I recently purchased the Just Flight Duchess for AF2. It is a lovely aircraft. However, I have found some troubling discrepancies the operation of the AP when compared to the real Century IV POH (March 1981 revision). Most of them are to do with which mode lights are illuminated, so shouldn't be hard to fix, hopefully in a future release (hint to any JF representatives...).
1. The ATT light is not always illuminated when the AP is in attitude mode. When the AP is engaged, it starts in attitude mode, but the ATT light is not illuminated. This makes it appear as though the AP is not controlling pitch, even though it is. The correct behaviour would be to illuminate the pitch mode light corresponding to the current pitch mode when the AP is engaged.
2. If you press a pitch mode button when already in that mode (for example, press ALT while already in altitude hold mode) the pitch mode light (e.g. ALT) extinguishes and the aircraft appears to revert to attitude mode, but the ATT light does not illuminate. The correct behaviour is that pressing a vertical mode button when the AP is already in that mode should do nothing. If you want to select a different mode, you should push the button for the desired mode.
3. If the AP is in Heading mode and you press the NAV button to intercept a VOR radial, then if the CDI deflection is more than half scale, the AP remains in heading mode even though the HDG light goes out and the NAV light illuminates. It will only change into NAV mode when the CDI comes within half scale (possibly never if the heading is away from the radial). The correct behaviour is that it should switch to NAV mode immediately and fly an intercept course for the selected radial (POH p. 5). Note that the Century IV does have a mode where it will fly the bug heading until a radial is intercepted, but (a) that mode is initiated but pushing the NAV and HDG buttons simultaneously, not just the NAV button; and (b) both the HDG and NAV buttons are illuminated until the CDI comes within half scale and the AP changes to NAV mode with only the NAV light illuminated (POH p. 6). If the correct behaviour is difficult to emulate, then it would be more acceptable to illuminate both HDG and NAV lights until intercept as described above, i.e. behave as though the HDG and NAV buttons had been pushed simultaneously even though only NAV was pushed. But at present it is confusing as only the NAV light is illuminated even thought the AP is not actually navigating to/from the VOR.
4. When performing an ILS approach, the pitch mode light (ALT or ATT) goes out and the GS light comes on as soon as the glide slope is detected. The correct behaviour is that the GS light should come on when the glideslope is detected, but the current pitch mode light should remain illuminated until the glideslope is intercepted (POH p. 28). The danger here is that you could be in attitude mode, descending, below the glideslope and when you get in glideslope range, the ATT light goes out and GS comes on, making it seem as though your descent is being controlled according to the glideslope when it is not.
5. When no lateral mode is selected, e.g. when AP is turned on, the AP operates in "wings level" mode with no lateral mode light illuminated, which is not documented in the POH. According to the POH, the AP should be in HDG mode when powered up, and should retain the lateral mode when disengaged and re-engaged (p. 5). Although the undocumented "wings level" mode is actually quite useful, in the interests of simulation accuracy it should not be available. In which case, my comments in point 2 would also apply to the lateral mode lights.
6. The mode lights for modes that are not currently selected, are fully turned off. According to the POH they should be dimly illuminated, presumably so one can see the legends in the dark (POH p. 4). This is a minor point.