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Guest Harkonnen

Problem with long flights, my 747 slows down then stops

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Guest Harkonnen

HiI just bought this nice plane, but I've got a problem with it : when I do short flights (like Marseille-Lille in France), all is good, but with long flights, such as LA-New York or Beijing-Hong Kong (FL350, M0.86), about in the middle of the flight, the 747 slows down although my FMC is set to M0.86M0.85...0.80...0.75...etc... until it stops ! I've got enough fuel, so I don't understand what happens... Could you help me ?Thanks in advance

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Check your weather simulation. My first thought in response to your description was due to conditions I've observed on occasion, the TAT is +15C instead of what you expect at flight levels and the air model is reacting to the unrealistic condition.


Dan Downs KCRP

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Guest Harkonnen

>Check your weather simulation. My first thought in response>to your description was due to conditions I've observed on>occasion, the TAT is +15C instead of what you expect at flight>levels and the air model is reacting to the unrealistic>condition.I have no weather simulation, I only use FS 2004 integrated weather (realistic weather, updated every 15 mn)

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Guest Harkonnen

>Stops? Your speed is ZERO??no, but it slows very down and I have to set flaps to 25... but engines don't stop although. I don't understand...

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You weather simulation is FS2004, okay.. now check the conditions during your flight that is having problems. Is TAT appropriate for your flight level (e.g., at FL300 TAT should be in the minus territory maybe -12 or -20 or whatever)... I've seen the same symptoms that you describe when the TAT was +15 and for unknown reasons FS9 was failing to provide a realistic scenario. I think it had to do with lack of reporting stations but I never nailed it down.What is the TAT and winds when your problem happens?


Dan Downs KCRP

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Guest Harkonnen

My TAT is -7 @ FL330

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>My TAT is -7 @ FL330That TAT is a bit high for FL330. The standard ISA OAT for FL330 is -50C. Taking into account a ram rise of 33C for that Mach number, the TAT under ISA conditions should be -17C. This is indicating that you are flying in air that is +10C above ISA conditions. That being said, it shouldn't affect your speed that drastically.Did you by chance hit the spoiler key? Are the engines developing max CRZ power when this happens?Cheers,JohnBoeing 727/737 & Lockheed C-130/L-100 Mechanichttp://www.sstsim.com/images/team/JR.jpg

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>>Stops? Your speed is ZERO??>>no, but it slows very down and I have to set flaps to 25...>but engines don't stop although. I don't understand...>I had the opposite happen to me once when flying from egll to ksfo. I started to overspeed. I think I hit mach .92 before I realized that VNAV had disconnected. I clicked on it, and it re-engaged, and brought the cruise speed back to normal. Haven't seen it happen since.- John

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I have had the exact same problem as Harkonnen. VNAV Commanded speed drops to the very minimum (like mach .300, or something) and then the a/c just chugs along, with the stick shaker screaming, losing altitude slowly, and burning huge quantities of fuel.Does it on every long haul flight, and always while I'm fast asleep. I cannot figure it out because I don't witness it happening.I'm going to try ALT hold and SPD hold and see if it is something to do with VNAV.James

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Guest Miguel_Correa

Hello,I

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It sometimes happens if the winds aloft change suddenly (as fs is wont to do). If you go from a strong head-wind to a strong tail-wind, the airpeed suddenly drops and the aircraft is too high/heavy to properly accelerate, expecially at CRZ power setting. The aircraft flies at an excessively high pitch attitude to maintain altitude and is therefore very draggy. You can usually salvage the flight by changing the thrust setting (using the fmc) to CLB or CON to enable the a/c to get back to proper cruise mach.regards,Jeff


Jeff Hunter
 

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I can certainly believe that rapidly changing winds would do it, but I use ActiveSky with wind smoothing enabled, and my wind shift are VERY slow and controlled.So, I think it might be something else.James

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Guest Miguel_Correa

I also think it has to be something else. I use ActiveSkyAny other idea ? :-zhelp Cheers

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