Autopilot Gone Mad. B747-400x Problems

[Guest lorenz88]

I was doing another test flight with the B747-400X while, in cruise, with left autopilot engaged and LNAV and VNAV activated the aircraft started to roll abruptly until reaching a bank angle of 50

[tcas_climb 45]

Sound like your joystick is spiking, do another flight, if it happens again quickly take a screenshot of the EICAS STATUS page and post it here, btw does the autopilot disconnect?

[downscc 1,327]

The stall warning might be a clue. How are you simulating weather? Rapid variations in airspeed can be fixed with a registered copy of FSUIPC and its weather smoothing abilities.

[Guest lorenz88]

The autopilot doesn't disconnect. I have a registed copy of FSUIPC and I have selected the options "control spike elimination" and "fix control accelerations".If it happens again I will take the screen.Thanks a lotBest RegadsLorenzo ChiovaroThe airspeed was quite stady. It was not caused by the wind. I have the "wind smoothing" option checked in FSUIPC and anyway I think that the stick shaked starded because reaching a bank angle of 40 or 50

[Guest lorenz88]

It happened again. Always with those abrupt turns...now at FL390. This time, anyway, were less violent and had a durention of only a couple of minutes. I have made a screenshot of the EICAS STATUS page. I will post it in this forum just after landing. It seems that both ailerons and elevatos work in the right way...trying to maintain the arcraft on a steady flighpath.Best Regards and many thanks in advanceLorenzo Chiovaro

[Snowfalcon 18]

Could it be an icing problem?Chuck

[Guest lorenz88]

I don't think so because I tried to solve by switching on the engine and the airfoil anti-ice systems and it gave no results. I was completly outside any visible water vapour formations and with a very low external temp. I know that ice can be even formed with very low temp in exceptional way but I don't think this is the case.It must be underlined that even HDG SEL function of the autopilot didn't respond correctly when the aircraft starts to roll not only the LNAV.I also noticed that once I have engaged an autopilot (suppose the Left one) I cannot switch to the other systems (Central or Right) simply by pressing the relative buttons but I have to disengage the Left one and then connect the central or the right. Is this correct?Best RegardsLorenzo Chiovaro

[downscc 1,327]

Screenshots might be helpful.

[shortspecialbus 13]

I once had this problem. It turned out my weather program was having some sort of issue where the OAT would spike up to something obnoxious like 38C at FL370. This increase in pressure would completely kill the engine performance and would lead to those big rolls for whatever reason. Once I made FSUIPC limit pressure changes, everything got better. So, I would investigate that.-stefan