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guy

What is ACT LIM SPD CRZ ?

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Flying on cruise at 33000ft through a thunderstorm with the 777, I suddenly had an abrupt decrease of the airspeed.It went down to under 180 kts. I switched the autothrottle off and on, but no means to make it increase again.I looked then at the Vnav page of the FMC. It indicated still Econ speed but was entitled "ACT LIM SPD CRZ" .What does that mean?I didn't find any explanation in the documentation.Guy

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What most likely happened actually is due to weather, not PSS related. What happens is that in FS, unlike the real world, wind shifts will have an impact on TAS. This is not realistic, since winds do not alter true airspeed. Winds aloft only alter your GS and will only have an impact on TAS if you encounter windshear which is a sudden and momentary shift in wind velocity or direction.Since you were flying inside a CB (you really whouldn't though!) what most likely happened is that you encountered significant windshear which immediately altered you airspeed. It's kinda like hitting a brick wall. One minute you're cruising at say 485 kt TAS with a slight 20 kt tailwind. Then you enter the CB and inside it there is a very strong 100kt wind opposite to your direciton of flight. When you "step into" that turbulent air pocket with the 100kt headwind, you will have hit a "brick wall" which will obvisouly immediately slow you down and which could even bring you close to a stall. However, thunderstorm clouds are more known for violent vertical currents, and not lateral ones.Why your Vnav profile was set to Active Limit Speed Cruise though I'm not sure though. I'll read the plane's real manual and try to sort it out for yaBest Regards,Victor Limahttp://www.precisionmanuals.com/images/forum/800driver.jpg

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You could avoid this unrealistic phenomenon by limiting atmospheric data change rate via FSUIPC (full version).

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Guy,Saw this a couple of months ago when we were looking at the affect of inadvertant flap lowering. For example, if you are in a normal cruise mode with an IAS below the the flap limiting speed - this occurs at high altitude - and you lower 1 flap, the FMC will reduce to the scheduled IAS for this flap selection and the mode will change to Act Lim Spd Crz. In this case, if you then raise the flap, the FMC reverts to normal crz mode and the aicraft accelerates back to the correct speed.In your case, perhaps the FMC was fooled and "saw" the speed reduction as caused by a flap lowering???John Rooum

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Thanks to all for the information.I know the issues with the FS weather.John,I think the flap thing would be a logical explanation. I will try to lower the flaps on the next flight , just to see. Guy

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John,You were right.At cruise I lowered the flaps to 1, the speed went down to 260 kts and ACT LIM SPD CRZ appeared on the Vnav cruise page.Guy

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