So I’m not really sure if this is me or the sim. After using Airbuses for long enough I decided to dabble in the PMDG for a bit. Set up a really non detailed flightplan, with a little bit of guessing here and there and went flying(aka I didn’t know the cg so I plugged a random number in so I could get v-speeds and low fuel reserves plugged in). Sometimes I like to do dirty zero prep flights just to make sure I can at least use the airplane on a basic level, and won’t be wasting time later figuring it out with GSX and ATC and trying to do everything by the book. 

Took off fine, turn the autopilot on at 1000ft because I’m lazy. And suddenly the autopilot gets a mind of its own. 

 Lnav and Vnav would light up, like it’s supposed to. VNAV would descend at a pretty hard rate from a pretty solid climb(note: there was no altitude restriction in the FMC and during the descent, the airspeed would be pushing towards overspeed. It also wasn’t temporary, it would do this for a minute or more).

LNAV would go straight into a 360, and would start to do some harsh banking, sometimes even a steep turn, even when a green Lnav was shown on the PFD. I was sort of able to figure that one out, since I had forgot I had not hit a direct and it was on vector, but even after I hit direct to the first waypoint the issue would persist on and off.

In other words, with both LNAV and VNAV on, the airplane would go into a diving spiral. 

I eventually decided to take the autopilot off and hand fly it where I wanted it. Once I got stable I turned it on again, same problem. Paused it for a second to look this problem up. No luck. Unpause the sim, autopilot suddenly works perfectly. It Climbs normally, holds altitude, stays on course, like nothing had happened, at least until Approach.

On the Approach I made a quick change of the route save time, deleted the initial fix for the ILS, still in a bit of a descent, LNAV and Vnav are still working fine.

It flies fine for a minute or two, then boom exact same problem. Aircraft goes into a hard right turn(turning straight off the magenta line even though the waypoint was straight ahead), VNAV starts descending hard, busting straight through the MDA which was set in the fmc. Go overspeed again, and again I turn the autopilot off, and hand fly the rest of the Approach and land it. 

Not sure if I set something up wrong, or if it’s something with the sim or my joystick(I do know my joystick has a slight calibration to the right), or me just not knowing the airplane, any help would be greatly appreciated. 

Edited March 7, 20206 yr by Littleferrhis

15 hours ago, Littleferrhis said:

...Set up a really non detailed flightplan, with a little bit of guessing here and there and went flying(aka I didn’t know the cg so I plugged a random number in so I could get v-speeds and low fuel reserves plugged in). Sometimes I like to do dirty zero prep flights just to make sure I can at least use the airplane on a basic level, and won’t be wasting time later figuring it out with GSX and ATC and trying to do everything by the book...

 Lnav and Vnav would light up, like it’s supposed to. VNAV would descend at a pretty hard rate from a pretty solid climb(note: there was no altitude restriction in the FMC ...

Not sure if I set something up wrong, or if it’s something with the sim or my joystick(I do know my joystick has a slight calibration to the right), or me just not knowing the airplane, any help would be greatly appreciated. 

I clipped most of your post, leaving the parts I'm going to comment on.

Why a non detailed plan? Using a real one is super fast and easy. Use Flightaware's 'Flight Finder' (or similar) to find a real airline flight flying your route, and copy it's flight plan. Better yet, copy the flights information into Simbrief and export the resulting briefing directly into your EFB's library so that you can pull it up on the EFB where you need it. I can go from start to having a ready to fly flight plan and saved briefing in 5 minutes or less. This probably isn't what is causing you issue, but it might be since LNAV and VNAV are trying to fly whatever you put in. Garbage in, garbage out. Here's American Airlines 273 from Miami to New Orleans:

• WINCO2 WINCO CHRRI DOLIE Y280 REDFN Q105 HRV
• Flight Level 340 / Cost Index 80

Next. Guessing on the CG is not necessary and could also be part of the reason the plane nosedives. Use the payload menu in the FMC to set a random payload (there is a 'random' button). Then, when you are in the perf screen, just click the button to the left of the line where you enter the CG. When you click the button, the aircraft's current, actual CG will populate into the scratchpad line at the bottom of the FMC. Click the same button next to CG again, and the correct CG will be entered for you. It will also give you the proper trim setting.

I hate to be that guy, but you really need to read the manual(s) to be able to enjoy the airplane. At least read PMDG-737NGXu-Introduction.pdf and then fly the tutorial flight in PMDG-737NGXu-Tutorial-1.pdf. Then you'll know enough to be dangerous. lol

Edit: There is a joystick calibration routine available under options -> controls. You should run it. It will give you visual verification that it is working correctly.

Edited March 7, 20206 yr by MDFlier

One of the most time-honored principles of computer programming:  garbage in--garbage out. 

You can't just do a left-hand extraction on the input data (flight plan, weights, fuel load) and expect a smooth flight.