesh as well as
flying a standby instrument identical to the PFD.
Electronic standby packs like those from Aspen, Honeywell
and L-3 take the evolution of redundancy up a notch or two to
give its newest electronic standby instrument a higher degree of
The most-attractive are basically like adding a stand-alone
PFD for little more than the cost of an old analog, spinningmass electric attitude indicator for an attitude-driven autopilot.
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