Is Your ILS Approach Too Perfect?

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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 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.