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I've noticed that my Garmin GFC 700 autopilot is 'hunting' on ILS approaches. To be absolutely clear, I mean an approach using a localizer with glideslope. It is rock solid on RNAV, GPS, and other approaches, enroute navigation, etc. I've tried using both navigation radios and it hunts more than I do when I fly manually, regardless of what the winds are. The descent itself is perfect; I'm only talking about lateral navigation here.

What would cause that behavior?

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    $\begingroup$ Talk to your avionics shop? Regardless of the advice you can get here on the internet, its going to have to be fixed by an A&P anyway. $\endgroup$ – Ron Beyer Dec 6 '16 at 3:12
  • $\begingroup$ I've already done that. Curious on the thoughts of the Internets... $\endgroup$ – P Marx Dec 6 '16 at 3:13
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    $\begingroup$ I'm guessing that this is part of a G1000 installation, when running an ILS approach does the needles on the G1000 move around and the autopilot is chasing it? In other words, is the autopilot chasing the needles, or does the autopilot lead the needles? $\endgroup$ – Ron Beyer Dec 6 '16 at 3:30
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    $\begingroup$ Have you flown approaches at more than 1 airport? Despite the 'theory' localizer paths are not perfectly straight due to multipath and some are worse than others. When you say it's tracking well on RNAV and GPS, does that include LPV approaches? LPV mimics the beam width of a LOC so the response should be similar. If that's good, I'd focus on the LOC receiver or the LOC signal itself. $\endgroup$ – Gerry Dec 6 '16 at 13:06
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    $\begingroup$ That should be telling. Since the LOC and LPV 'beam' widths are smaller than enroute/terminal, most autopilots reduce their gain to avoid over-correcting/hunting/chasing. If it's good on the LPV, have the LOC receiver checked out. If the problem does occurs on the LPV, I would suspect a gain issue in the autopilot. $\endgroup$ – Gerry Dec 8 '16 at 13:44

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