Avionics News May 2018 - 38

THE TECHNICIAN'S GUIDE TO WEATHER
RADAR TROUBLESHOOTING
Continued from page 37

Know thy system
For the sake of this article, let's assume pilots know
how to use their radar - most don't - and that any
issues they report are technical in nature. While that
eliminates a good percentage of problems, there are
still plenty more for the unwary technician to sort.
Unlike most other avionics, the typical radar is not a
stand-alone piece of equipment, and therefore, needs to be
looked at differently than a GPS or FMS. As an example,
the components that make up an older radar system
consist of a receiver/transmitter, an antenna drive unit, a
parabolic dish or flat plate antenna, and a stand-alone plan
position indicator that contained the radar display, as well
as the range and mode selection controls.
The only major interface with other aircraft systems
was with the pitch and roll attitude gyros. Therefore,
most of the radar troubleshooting was within the radar
system itself. Still, it is a system.
Newer digital weather radar systems crank the
systemality aspect up a few notches by being extremely
integrated with many other aircraft systems.
"As an example, our RDR-4000 weather radar
interfaces with such aircraft systems as the navigational
display, the audio system, the air data inertial reference
system, the flight management system, the enhanced
ground proximity warning system, the radio altimeter,
ACARS and the central maintenance computer," Syms
said. "Therefore, when troubleshooting (the radar),
several other systems must be taken into account
during the fault isolation process."
"The problem could be with any of the components
in the chain or the connectors or wiring that runs
between them," explained Venkata Sishtla, senior
systems engineer for Rockwell Collins. "The technician
needs to look at the system as a whole, and that often
starts with a review of the aircraft's wiring diagram.
I've seen cases where the problem was as simple as the
way the crew is operating other avionics in the system.
Just turning other equipment on or out of order can be
a problem. Because the radar may require input from
other sensors or systems, if you turn it on before the
other systems are operating, the radar can look for an
input that it's not seeing, and that can display as a fault
on the radar. The technician will then troubleshoot a
problem that doesn't exist with the radar. I can't stress
enough the need to follow procedures when powering
up the radar and other connected systems."
38

avionics news

may

2018

The tactical approach
to radar troubleshooting
So you have yourself a "sickly" radar. Where do you
start with your hunt for a cure? Sure, you can use the
information from the pilot, but as we discussed earlier,
you often must take it with a grain of salt.
With that in mind, the first place to start is analyzing
any built-in diagnostics or self-testing capabilities
the system offers. But as helpful as they are, even
these new technology tools aren't immune to
leading a technician down a false rabbit hole. Never
underestimate the value of personal observations.
"My experience has been that you can typically find
a lot of problems with a thorough inspection of the
system's components," Cahoon said. "In their rush
to find a 'big' problem, technicians often forget to
start with the little things. It could be as simple as a
problem with a discrete connector pin coming loose,
or maybe it's a strapping problem. A lot of systems
have strapping for the IRS or AHARS inputs and other
sources. If that strap is broken, you can get intermittent
results. Just pop off the connector and look at the
strapping on the input cables to make sure it's solidly
connected."
Also, don't overlook the condition of the drive gears
and any interconnect cables and connectors, including
the connectors that go through the pressure vessel. This
is a known area for moisture and corrosion build-up;
either of which can cause intermittent problems.
While we're on the subject of "simple" things that
surface as difficult problems, Cahoon also stressed the
need to talk to the aircraft's maintainers to find out if
they've done any recent work on or around the radar
antenna or radome.
"If they take the antenna off and then remount it
on the bulkhead, it must be aligned within a certain
tolerance in the pitch and role plane relative to the
aircraft's nose," he said. "It's usually within a quarterto a half-degree of center. It that's off even slightly,
the radar will give you strange results. You have to
validate the antenna's installation with an inclinometer.
Technicians often overlook that type of a problem."
Another thing to consider is the environment the
radar was operating in when the failure occurred. It's
a good question to ask the pilot, just in case they made
note of it.
"Most of our radars are mounted in a non-heated,
non-pressurized radome," Cahoon said. "And while
we spec those boxes to operate at minus 55 degrees
centigrade, we can still find instances where it gets
much colder than that at altitude, which can lead to

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