What's wrong?

[ideoplastic 86]

Let me start by saying that I am not an expert when it comes to aviation. However I have flown 2000 hours plus in VATSIM usingall kinds of payware aircraft. So let me explain what the problem is and perhaps someone can tell me what I have done wrong or notas the case may be.After loading the fuel and wt with the configurator and programming the FMS I took off from KSFO for a 10 hour + flight to GCTS.I engaged TOGA for take off and at approx 4000 ft engaged LNAV/VNAVand the aircraft climbed to its programmed FL360 and upon reachingCRZ mode was engaged and plane was on SPD ALT VNAV PATH. After approx an hour into the flight I disconnected from VATSIM and went to dosome chores, some 2 hours later upon my return I was surprised tohear the stick shaker and the aircraft struggling at 210KTS.LNAV/VNAV was disengaged and aircraft was several miles from itsprogrammed route. Also the nose was up and I just couldn't engagethe THR or SPD buttons. The only thing I was able to do to recover from this was to take aircraft into a dive to increase its speed and then re-engage the Autopilot, LNAV/VNAV and get the aircraftto climb again. It did all this flawlessly and afterwards trimand everything else was OK for the rest of the flight. This verysame thing has happened to me on various ocassions.Anti-ice was engaged and the weather was being produced by ActiveSky 6. I have read the post concerning throttle management andbelieve I have followed its recommendations accordingly.Would be grateful for any input/advice.

[rsrandazzo 6,353]

Joaquin-A couple of things for you:1) If you climbed to FL360 on a 10+ hour flight- chances are very good that you were overweight for that altitude? There are some conditions in which you could reach FL360- but the rest of the airplane would have to be relatively empty.... Worth thinking about....But i don't think that's what caused your problem:2) Do a search in this forum on WEATHER. You will find some very long post (written by me) describing how rapid changes in atmpospheric conditions caused by MSFS can create havoc for the flight model. Sounds to me like you got bitten by a rapid change in temperature- which your airplane is responding to realistically... Fortunately that rate of change isn't possible in the real world...This is yet another MSFS sim'ism that becomes more pronounced the larger the airplane is...But its a problem with MSFS and imported weather...

[Guest SKireyev]

Captain Randazzo is on target on this one, I believe. The modeling of the aircraft is far more sophisticated than the weather implementation of the simulator. I think you've mentioned ActiveSky? I use it as well, and stand by it as the most authentic weather program available for the sim. Please go under options in AS6, and check that options to force constant pressure and temp at altitude are checked. Those will prevent anomalies of the sort, that will make your plane practically unflyable even at optimal altitude and well within it's flight envelope. Even with those options checked, I can give you a pretty good example of what havoc the weather in the sim can wreak. I was cruising at 36000 on my regular Quantas route from YSSY to EGLL via VHHH. Somewhere around Novokuznetsk, Russia I have encountered a severe cold front, with winds as high as 120 mph at cruise level and multiple thunderstorm cells for as far as eye could see (or sim would draw). Since we have no ability to have an accurately modeled weather radar (courtesy of our friends at Microsoft), I was practically flying blind, not even knowing that the front was stretching for a good 250-300 miles. In any event, the buffeting of the wind and turbulence had cause my speed to fall into the range where shaker was on constantly. The plane was trying to maintain altitude and stuck the nose up at about a 4-5' angle, creating even more drag. I had to disengage the vertical navigation and any altitude controls and maintain level flight just to keep speed from dropping further. The wings were flexing violently and LNAV had to constantly keep banking left and right to keep plane on course. I engaged CON A/T mode, and after 40 minutes or so of flying in that manner I lost nearly 18000 feet of altitude and was constantly within 15 or so knots of stall speed. I was close to St. Petersburg, Russia when I was able to finally re-gain my altitude and speed. There was another front over Northern Poland and Germany, but it was well below me at that point. I really wish I saved the screen capture of the lightning reflecting off of the Quantas livery by a thunderstorm cell. But at that point I was more concerned about keeping her afloat and letting 300+ FSPassenger souls on board live to fly another day (unless the swear off flying forever after that ordeal).Thanks to Capt. Randazzo and PMDG crew for a great model that's an experience in itself. (No one can say that tech support is amiss at PMDG! Just look at the timestamps on the question and the reply! Cap, you have an implanted chip in your skull that tells you when someone is posting with tech issues??? :))Cheers, and enjoy her!Best regards,

[lostmoon 0]

all i know is, when you are flying and you go to the map menu, on FS9, and once you exit out of the map (with or without changes), auto thrust on PMDG disconnects.i have a feeling your autothrust disconected when you disconected from vatsim. maybe?

[ideoplastic 86]

Wow! Well many thanks for the great replies. I have read your postingon weather Mr Randazzo and I think that weather is what causesthe problem, for going from memory when this problem occured I noticed the weather changed suddenly followed by loss of speed,etc. Have also ticked the items Skireyev suggested in AS6.I must say I have noticed this sudden weather changes before butnever imagined it could affect the aircraft systems.Lonemoon, I cannot remember last time I used map mode but I willkeep in mind your comments if I ever do.Will see how things go and will let you know of any developments.

[Guest SKireyev]

Joaquin--Another thing, about what lostmoon was talking about: that doesn't just apply to going to map view. Generally, that happens to me in any situation where I am navigating a menu item for a long time, such as settings, nav log, etc. Also, I do save flights in progress from time to time, and when I resume them, it is guaranteed to happen: the AP usually disconnects, so does the A/T, all screens turn off for a sec then come back on, gear lever might find itself in a different position than what was left before, and engines power down to idle for a split sec. During that time, the plane can veer off course and climb 1000 ft or so with loss of airspeed, but that's it. Then 4-5 seconds later all items are back to norm, and the plane returns to its path. That's why before I save a flight, if I have to leave in the middle, I pause the sim before saving. It does nothing more but giving you time to be ready for the antics. I know, I know we're not supposed to leave the flight 'cause-real-world-pilots-can't-do-that-blah-blah-blah' , just like using time accelleration, etc. But then again, it's their job, they are getting paid for it, and if I have to save the flight from my 'pretend-job' to go to bed so I can be awake in time for my real job, so be it.Cheers,