Avionics News February 2015 - 20

ROTORCRAFT AVIONICS
Continued from page 19

missions. All patients, pilots and medics on an EMS helicopter, for example, stand to benefit from the depth of today's glass-cockpit options
- particularly technologies specific to their operations.
"Many of these special-use helicopters also require special-mission
avionics," said Wysong, who is a member of the AEA's board of directors. "Sometimes, these radios are not produced specifically for aviation purposes and, therefore, require special evaluations and approvals
for installations as well as consideration to assure that their frequencies
do not interfere with critical in-flight communication and navigation
systems."
What these missions share in common is the helicopter's unique
ability to meet the need. Conversely, earning the airworthiness certificate is a step common to almost all general and all of commercial aviation, with rules specific to the aircraft type.
For example, Part 23 and Part 25 regulations cover light, mostly
general aviation aircraft of the fixed-wing persuasion and large aircraft
predominantly used by commercial airlines - though commercial and
GA operators use some of both.
Airworthiness regulations for helicopters split along similar lines:
Part 27 regulations proscribe certification requirements for normalcategory helicopters, and Part 29 covers transport-category helicopters. The basic instrumentation required under Part 27 echoes that of
elementary fixed-wing panels: airspeed indicator, altimeter and a magnetic direction indicator - typically the venerable whiskey compass.
Part 29 requirements step up the stack with these additional requirements: vertical-speed indicator, nontumbling attitude gyro, turn-rate
gyro, an outside air thermometer and clock. Any that use electricity
must have an independent reserve capable of powering those devices
for a minimum of 30 minutes after loss of normal electrical power.
But at their foundation, the two rotorcraft approval options deal
largely with performance, handling and reliability issues specific to helicopters. Similarly, helicopter avionics face some different standards
in recognition of the different type of machine.
And then there's the equipment itself. Another area of particular
focus is avionics' and instruments' tolerance of - and resistance to
- vibration.
Some kind of shaking
Not all those spinning-rotor-blade machines hum with the
smoothness of silk at all operating loads and airspeeds. Main rotors, even perfectly balanced main rotors, may still produce noticeable vibrations in various flight modes.
Variables include the number of rotor blades - two are often the
worst - and flight mode, with hover often resulting in increased
shaking. Depending on the number of blades in the main rotors,
some ships hover produces thumping comparable to the thump
of tires speeding over symmetrically spaced expansion joints in
roadway pavement.
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Sensitive electronics - particularly some sensors - require extra protection from the shakes. Additionally, everything from the
mounting of the boxes to electronic shielding and antennae placement require considerations specific to both the helicopters and
the model.
For the electronics themselves, the environmental-tolerance
standard is DO-160, the latest incarnation, which sets minimum
standards of tolerance for a range of environmental conditions -
among them standards specific to enduring helicopter-operation
vibrations throughout the flight spectrum.
Hence the "H" designation many avionics makers add to model
names to designate adherence to helicopter-approval standards,
particularly when externally identical to fixed-wing products.
Typically, a company first develops a fixed-wing version of a system - the fleet differences are significant - before or while working on an adaptation to meet the specific rigors of the helicopter
environment in terms of both physical and operational demands.
Getting static and avoid getting all charged up
The standards for resistance to magnetic effects can be complicated by the higher static electricity charge helicopter rotors may
build, while other standards set limits for any impact rotors may
have on magnetically sensitive devices such as flux gates for stabilized gyros and microelectronic mechanical devices that inform
PFDs.
Glass increasingly dominates helicopter stacks - even among
the smallest models with the most-limited space for avionics. Installers at several avionics shops with a heavy helicopter focus
stressed that understanding some issues specific to helicopters is
a must.
Bonding designed to help protect airframe, avionics and occupants in the event of a lightning strike, also assures an easy
path for electrical charges to equalize and eventually discharge.
Bonding complications vary with the airframe materials, the surfaces being connected, varying preparation requirements and the
amount of current the bond must handle.
"It's best to understand what the OEM requires and stick to
those instructions," Robinson said, summing up the perspectives
of several other installers.
What's true for component mounting is even more so for antennae, both the bonding issues inherent to aluminum and various
composites, but in terms of location. Just in terms of locating VHF
comm antennae, attention to the airframe manufacturer's recommendations is critical.
Spinning rotor blades can attenuate signal strength when transmissions have to pass through the blades or associated hardware
and engine enclosures. Often on top ahead of the rotor or on the
belly is recommended, depending on the machine.
"All of these things can have an impact on the performance
of the system if not installed in accordance with the specific airframe," Robinson explained.

Avionics News February 2015

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