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Garmin GI 275 interface with Century 21 AP. Coupled approach issues.

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Oct 29, 2019
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Dear Piper People,

I have a 1984 Piper Dakota and I recently installed a GNX 375 transponder/IFR GPS and a GI 275 ADAHRS+AP HSI unit. The GI-275 drives my original Century 21 AP - and in general, it's a great thing! Here's my question and quandary... After initial installation and checkout, it was found that the heading adjustment screw on the front panel of the Century 21 needed to be adjusted so that AP would hold the "bug" heading set on the GI 275, as well as follow the GPS HSI course. It does both now with great precision. The AP is left in the HDG mode to allow the GPSS to work its magic... The issue takes place when I switch the CDI over to VOR/LOC for ILS approaches. The indicator on the GI 275 tracks the ILS LOC very accurately when compared to the Nav 2 LOC receiver and Nav 2 mechanical CDI system. The needle centering also appears to be very accurate when compared to the Garmin Pilot ground track of the approach plate, and also when visually verifying the approach by looking out the windscreen. Here's the problem: When I engage the APR mode on the Century 21 AP to do a LOC-coupled approach, the AP places the aircraft on a course slightly to the right of where it should be - by about 1/2 of a dot, and the approach angle looks "off center" to me visually. This is consistent no matter which side of the LOC I'm on when I intercept the localizer. Note that this mode of coupled ILS approaches worked great - and was "spot on" before I had the new avionics installed. It looks like I should be adjusting the heading adjustment screw on the AP again to correct this, but doing so will cause the bug tracking and GPSS tracking to be off in the opposite way (there's only one heading voltage adjustment potentiometer.) Has anyone else ever seen this? If so, any ideas? There doesn't appear to be an adjustment on the GI 275 to independently trim the GPSS, HSI bug and Composite LOC outputs to the legacy AP.

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