Narrative:

The plane had MEL for localizer inop and approach mode button inop. We flew to ZZZ as normal but communicated we are unable for ILS and needed the RNAV xxl. We set up and briefed the RNAV approach in accordance with [company] SOP. We were given a 110 heading to intercept the xxl final approach course. The first officer selected heading mode and captain activated the approach on the FMS. First officer selected navigation mode so it would capture the final approach course. Ap then went turn further right than the 110 heading to intercept the course. Captain tells first officer to return to heading mode at 110 and wait till we are more aligned on final approach course. First officer complies; but when he rearms navigation we can see that LNAV is boxed and captured. However; the ap/FD remained wings level.captain starts scanning and cross checking as to why we aren't joining the course properly. Then; captain takes controls from first officer via the tcs button to manually turn back on course. As this was happening ATC (ZZZ approach) tells us to turn left 070 to now join the course learned RNAV xxl. Captain returns controls to first officer and tells him to not use ap; just hand fly the approach.the MEL 22-xx was for not using the apr mode button; as a crew wrote up the ap/FD behaved erratically when using it. We used navigation mode navigating off of the GPS only and had no issues till using navigation during approach to landing. I believe there is a deeper problem with this plane's automation system. We landed without incident and wrote up the issue in the mx logbook.ap/FD malfunction. We took manual corrective action when we noticed the discrepancy between what the automation said it was going to do vs. What it did do. I; as captain; should have been quicker correct the automation error.as pilots; I feel we need to be more descriptive in writing up our mx problems. I feel this can help mx fix write ups more effectively; to the deeper root problem as opposed to surface problems.

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Original NASA ASRS Text

Title: Air carrier pilot reported AP/FD malfunction which led to them flying through the localizer and having to fly the rest of the approach by hand.

Narrative: The plane had MEL for localizer inop and approach mode button inop. We flew to ZZZ as normal but communicated we are unable for ILS and needed the RNAV XXL. We set up and briefed the RNAV approach in accordance with [Company] SOP. We were given a 110 heading to intercept the XXL final approach course. The First Officer selected heading mode and Captain activated the approach on the FMS. First Officer selected NAV mode so it would capture the final approach course. AP then went turn further right than the 110 heading to intercept the course. Captain tells First Officer to return to heading mode at 110 and wait till we are more aligned on final approach course. First Officer complies; but when he rearms NAV we can see that LNAV is boxed and captured. However; the AP/FD remained wings level.Captain starts scanning and cross checking as to why we aren't joining the course properly. Then; Captain takes controls from First Officer via the TCS button to manually turn back on course. As this was happening ATC (ZZZ Approach) tells us to turn left 070 to now join the course learned RNAV XXL. Captain returns controls to First Officer and tells him to not use AP; just hand fly the approach.The MEL 22-XX was for not using the APR mode button; as a crew wrote up the AP/FD behaved erratically when using it. We used NAV mode navigating off of the GPS only and had no issues till using NAV during approach to landing. I believe there is a deeper problem with this plane's automation system. We landed without incident and wrote up the issue in the MX logbook.AP/FD malfunction. We took manual corrective action when we noticed the discrepancy between what the automation said it was going to do vs. what it did do. I; as Captain; should have been quicker correct the automation error.As pilots; I feel we need to be more descriptive in writing up our MX problems. I feel this can help MX fix write ups more effectively; to the deeper root problem as opposed to surface problems.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.