Avionics News May 2018 - 39

problems. On the other side, it can also get too hot,
and that can cause an issue."
"The problem is these types of failures cannot be
duplicated at ambient temperature," Sishtla said. "On
the ground or on the bench, you often have a system
that operates perfectly. The only way to uncover
temperature-related issues is by putting the unit
through a temperature cycle to see if you can duplicate
the problem."
Radome love
While we're up at the pointy end of the airplane,
let's take a minute to discuss the role the radome
plays in the radar's performance, or lack thereof.
While radomes are normally passive parts of the
radar system, damage, erosion and water ingression
can make them active participants in many recurring
problems.
But what can lead a good radome to behave badly?
Typically, it starts with water getting trapped between
the composite layers of the radome itself. If you get
enough moisture trapped in there, it will show up as a
spot on the display.
All that trapped moisture also will lead to a freezethaw cycle that will not only cause considerable
damage to the radome's structure, but the ice trapped
at 25,000 feet will cause problems for the radar.
Like the rest of the radar system, it's relatively easy
to spot potential moisture problems in a radome. Begin
with a good, thorough examination of the outer skin
and paint. Nicks, scratches, dings or any damage can
let moisture in.
And remember, a good-looking radome doesn't
mean it's free of faults. There are plenty of instances
where an improperly repaired or incorrectly painted
radome is at fault with recurring radar problems.
According to Syms, another radome problem that
leads to misdiagnosis by technicians is when the
lightning diverter strips in the unit's structure become
disconnected from their points. This poor electrical
connection can lead to spoking on the display.
Safety first
While technicians are faced with several options
when troubleshooting radar issues, there's one area
where you don't want to stray from the proverbial path:
safety. Radars can be dangerous beasts to deal with.
The best place to start any troubleshooting project
is by rereading FAA Advisory Circular AC 20-68B,
Recommended Radiation Safety Precautions for
Ground Operation of Airborne Weather Radar.

Although it was written in the predigital radar age,
it is loaded with valuable safety information about
the dangers associated with microwave radiation. The
AC also includes guidelines on how to calculate a
recommended safe working distance from the antenna
for a particular radar system. It's the same information
found in the radar's manuals, but with the age of some
systems, it's likely those documents are long gone.
If you are working with an older high-power
transmitter system and want to take an even more
conservative route, follow the guidelines for maximum
personal exposure limit radius that you can get from
the IEEE C95.1-1991. These guidelines are the ones
followed by the FCC, OSHA, NIOSH and others.
Always assume the radar is transmitting (even in
the "test" mode), so it should never be pointed at
buildings, vehicles or people.
Radar troubleshooting tips
Here are some tried-and-true radar troubleshooting
tips from the experts at Honeywell and Rockwell
Collins.
* Talk to the pilot(s) to get all the details
pertaining to the radar problem. They may have
overlooked a critical detail when filling out the
squawk form.
* Extract the system's fault memory or BITE
data as soon as possible. Other information can
overwrite this data during subsequent flights.
* Always check the RF power output and
sensitivity of the radar.
* Visually check all the connectors, grounds and
cables for wear, damage or corrosion buildup.
* Do all troubleshooting at normal aircraft power
input levels. Operating the system at other than
normal power can cause problems.
* Check the strapping on the IRS and AHARS/
ADHARS units for damage or corrosion.
* Run the maintenance test though the aircraft's
maintenance computer. On older units, run the
manual test mode while the system is working.
Make sure the aircraft is in a safe location
before operating the radar.
* Review the aircraft wiring and schematics
diagrams for the radar system to familiarize
yourself with all the other systems that integrate
with the radar.
* If the radar has multiple displays, always check
to see what they are showing at the same time.
* Make sure you have all the latest manuals for
the system you are troubleshooting. q
avionics news

may

2018

Table of Contents for the Digital Edition of Avionics News May 2018