Why should I lose VNAV?

[Guest cliffie1931]

When on a long flight in the 747-400 I sometimes leave the computer and return some while later.Sometimes VNAV is no longer operating and the plane has descended way below cruise altitude.What could be the reasons for this happening?Cliff

[Guest D17S]

Was your autothrottle engaged? When you came back and the airplane had 'moved', what was in the PFD's far left (AT) and far right (pitch) FMAs? That'll tell the story.Just to speculate, if the AT disengaged, VNAV PATH will try to maintain altitude with pitch. If thrust is too low, the airplane will start to slow down. VNAV PATH will compensate for this lower airspeed by rocking the nose back to maintain altitude. The airplane will slow even more quickly now. Eventually, the speed will get to V-stall. At this point the AFS will pull VNAV PATH off the mound and bring the relief pitcher FLCH. This mode will forget about trying maintain altitude with pitch (for the moment) and target airspeed with pitch. It will recognize the airplane is too slow and pitch the noise down to regain airspeed. A descent will occur. Once it gets some airspeed back, it will try to use that airspeed to get back to the target altitude with a climb. But thrust is still too low and the airplane will slow back down. So FLCH will repeate the slowdown/pitch down manuver to once again regain airspeed. I've never just let it go, but I imagine it would finally settle into an altitude where thrust could maintin both altitude and speed. That might be where you're ending up. Great fun to play with. Especially when you have that reset button handy! Could be, really don't know. Just a thought.

[rsrandazzo 6,353]

Cliff-You may also be getting victimized by the horrendous manner in which MSFS deals with dramatic changes in temperature/pressure/wind.This is covered in great detail in a number of other posts- so i won't rewrite it here- but suffice it to say that if your airplane flew through an over-dramatic (read: completely bugus and unrealistic) change in atmospherics- it may have found itself unable to maintain the previously selected altitude, and rather than stall- it sank.... and was unable to return to the altitude you selected because the new temperature difference...Just another thought....(This is pretty rare tho.... I've been hauling this thing all over the planet looking for flaws lately- and haven't run into this problem except VERY VERY rarely... depends on your weather methods/importing, however.... some are just better than others.)

[Guest cliffie1931]

Thanks for your interesting and informative reply Sam. And my thanks for your input too, Robert. However in my problem flights there were no dramatic weather changes of which I was aware.I'll try a long flight, remaining at the computer, and use your information to try and pinpoint the cause.One question though.....................what do you mean by "the reset button"?Cliff

[hvw 381]

Cliff,Are you sing FSUIPC 3.51? I had the same problems that you are reporting when I used that particular version of fsuipc. The problems were solved here with later versions of Pete Dowson's FSUIPC.Hans

[Guest cliffie1931]

Thanks for the tip Sam as I indeed had been operating with Version 3.31 of FSUIPC.I've now upgraded to the latest 3.53 and we'll see if that does the trick.Regards,Cliff

[Guest D17S]

That "reset button" comment was just my sense of humor. We have some real pilots on the board and we really can't ask them to get into a VNAV cruise, then disconnect the AT, pull the power back and see what happens. As much fun as they might have with playing with this maneuver, their pax might not share the sense of adventure. We, on the other hand, are free to play at will . . . and if things get out of hand, that reset function we have as simmers is real handy feature. Try an XB747 test flight. In VNAV PATH cruise, either 1) Disconnect the AT pull the thrust back to idle or 2) Leave the AT engaged and tap the airspeed knob to open the window. Dial down airspeed to the top of the yellow band. Stabilize. Then keep bring it down 5-10 kts at a time. Let the airplane stabilize at each airspeed. Don't mind that stick shaker. Actually, just turn the sound off . . . unless you get sleepy. Then, just sit back and watch the action. The airplane really puts up a good fight. But a nose high, full EICAS thrust limit power, airspeed in the bricks, stick shaker going like mad (that thing would wake the dead!) descent is a certainly a sinking feeling for sure, but wouldn't this also be called a stall?