Localizer interference
 

Hi All,

I have another issue with Bell 412EP. The localizer deviation bar oscillates about one bar thickness from the center. The situation is worse if ILS approach is made to cross-wind and the autopilot turns the RC to compensate the wind (the RC is approaching the runway nose slightly sideways) .
The VOR/LOC antenna is in the tailboom ("the thick handles beside both sides"). Would this be issue with the antenna as the loc deviation oscilaltion seem to be strong enough that the autopilot starts to make corrections in roll channel causing the whole rotorcraft to swing. The autopilot is SPZ-7600 (3-axis & dual FD) and the navigation radio is Garmin GTN750. Antennas are original from 1996.

BR;
Karote

This is new developed issue or avionics is just recently upgraded with GTN750 navigator?

My experience is that GTN750 like only Garmin brand of autopilots and all other are supported with reduced
performance and protocols.

Recently upgraded. Must say that the localizer interference was not so obvious when test flights started. Now, later on as the R/C has been in operation couple of year, the interference seems to increased. What part of the system would be degraded? Antenna, may be, but I do not believe it, since the reception is good and there are no warning flags. Only that the deviation bar fluctuates.

My guess going on protocol, not on antenna.
Have experience on one now 50 y old C172 avionics upgrade done 2 years ago:
System consist of GTN750+G5+G5+GMU11+GAD29B+Trig31+old AP+and the rest of old steam gauges without vacuum.
For example GTN was not able to communicate Alt on non Gar. dedicated protocol to Trig in way to complete ADS B out.
Position yes, Alt no... :)
Therefore an extra RS232 line going from G5 was necessary, but that limits other functions.
GAD29B interface to non Garmin AP was another trouble, still need to be resolved by replacement of whole AP ;) - paper work is another trouble/major modification
Conclusion from that, yes you can mix Gar with other, but regarding the compatibility level, be ready on some $$$ surprises...

Try checking all the connections and the wires to the antenna in the tail - it could well be corrosion, water ingress or a loose plug.

Does the aircraft have 1 Navigation radio or 2?

If 2, do both of them have the same issue or just 1?

This is on an ILS or LOC approach, (GTN in NAV) not a GPS overlay approach, correct?

If the answers are 2, both and yes, I would go towards the antenna, coax, antenna combiner (if installed, typically at the antennas), antennas splitter (if dual navs).

If the answers are 2, no and yes, or 1, N/A and yes, a bad nav first, then most of the above.