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katrin_e

Nosedive tendency with AP engaged

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Hey, 

So I and some people I talked with experience very similar problem with DC-8 autopilot. Basically, when we engage heading select and vertical speed and the plane initiates the turn during initial climb, AP seems to be unable to hold selected VS properly and forces the plane into quite a nose dive. 

I could accept realistic oscillation between -300/+300fpm but going from 2200 fpm climb into 3000 fpm descend when the AP is turning to the selected heading is just crazy. The issue occurs across all phases of flight, just not with 100% repro rate. 

Bascially:

1. Take off.

2. Engage VS to achieve ~2000-2500 fpm climb.

3. select random heading to turn the aircraft to.

4. Engage HDG select. 

 

Can't make a video right now because I am not around my PC. Cheers. 

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I haven't seen anything like this in 100s of hours of testing, so it's not easy to know what the problem is.  The only difficulty with the AP during beta testing was a tendency to oscillate more than it should for a time before settling down in altitude hold mode at low speeds, such as during the approach phase, and that was addressed during the beta. I need more information to determine what the issue is. 

 

Do you have XMLTools installed and working?  You can test this by seeing if the inboard engine only thrust reverse works.  Click on either of the 2 or 3 reverser handles.  If XMLTools is working engines 2 and 3 should go into reverse thrust.

 

The stabilizer should be trimmed for hands off flight before engaging the autopilot.  If you trim it according the takeoff stabilizer trim chart you will be close enough during the initial climb out. 

 

Is your speed appropriate for the phase of flight?  Are you setting the speed bugs before takeoff?  V2 plus 10 knots and then V2 plus 25 when you go to 10 degrees flaps, etc.  Ideally you should hand fly at least to 3000 feet above ground and full flap retraction before engaging the AP.  Is the stall warning going off during your turn? 

 

You say it happens in all phases of flight? So if you are in cruise at say Mach 0.8, and initiate a turn with heading select the aircraft might start descending at a high rate?  Do you have any hardware controllers mapped to autopilot or VS functions?  Are you trying to use keyboard or hardware controller commands to operate the AP?

 

If the autopilot is working correctly, when the AP is off, the VS wheel will rotate to reflect your vertical speed. The pitch knob will always be in V/S mode unless you put it into pitch damping, in which case the AP will not control pitch at all. With the knob in the VERT speed detent, when the AP is turned on using the AP switch (OFF/YAW DAMPER/AUTO PILOT) the AP VS speed will be set and locked in to whatever the aircraft V/S speed is when the AP is engaged. Then you can fine tune your V/S using the V/S wheel.  Does it appear to be working like this generally? 

 

 

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I wasn't precise enough, so excuse me - actually the issue occurs more than just with heading select - it just that it was the easiest way to reproduce the issue. 

Even if the AP follows the AUX nav source like CIVA, I can still experience a sudden attitude change and see my airplane dive towards the ground. 

 

The speeds are well within their normal range for all phases of flight, the plane is reasonably trimmed before engaging the AP. I use mouse to manipulate AP controls and didn't use any sorts of custom or keyboard binds. Everything else seems to be working quite fine, so probably not an issue with XMLTools. 

 

I will be able to provide more information when I am back home. 

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The VS and altitude hold functions make use of the default autopilot (with some extra code to help it for altitude hold) so it's not obvious how this could be DC8 specific.  The next time it happens or you are able to induce it, check the following:

 

The autopilot uses trim for VS, and you can see what the autopilot is doing by looking at the trim indicator on the pedestal.  Is it going all the way forward when this happens? 

 

When the nose down behaviour starts, what is the VS wheel doing if anything?. Staying where it was in spite of what the aircraft is doing? Or is it moving to reflect the aircraft's descent so the white stripes are rotating out view? 

 

Finally, are you using the keyboard for some other app or addon around the time the nose down behaviour begins?  Some kind of unintended command would still seem to the most likely cause, although I am keeping an open mind about all possible causes.  Are you using any addon that commands the AP? 

 

 

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I had a similar experience once. I think it was weather related, there was some turbulence at cruise altitude. In calm weather, the AP works flawlessly.

 

Peter

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I haven't noticed any problems to that extent so far.

 

The one thing I did notice, is that if you engage LOC/GS mode on the AP, and your aircraft is not quite configured properly (i.e. you are a little fast, or a little too much descent, for example), then the AP may engage, but the aircraft may oscillate in the pitch axis.  This is user error on my part I think -- because when I have an aircraft properly set up for approach, and THEN enter LOC/GS mode, it never happens.  Only when I try to "force" the AP to calm an incorrect config, do I see this behavior.   

 

This could be related to what the OP is seeing -- that they are entering AP too early (I always hand fly at least until final flaps up, and usually later than that). Their aircraft could be in a 15-deg nose up (like you would get after takeoff), or, the aircraft is climbing at a very high rate (3500+ fpm), and the AP can't deal with it well.

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21 hours ago, Mathijs Kok said:

What weather engine was used?

Active Sky for P3D v4.

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I have seen Active Sky cause some windshear, check if you have the nosedown effect when you change altitude or when the weather is refreshed. 

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