Avionics News July 2020 - 56

RESOLVING SOME
CONFUSION ON
GYROSCOPIC
INSTRUMENTS
While much of 14 CFR ยง
91.205 is straightforward and
self-explanatory, there are few
areas that leave some room for
confusion and/or interpretation.
One persistent concern relates to
confusion over the requirement for
"gyroscopic instruments."
Until the advent of digital flight
instruments, little confusion
existed about what constituted
a "gyroscopic instrument"
because spinning-mass gyroscopic
instruments were all we had.
The confusion came with
the advent of ring laser gyros,
accelerometer-based instruments
and MEMS - those microelectromechanical system miniature
machines that have both mechanical
and electronic components - used
in today's modern glass-cockpit
components.
With the federal aviation
regulations lacking a definition
of a "gyroscopic instrument," the
Experimental Aircraft Association
- the main organization supporting
EAB builders and pilots - sought
input from the Small Airplane
Directorate in Kansas City, Missouri.
The Directorate's response:
Any instrument performing the
function of the required gyroscopic
instrument which presents its
information in the same manner as
the gyroscopic instrument meets
the requirement in 91.205 - and
that's regardless of the means used
to generate the information and
display, mechanical or electronic.
What counts is the function and
presentation. q

56

avionics news

*

july

2020

EQUIPPING EXPERIMENTAL,
AMATEUR-BUILT AIRCRAFT FOR IFR
Continued from page 55

Navigation Satellite System Equipment," AC 20-138A.
The purpose of this AC is stated as providing "guidance material for the
airworthiness approval of Global Navigation Satellite System equipment.
Now we all should know that advisory circulars are not regulatory
documents. The FAA issues ACs to provide guidance, sometimes
specifically to provide a method of compliance with the federal aviation
regulations.
But following the guidance of an AC is optional. Should the builder
elect to take a different path and follow an alternate method, the FAA
must anoint that alternate method as an acceptable means of complying
with the requirements of the federal aviation regulations (Title 14 of the
Code of Federal Regulations, 14 CFR).
The guidance contained in AC 20-138A is based on FAA regulations
contained in Parts 21, 23, 25, 27, 29, 43, 91, 121, and 135. Of these
regulations, only Part 91 applies to homebuilt aircraft.
And regardless of the approval used, AC 20-138A provides valuable
information for the builder since it contains accuracy and testing criteria
that can be used to verify that the installation meets the performance
requirements acceptable to the FAA.
As with transponders and other equipment discussed previously, GPS
equipment must meet the performance requirements of the applicable TSO
(in this case, C129), but there is no specific requirement for the equipment
to be built under a TSO authorization.
However, if the equipment is not built under a TSO authorization, it
is up to the owner/operator to verify and document that the equipment
performs within the required specifications. It is also the owner or
operator's responsibility to document the necessary flight-test data
showing that the installation performs within the required accuracy
parameters.
The bottom line
All of this should be enough to resolve the TSO or no TSO debate for
EAB and factory-built airplanes undergoing an avionics upgrade. None
of the equipment installed in a homebuilt aircraft is required to be built
under a TSO authorization. Additionally, meeting the TSO performance
standards allows non-TSO'd avionics to be used in factory-built aircraft
with standard type certificates.
But, again, it's the builder's responsibility to document and confirm the
equipment meets the performance standard of the applicable TSO - and
avionics makers are the source to turn to for that technical documentation.
Admittedly, installing TSO'd equipment has the advantage of providing
that performance-compliance documentation. And FAA guidance aimed
toward type certificated aircraft can be used by the homebuilder when
installing equipment in an EAB - even though many of the regulations
referenced in the FAA guidance do not directly apply to EAB aircraft. q



Avionics News July 2020

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