Finally got an ILS Issue!
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I finally encountered (possibly) the "ILS bug" that I've heard many complaints about. I was flying into EGLC, ILS rwy 9 LCY (D108E) NDB transition flying the turn with GPSS on NAV mode, near the end of the turn, I manually switched the CDI source to NAV (VLOC) with the TDS 750 which initially captured the LOC right away. With glide slope alive, I selected APPR mode: this annunciated HDG and APPR/GS. Despite APPR/GS annunciations, the FD/AP refused to capture/track LOC and instead remained on HDG hold despite the CDI being centered on the localizer. Switching back to NAV mode, it would happily capture and track the LOC on NAV mode. Since this was a 5 nmi final, I hand flew the APPR using the HSI with the FD off. I am curious if I mishandled anything with the Century IV in this example. I tested the APPR again once intercepting with HDG mode, and again with the same method I described above, but was not able to reproduce the issue... It seemed to have fixed itself once reloading the flight.
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So you had it with the TDS? I have this problem now after using the KNS 81, but when I tested with the PMS50 it captured fine. I think using the standard Nav radios for VOR-to-VOR navigation, I've been fine.
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Yes, this appears to be the bug I've also reported here. I do not have the TDS and I never encountered it with the PMS (free version) or the GNS 530/430. Based on my personal experience, the bug affects the Black Square KNS 81 RNAV system and the 3rd party KLN90 GPS unit. Now you are also confirming the TDS.
The KLN 90 and the TDS work without these issues elsewhere. The Black Square RNAV obviously can only be tested in a Black Square environment, but the conclusion to me is that this is something Black Square would need to look into. I stopped using the KNS 81 and the KLN 90 as they almost always lead to the same outcome in terms of ILS functionality.
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Since this comes up every once in a while, I will just add that the commonality you're seeing is that all of the avionics in my aircraft, with the exception of the GNS 530/430 use the native autopilot controller for all modes, including approach capture.
While I don't completely disregard your report that you see this only in my aircraft, I spent a day proving to myself that it was present in all the default aircraft in MSFS 2020, at least a year or two ago. I've had occasional trouble getting the AP to capture the glideslope in the past, but I've avoided it by habitually arming approach mode only at glideslope intercept. Using this procedure, I don't think I've ever seen the issue. There is also another limitation inherent to the native autopilot's approach mode that I know for certain is present in all aircraft that use it (again, not including the WT avionics). I forget the details, but I believe the target intercept angle in approach mode is constant, and can cause error accumulation down the glideslope, which a control system with effective angular control would alleviate. I explored this with an airline pilot when I released my TBM, I believe, so the native autopilot does continue to produce surprises ignored by probably most simmers.
In summary, I think this is an issue with the ~30 year old native autopilot logic, and not necessarily my aircraft; however, it's always possible that there is something that I'm not aware of.
