HelloFirst, I want to apology for my poor english.I have a very annoying problem with the LNAV and VNAV autopilot in PMDG 737-800 NGX- I set the flight plan, climb detail information etc. to FMC.- Enable the both FD and ARM the autothrottle, LNAV an VNAV before take off- Proceed take off with TO/GA, after a while about few feet above ground, the VNAV engage (button green light on), and later LNAV engage (but button light is off)- I am enable first CMD A autopilot, then LNAV- But first waypoint in SID procedure LNAV autopilot didn`t make it (LNAV disconnect), I am trying enable again LNAV but he ALWAYS disconnect after very short time (2 - 10s), VNAV follow- OK, then i fly manualy to last point of SID, again engage LNAV and VNAV, short time work, but very soon problem is repeate- later before slight turns (10°) between waypoints in cruise flight level LNAV again disconnecting.I try same flight route with same airporports and every time is a same problem (but always little different, in time especially). Once time, the LNAV and VNAV works almost fine, another time is a horrible.I`m study original flight tutorials, view many videos and this is not to be happend (i hope so).Please help, I don`t know, what I`m doing wrong.Thank YouSten.

do the fmc display any message after LNAV/VNAV disconnected. did you see the red warning light blinking after VNAV disconnected. do any other lateral navigation system engage.

Edited February 10, 201214 yr by ctdesing

Sounds like possibly signal spikes from your yoke/stick

So, i think thats problem solvedI shut down every weather addons and reinstal FSUIPC module, but problem is really in joystick. I use Logitech force3d pro with force feedback. I think this f.f. causes undesirable force to plane control , that`s why LNAV and VNAV always turn off. But I disconnect joystick and all works OK,or turn off force feedback function and all works OK:)However, In FSUIPC setting allows joystick calibration etc., maybe this is a better way, but I don`t know how yet.