Heading bug vs Course knob

[AKar]

This bit made me think, and I wonder if it's the autopilot's capability in HDG mode once NAV is armed to work out and fly a 45° intercept which is significant. At any rate, in the brief test I did in P3D earlier, if the HDG bug is rotated off-course (say left) by more than 45° once we're tracking the radial in NAV mode, then it seems we'll end up flying a course 45° right of the bug. For instance, if our radial is 90° and we spin the bug to 30° for some inexplicable reason, the resultant course will be 75° and obviously we'll no longer be tracking our radial. However, if the bug is rotated off course by less than 45°, the autopilot will eventually regain the correct course after its turn towards the bug, but it'll no longer be able to re-intercept the radial with any accuracy because it doesn't know exactly what heading to fly anymore.

Yes, the intercept angle was 45°, not 30° which I had in my head from the crosswind spec. Honestly it has been years since I actively outed the A2A Cessnas, I've mostly flown the Pipers. But regardless, the behavior you are describing is precisely what I was after: moving the bug some distance off the nose will likely bring the autopilot into its intercept logic.

In other words, it seems the AP algorithm always needs to know the heading (from the bug) as well as the turn rate and CDI deviation. In fact, I guess it's possible that interacting with the heading bug (even just turning it by 1°) actually triggers a bit of code which starts the 45° intercept routine, rather than the bug simply providing a 'voltage' to the autopilot's brain like the real thing (if I understand it correctly).

This part of the behavior is something I'd intuitively guess wrong, but would need to test (or measure) tu be sure. I would think that heading bug offsets smaller than likely the crosswind limit 30° are simply ignored, as they are redundant to the tracking function.

Well, in this case perhaps it's better-than-life™ then. The fact remains that if you do arm the NAV intercept in HDG mode at a 30° angle to the radial course, the autopilot will just increase the intercept angle to 45° anyway. Does the real unit act differently in this respect?

It shouldn't do that if the other criterion are met - namely, localiser bar alive, and moving and the prevailing conditions not causing the aircraft to over-correct. If you want to fly an intercept from further out, then HDG or ROL mode is the way to do it, switching to NAV/APPR mode only when the criterion are met, should do fine.

The function of the NAV ARM mode is to "lock out" the tracking function until the needle becomes inside the capture criteria. Without it, the autopilot would just turn towards the needle, potentially in endless circles. In heading mode, the NAV ARM does increase (or decrease) the intercept angle to 45° (unless of course the needle was immediately captured), whereas in ROL mode the current heading is approximately retained until intercept.

As in the real world, what is claimed as 45° is better thought of as 30°, as anyone who has ever used a KAP 140 outside of a simulated environment will tell you. I always regard the A2A version as being true-to-life, not just in keeping with the manual.

This figure is precise, as it signifies the angle at which the autopilot flies the intercept. In that sense there is no reason to think it as anything else to be safe or anything. What remains uncertain is what makes the 30° crosswind correction figure significant - if there is anything behind it, sometimes manufacturers just throw figures out of their Stetsons so that they feel comfortable. I do presume that this is the offset from the bug where the signal comes active when under NAV tracking (the interception logic of 45° would need to have some buffer so that it is not exactly on the borderline of having the bug active.

-Esa

[Nick - A2A]

It shouldn't do that if the other criterion are met - namely, localiser bar alive, and moving and the prevailing conditions not causing the aircraft to over-correct. If you want to fly an intercept from further out, then HDG or ROL mode is the way to do it, switching to NAV/APPR mode only when the criterion are met, should do fine.

I will check it out on my own system later, but I don't recall having issues with it increasing the angle of intercept on mine, but then I tend to fly onto the radial or localiser by hand most of the time.

Yes, As Esa mentions above, even with a live CDI (let's say at half scale) a NAV intercept in HDG mode will increase any intercept angle which is less than 45°, to 45°, unless you're already more-or-less on top of the radial/localizer in question. I highlighted this above partly because when I first started to use the A2A KAP 140 simulation, my understanding of the NAV arm in HDG mode was a bit shaky to say the least: I assumed that the "45° intercept" meant it could only intercept from an angle of 45° or less (c.f. the "all angle" intercept from ROL mode). So, I duly set up an intercept onto a live localizer at a 'sensible' angle of something like 35-40° only to mutter an oath along the lines of "what the hell's it doing now?"* when the autopilot turned in (what seemed to be) the wrong direction for a nice smooth intercept from my chosen heading.

*Quite often, a pilot's last words I'm led to understand...

It was also known for doing strange things of its own volition with notable variations in HDG mode, so perhaps A2A are simulating this, too..?!

I understand for users with FPS in the low teens, they do indeed!

Nick