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Tom Rogers, Ph.D. |
This article originally appeared in CESSNA PILOTS ASSOCIATION MAGAZINE.
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| About the Author ... |
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Tom Rogers is Avionics Editor for AVweb.
Tom operates
Avionics West, Inc. at Santa
Maria, California, one of the finest radio shops on the West Coast. Tom is an
instrument pilot, an FAA Designated Engineering Representative (DER) for
avionics, and has a Ph.D. in nuclear physics. (We're not sure why he got the
doctorate, but we call him "Dr. Tom," and he seems to like that.)
You can send Tom your avionics questions at
avionics@avweb.com.
Tom's company, Avionics West,
is one of the worlds largest discounters of handheld and
panel-mount avionics for general aviation. They offer all leading brands of
aviation electronics at deep-discount prices. Every item sold by AWI is
covered by a 30-day no-questions-asked return privilege.
Avionics West features handheld GPS receivers from Garmin and Lowrance;
headsets from David Clark,
LightSPEED and Telex; and panel-mount GPS navigators from II Morrow. AWI
stocks these units and generally sells them at prices substantially below the
manufacturers' "Minimum Advertised Price" (MAP). Consequently, AWI is not
permitted to advertise these prices on its web site, but you can obtain them
by sending an email to the AWI auto-responder at
avionics-specials@avweb.com, or
by telephoning AWI at 1-805-928-3601 (M-F, 8-5 Pacific Time). |
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| Ask Tom an Avionics Question |
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According to my computer database, Cessna/ARC nav-comms fail 83% more often than the
King KX-155. Think about that next time you're plowing through a cloud on approach. Sure
you have two radios, a hand-held, and perhaps a Gideon Bible you stole from the hotel. But
that still might not be enough to keep you out of the rocks.
Now that I have your attention, let me tell you about a real nightmare we often find
with Cessna/ARC nav-comms. If you have a 1978-1982 Cessna single with the 300A autopilot
and the original Cessna radio switching panel, you could be in for a very rude awakening.
A common failure mode in this setup results in your CDI needle remaining centered even
though you're way off-course on a localizer or VOR radial. What's worse, the nav flag on
the indicator still shows good reception.
An ILS to Nowhere
Picture this: You're shooting an ILS to an airport situated in hilly terrain. You've
intercepted the localizer and glideslope, and are flying a good approach. Now the failure
happens. You still have a good nav flag, but your CDI needle now remains centered
regardless of how far off the localizer centerline you are! If you're lucky, you'll break
out of the clouds with no airport in sight. If you're not so lucky, well...
Two Cheap Relays
We've found that the cause of this problem is usually a stuck R3 or R4 relay in the
switching panel. The purpose of these relays is to provide reverse sensing when the back
course mode is selected on the 300A autopilot. Unless you select back-course mode, the
relays are never energized. The relays switch a very tiny current, about 150 microamperes
maximum, which means that a little corrosion on the contacts can prevent the current from
flowing and the CDI needle from deflecting.
Like many electrical components that Cessna used, these relays aren't exactly aircraft
quality (can you spell "cheap"?) If you've ever experienced intermittant VOR/LOC
needle movement, by all means replace those relays. It's usually an easy job.
Preventing the Problem
If you haven't encountered this problem yet, here's what I recommend to keep the relays
clean and working. Before every flight, switch both nav radios to any localizer frequency.
(It doesn't matter whether you can actually receive a localizer.) Now turn on the 300A
autopilot, push in the nav button and select Nav 1. Now push the Back Course button
rapidly ten times to exercise the relay. Now switch the autopilot to Nav 2 and push the
Back Course button rapidly ten more times to exercise the other relay.
Keep in mind that this routine applies only to aircraft with the original factory
Cessna switching panel and the 300A autopilot. It cures about 90% of problems with
"stuck needle syndrome." The other 10% is a mxiture of bad VOR/LOC meter
movements or a bad connector under the panel.
If you suspect you have an avionics problem, feel free to call me at Avionics West, telephone (805) 928-3601. I have
alerted the FAA about this dangerous problem, but I haven't seen any significant action
taken yet.
IFR flying can be safe and fun if the equipment is working properly and the pilot is
proficient. Please make sure both conditions are met before you enter the soup.
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