Speed loss in cruise cause stall

[Guest Schunken]

Hello!I like the 747 much, but in my 10 flights until now with her I have the follow problem....I be on the optimal cruise level show by the FMC.My gross weight is 711.0 and I was depart EHAM (Amsterdam) for a hop to KEWR (New york).At cruise at 32.000 I encounter bad weahter over GB with turbulences... VNAV and LNAV + Autothrottle (cruise) was active...My engines was run at 101% RPM but I can see I lost speed about 1 knot every 2 seconds in the turbulent area.... winds aloft is off, so it was not a sudden wind change......my speed goes back into the yellow (stick shaker become active) and I start a decent because meanwhile my aoa was at about 12 degree....I was forced to go down until 9000 feet... than I was able to recover, hain speed and meanwhile I be back to my cruise level again...Can turbulences cause this? Or Icing? Is there someting I should switch off via FSupic?Thanks much!Andreas

[Guest D17S]

Once you have established a VNAV PATH/SPD cruise, verify that that the AT (Auto Throttle) has cruise power set at least a tad BELOW the EICAS thrust limit. Zoom in and look carefully. Thrust must NOT be "on the stop."If cruise power is being limited by that EICAS thrust limit, the airplane will experience a painfully slow deceleration. You will hardly be able to see it on the airspeed tape . . . that is until you come back to cockpit after dinner and you are bouncing off the stickshaker. If this continues, it will indicate a weight vs altitude mismatch. As you are describing, use the FMC's recommended "OPT" (not "MAX") altitude. You got this from the Vnav cruise page? Right? ("OPT" is the altitude you want. That "MAX" is only for weather or other emergencies.) At the FMC's "OPT"imum altitude, "CRZ" EICAS thrust limit will be a high enough thrust limit to allow the AT to maintain the FMC's calculated ECON airspeed (no 'speed intervening' now!~). Now double check that the FMC's TO page weight data is all entered correctly. Just to get you going, try setting your EICAS thrust limit at CLB. That will give the AT a bit more elbow room. I see real life crews using CLB as their EICAS thrust limit for cruise phase operation all the time. There's not a thing wrong with this practice . . . that is unless this is the only way you can get your airplane to maintain ECON airspeed at OPT altitude. Check outside. Do all the big pieces appear to be there. Real 747s can dispatch without those big canoes (flap track farings). And I've seen them come in a time (or ten) with some really big pieces missing! (Tail pipes, LE flaps, canoes, to mention a few.) Bet those drivers were seeing something very much like what you are seeing.

[rsrandazzo 6,353]

Andreas-Take a few moments to read through Sam's reply- he has some good information.Then also consider the following: The implementation of MSFS weather can cause some pretty severe changes in the temperature/pressure when flying through areas where there are very few weather reporting stations and/or the weather is exceptionally severe and varies greatly across a short span of your route.If you run into this- it will be evidenced normally by a rapid loss or gain in performance of the airplane. In the real world, temperature and pressure changes like these are essentially impossible- and as such the airplane does not suffer such upsets.If it were theoretically possible to trigger such wild changes- the airplane would likely do exactly what you are seeing- or worse....I've made a number of very detailed posts on this topic- if you think this is what you are up against- try searching for "wind" in this forum and you should find my previous posts....

[Guest D17S]

Very good point. We

[Guest Schunken]

Thanks guys....Yep, I also "blame it on the weatherman" ;)Thank for your worthfull replies...Maybe I make some change via FSupic that makes weather changes more smooth....Cheers!Andreas

[jtsjc1 3]

Andreas, Active Sky will make your FS9 weather experience alot better. The default weather is very unpredictable. AS6 will give you realtime weather without the huge pressure differences. Joe