Glideslope Redux
It all started with the upgrade of my Garmin GNS430 to the new, WAAS-capable 430W. I flew a bunch of GPS approaches probing the new WAAS capabilities and was very satisfied. I never thought to check the ILS or glideslope functions; that’s pretty plain vanilla stuff, right? Not necessarily…
Slow to Notice, Suspect a Problem
Flying in Southern California it is not uncommon to get “last minute” vectors right outside the approach gate, and under those circumstances you wouldn’t expect to get a glideslope needle until right outside the approach gate - you are just getting established on the localizer, after all. So it took me a long time to notice that my glideslope needle was acting differently than it had, or than it should. A few times I noticed the glideslope needle either arriving late, or being twitchy once it had. This was new behavior with the 430W. This should have been, but still wasn’t, enough to get my attention.
Not until months later on a long cross country trip when I got a long straight-in on an ILS did I confirm definitively that my GS was arriving really late - in some cases inside the Final Approach Fix, necessitating a Localizer Only approach.
Tuning, Testing, Smoking Gun
On my return from that trip I started worked with Zenith Flight Support, the dealer from whom I purchased the 430W, to resolve the issue. We tried a number of things, tuning the glideslope receiver, sending it back to Garmin for factory work, all to no avail. Ultimately, inside the rudder cap Zenith found a coupler connecting the antenna to the cable that runs to the panel that was not the right coupler and was only letting a very small portion of the GS signal [I think just the highest freqs] pass through. It has been there since I have had the plane - over 4 years now. And yet, it wasn’t a “problem” until now.
Picky, Picky, Picky
Apparently the old [standard, original] GNS430 took the weak signal it got, accepted a lower strength signal than the 430W will, and made the most of it. I did have some trouble far out on some approaches but never so bad I thought “hardware problem”. I could always receive a GS needle outside the FAF. The new WAAS-enabled GNS430W is far pickier. If the signal isn’t strong enough this unit retracts the needle to indicate “no signal”. When I would get close enough to the transmitter, even my funky coupler setup provided enough signal to the 430W to make it happy. Interestingly the 430W loaner I had for a couple of weeks was able to “accept” the signal much further out. Clearly there is some variance between units.
New Antenna, Coupler Does the Trick
So we replaced the antenna - which was a little corroded - and the new one has the correct coupler built-in. Now everything works great. ;>)