Is Your ILS Approach Too Perfect?

A stuck relay in the audio panel of your 1978-1982 Cessna can lead you to disaster. It's easy to prevent this problem if you know the trick, but few do.


AvionicsAccording to my computer database, Cessna/ARC nav-comms fail 83% more often than theKing KX-155. Think about that next time you’re plowing through a cloud on approach. Sureyou have two radios, a hand-held, and perhaps a Gideon Bible you stole from the hotel. Butthat 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 findwith Cessna/ARC nav-comms. If you have a 1978-1982 Cessna single with the 300A autopilotand 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 eventhough you’re way off-course on a localizer or VOR radial. What’s worse, the nav flag onthe indicator still shows good reception.

An ILS to Nowhere

Picture this: You’re shooting an ILS to an airport situated in hilly terrain. You’veintercepted the localizer and glideslope, and are flying a good approach. Now the failurehappens. You still have a good nav flag, but your CDI needle now remains centeredregardless of how far off the localizer centerline you are! If you’re lucky, you’ll breakout 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 theswitching panel. The purpose of these relays is to provide reverse sensing when the backcourse mode is selected on the 300A autopilot. Unless you select back-course mode, therelays are never energized. The relays switch a very tiny current, about 150 microamperesmaximum, which means that a little corrosion on the contacts can prevent the current fromflowing and the CDI needle from deflecting.

Like many electrical components that Cessna used, these relays aren’t exactly aircraftquality (can you spell "cheap"?) If you’ve ever experienced intermittant VOR/LOCneedle 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 relaysclean 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 300Aautopilot, push in the nav button and select Nav 1. Now push the Back Course buttonrapidly ten times to exercise the relay. Now switch the autopilot to Nav 2 and push theBack Course button rapidly ten more times to exercise the other relay.

Keep in mind that this routine applies only to aircraft with the original factoryCessna 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 metermovements 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 havealerted the FAA about this dangerous problem, but I haven’t seen any significant actiontaken yet.

IFR flying can be safe and fun if the equipment is working properly and the pilot isproficient. Please make sure both conditions are met before you enter the soup.