Avionics News October 2015 - 45

position source is compatible with the Mode S transponder,
which involves more than just checking the manufacturer's
specifications. Currently, all ADS-B upgrades must be
performed under a supplemental type certificate. It is
critical for the installer to verify that all requirements of the
STC are fulfilled. This may involve firmware upgrades or
reprogramming the position source as well as assuring the
proper connection of the position source to the transponder
(and, of course, configuring each device properly to
communicate with the other).
Another, rather subtle "gotcha" in Mode S upgrades arises
from the fact that ADS-B offers a different form of traffic
reporting than the old Traffic Information Service. Operators
who have been getting TIS traffic may be disconcerted to find
that their newly upgraded ADS-B systems no longer display
traffic in the same way or on the same display. ADS-B traffic
reports are actually more accurate, but require separate ADS-B
In equipment. The transponder alone will not receive them.
UAT-based solutions may appear simpler, because most
(though not all) UAT devices installed for ADS-B Out have
a built-in GPS position source, and many of the newer UATs
offer a wireless interface enabling both traffic and weather
display on a tablet computer - a particularly nice option in
older piston aircraft that lack a display in the panel (or space
to install one). However, compatibility is still an issue in UATbased systems. The built-in position source will require a GPS
antenna, creating the usual issues about finding an appropriate
location and running the required cable to it from the UAT.

The FAA also requires aircraft using a UAT-based ADS-B Out
device retain a functioning Mode C transponder, which raises
two issues. One is where to install the UAT, since nothing
is being removed from the panel to make room for it. Not
surprisingly, most UATs are designed for remote mounting. The
second is that the FAA requires the same squawk code to be
reported by the UAT and the transponder; otherwise, air traffic
control will see what appear to be two separate aircraft in the
same location. Various vendors have solutions to this, in some
cases requiring a control head that provides a common squawk
code for both devices. In other cases, the UAT attempts to
detect and reproduce the transponder squawk code. Each raises
compatibility issues that need to be addressed.
My comments up to this point mostly assumed ADS-B was
being retrofitted into an aircraft with conventional, modular
avionics. Integrated glass-panel systems, which include built-in
panel displays and transponder functions, raise a whole new
set of issues. In these aircraft, the transponder is covered under
the aircraft type certificate. Unless an ADS-B Out solution
can be added that's physically independent of the integrated
systems (which may be possible, using a UAT with its own
position source), you're looking at a potential change to the
type certificate, with all the complications that entails. The best
move will probably be to contact the aircraft manufacturer to
see if an approved upgrade is available.
As always, performing a complete inventory on the
Continued on following page

Change 7-1 Flight International Dec 2014_Layout 1 8/19/2015 10:45 AM Page 1

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