Heading bug interferes with AP in NAV mode

[aurel42]

I'm recently experiencing some weirdness regarding the heading bug and the autopilot. I am not sure when it started, might have been when I upgraded from P3D v4.2 to v4.3, or after I removed a Flight1 GTN750 from the aircraft and "downgraded" to flying without GPS (or with the GPSMAP 295).

"Check for updates" tells me that I have the latest version of the C172.

- C172 (with GPSMAP 295 or no GPS)
- NAV1 tuned to a VOR
- I'm handflying along a radial, HDG bug is set to aircraft heading
- I enable the AP and engage NAV mode (and altitude hold)

The AP follows the radial, it displays NAV|ALT. It does not display HDG, it has captured the radial.

- Now I turn the heading bug to the left or right, let's say by 90°.

I expect the AP to keep flying straight and level and following the radial, since it's in NAV mode, no matter what I do to the heading bug.

But the actual result is: The aircraft starts turning towards the heading bug.

That's not working as intended, is it? Am I being stupid?

Anybody else experiencing this? Any idea what could be going on?

Cheers,
Marc

Win 10, latest Nvidia drivers, P3D v4.3, FSUIPC for input configuration, VR using Oculus Rift CV1

[Tomas Linnet]

I believe this is correct behaviour of the auto pilot

[aurel42]

I believe this is correct behaviour of the auto pilot

Huh, it appears you're right.

From the KAP 140 manual:

3. NAV Mode Selector Button -
PRESS. Note NAV ARM annunciated.
NOTE: When NAV is selected, the
autopilot will flash HDG for 5 seconds
to remind the pilot to reset the
HDG bug to the OBS course. Check
the heading displayed on the DG
against the magnetic compass and
reset if necessary.

4. Heading Selector Knob -
ROTATE BUG to agree with OBS
course.

So it was just me being stupid. I still don't understand why the AP would behave that way, but I see that it behaves according to the manual and matches real-world behaviour.

I guess I should take this to the Bendix-King forum.

Thanks, Tomas!

Cheers,
Marc

[Nick - A2A]

I still don't understand why the AP would behave that way, but I see that it behaves according to the manual and matches real-world behaviour.

Esa puts it rather well here.

Oh btw - I mentioned earlier that the AP doesn't know the airplane's heading - so how can heading select work?! The answer is that the offset of the heading bug generates an analog (voltage) signal that is put into autopilot's arse. That is, more the bug is to one way or another, more voltage is generated into that positive or negative way. It doesn't know the heading, only whether the bug is to the left or right, and for smallish offsets, like under 45° or so, how far it is.

Cheers,
Nick

[aurel42]

This explains how the heading mode works. I don't believe it explains why the heading bug affects NAV mode which should just "chase the needle", no matter where the aircraft is pointed (as the detailed post with the broken images explains in extenso).

[Nick - A2A]

This explains how the heading mode works. I don't believe it explains why the heading bug affects NAV mode which should just "chase the needle", no matter where the aircraft is pointed (...)

Well, to elaborate a bit more, the KAP 140 autopilot's NAV mode includes an intercept capability whose exact operation depends on the way the mode is activated. However, to be able to intercept the desired course at, say a 45° degree angle, the autopilot needs to know the aircraft's heading as well as the selected course. With the basic heading indicator installed, we have to tell it this by moving the heading bug as Esa describes.

Thanks,
Nick

[aurel42]

Now it all makes sense to me, tyvm!