high Latitude problem

[ivi96 0]

Hi everyone,

I had some issues with the autopilot during my previous flight, from Glasgow to Seattle with PMDG 747. I was cruising at  FL380 with LNAV,VNAV and CMD set and after 3 hours flying normally the AP went crazy: 

The aircraft started to turn to the left with constant bank angle of 5* ( F/D was inviting me to turn to the right following the FMC) and the altitude was fluctuating between FL380 and FL370, I tried to disconnect the autopilot but It didn't let me take control of the aicraft, so I renstalled PMDG 747 and made a test flight along another route ( Milan LIMC - Male VRMM) and the aircraft didn't have any problem for the whole flight. I tried again Glasgow - Seattle: same issue as before. I noticed It happens always while I'm closed to the 70N parallel. 

Any ideas? I really want to fix it....

[Biggles2010 343]

First I must say I know nothing about 747 operation. However, for something to start investigating, remember that magnetic compasses become increasingly unreliable at higher latitudes.

In older aircraft we used to switch from compass mode to directional gyro mode at high latitudes. There are then procedures needed to correct for gyro drift, because that always occurs.  The fact that your aircraft starts turning unexpectedly at 70 degrees north suggests that it is using the compass as the direction source, and getting high latitude compass errors.

PMDG program their aircraft with great realism, so this behaviour would not be surprising. I know they have very comprehensive manuals, so there may be something in there. Hopefully a 747 expert will be able to give more detailed advice.

Hope this helps.  I suggest deleting your original post. Double posts are not allowed.

Edited April 4, 2020 by Biggles2010

[DAH4062 45]

Have you tried switching the heading mode from MAG to TRUE? It's a flip switch just below the glareshield, above and to the left of the gear lever. Switch it across at about 68N.

 

Don't know if it'll help but might at least narrow down the cause

[ivi96 0]

Yeah I thought for a Second might be something related with direction indication. Thanks for you replies Guys. I’ll try to do  what DAH4062 said 🙂

[Biggles2010 343]

The high latitude effect on a compass system is caused by getting much closer to the north pole, so the magnetic force becomes more and more downwards, with an increasingly weak directional signal. This leads to erratic compass readings. The extreme case is when over the north magnetic pole. Then the effect is vertically downwards with no directional signal. It does not matter if a magnetic compass is set to magnetic or true, the same magnetic forces will affect it and north will always be directly below you. This is why we do not use magnetic compass systems at high latitudes. They give false readings and can cause problems with autopilots.

Now it's possible that setting the 747 mag/true switch to true has some other function and changes the signal source to a different kind of input, possibly from an inertial nav system. I don't know, but I would have expected that to be indicated differently.

 

[ivi96 0]

ok so could be the wrong option for my problem... I just tried a second time, the aircraft was not deviating from its route, but started descend to a lower altitude, despite the autopilot was active. I tried to disconnect it and again, I couldn't control the aircraft..... any other ideas?

[Biggles2010 343]

It suggests it's something other than the compass system. Unfortunately I'm not familiar with any Boeing models, other than what I read on here. There are people on Avsim who know every detail of every Boeing, especially PMDG models. If you can't get answers here, then try again on the P3D forum, which is always active (if you use P3d, otherwise FSX if you use that). Just post under a descriptive topic title referring to the aircraft type, like '747 autopilot problem'. Try to give as much detail as possible. I'm sure someone will have the answer.

Just a thought, from your first post, the problem occurred after 3 hours. Are you getting any Out of Memory indications? It would depend on what version of sim software you are running. If it's P3d 4+ it's not relevant.

Edited April 4, 2020 by Biggles2010

[ivi96 0]

I found out is not about my pmdg! I was flying another aircraft*blackbox A330 and it happened the same thing, the aircraft started to turn to the right this time and I had the same issue with the autopilot. it seems after I cross a certain distance from europe the aicraft wants to turn back and the autopilot gets stuck