Sign in to follow this  
Guest Harkonnen

Problem with long flights, my 747 slows down then stops

Recommended Posts

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

Share this post


Link to post
Share on other sites
Help AVSIM continue to serve you!
Please donate today!

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.

Share this post


Link to post
Share on other sites

>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)

Share this post


Link to post
Share on other sites

>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...

Share this post


Link to post
Share on other sites

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?

Share this post


Link to post
Share on other sites

>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

Share this post


Link to post
Share on other sites

>>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

Share this post


Link to post
Share on other sites

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

Share this post


Link to post
Share on other sites

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

Share this post


Link to post
Share on other sites

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

Share this post


Link to post
Share on other sites

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

Share this post


Link to post
Share on other sites

a. This was discussed at length when the Queen first came out. A search might find it.b. Set engine rating to CON to allow more excess thrust, to better deal with weather transients. PMDG did not agree with me on this at the time, but I feel their analysis wasn't accurate. Flight test has been my expertise before retiring, and still is.c. FSUIPC full version enables proper control of those transients, limit them to 1Kt/sec and 1 degree/sec wind velocity change. Even then you would see pressure changes coming in that drop the aircraft 100 ft in 1 second.Edit: I did not want to get you all into a detailed flight mechanics discussion, and therefore wrote just what to do with little reasoning.

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
Sign in to follow this