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jspringe

747 Can not maintain speed at cruise altitude

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I have been having a lot of trouble with the 747 lately (since applying the service update). I can not maintain the requested airspeed while flying level at cruise altitude. For example, I am currently flying at FL380. FMC indicates that Max Altitude is FL393. But, I started loosing airspeed and now am stuck just above stall speed.This has happened several flights in a row over the past couple of weeks.I used the 744 Load Manager (2/3 full) just prior to the flight and I used the PMDG Fuel menu to load the fuel (190,000lbs).What am I doing wrong?Thanks,Jason

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Hi Jason,Yes, it looks like you're way too slow. I'll try the flight myself and let you know what it does. You may need to do a reinstall. Maybe someone has expirenced this before and could let you know something. I'll let you know how mine performs. I don't know what to put into the FMC since I don't have your route of flight but I can put in the weight and fuel that you indicated. By the way, are you using real weather or just weather themes? Ken.

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The problem is not route dependent (I have been experiencing problems on several different routes).I was able to recover from this one by changing my Thrust LIM to "CON" and applying full throttle for several minutes.BTW, I'm using real weather.What is a good CRZ CG %?Thanks,Jason

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HiI am hardly an expert on real world 747 characteristics. But I find with the pmdg 747, it is better to climb to the optimal (OPT) altitude given by the fmc first, and as that increases follow accordingly until you reach you desired cruse altitude. If I try to go straight to the max altitude I almost always get problems like stalling. Depending on my weight and winds.Neil

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Hi Jason,Yes, you're right. It's not route dependent. I did this flight from KSFO to KDFW. I starte my flight with the airplane already on the runway at San Fransisco on runway 10R. I loaded it with 190,000 lbs of fuel and my ZFW was 516,700 lbs. Of course, I selected 2/3 full in the load manager. When I loaded the FMC I had to enter the ZFW. I took off and climb to 38,000 feet without the airplane slowing down to 212 knots. My indicated airspeed stayed at around 274 knots/Mach 0.854. Did you climb in VNAV or did you climb in another mode? The reason I ask is becaue if you climb in any other mode, you must remember to adjust your airspeed to the encono speed, after passing 10,000 feet, which in my case was around 329 knots. But I climbed in VNAV. If you wait until you get at a high altitude to increase your speed, you will not be able to accelerate up to that speed, because the air is too thin for the engines to produce enough thrust to get you back to that speed. If you are climbing in VNAV mode, I would not know what's causing your problem. Ken.

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Hey Ken,Well I used vnav for climb above about FL240. I was at FL380 and at the specified .854 mach for quite a while before the problem started. Once it happens (slow down) the only way to recover is to change to CON and full power. It may be gusts or crossing ActiveSky zones.Jason

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>Hey Ken,>>Well I used vnav for climb above about FL240. I was at FL380>and at the specified .854 mach for quite a while before the>problem started. Once it happens (slow down) the only way to>recover is to change to CON and full power. It may be gusts>or crossing ActiveSky zones.>>JasonHi Jason,Yes, it could have been. So, you must have been climbing in V/S or FL/CHG from the time you switched on the autopilot up to FL240. What was your airspeed when you crossed FL240? I did thought the problem started on the way up to FL380, in other words, the airplane got slower and slower on the way up to FL380. You said your speed was mach 0.854 for some time. When the problem started, was the air getting turbulent at the time? John said in the Virgin Video that if the air is turbulent or there is rain, the pilot will go to mach speed of 0.85 and select CON. Ken.

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I've had issues with this but it's always been OAT related. It looks like the temp here is -43, is that within spec for that height? If memory serves, -59 is standard temp, so this is 16 above standard. Shouldn't matter that much, should it?

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>John said in the Virgin Video that if the air is>turbulent or there is rain, the pilot will go to mach speed of>0.85 and select CON. >>Ken. >That could just be it then. There does seem to be some light turbulence when these happen (where the auto-throttles speed up & slow down a bit and the trend indicators bounce up & down a bit). Then the trend is just down and the auto-throttles are not able to recover airspeed.Jason

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>I've had issues with this but it's always been OAT related. >It looks like the temp here is -43, is that within spec for>that height? If memory serves, -59 is standard temp, so this>is 16 above standard. Shouldn't matter that much, should it?>Hi PeterThe standard temperature at 38,000 feet is -56.48333 degrees C. This is based on standard atmopheric conditions. If you use the real weather in FS2004 as Jason was using, the conditions are not always standard conditions. You probably already know that the standard temperature at sea level is 15 degrees C or 59 degrees F, and standard pressure is 29.92 inches or 1013 hectopascals. But the weather is always changing. During the summer, temperatures are usually above standard, and this also applies at higher altitudes. Ken.

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not exactly sure, but isn't 190k a lot of fuel for that flight?Also, going to FL380 right away is not something you would want to do. step climbing is the way to go. even for semi short flights

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Hey Jason, Looked interesting so I did a couple of test flights. I set my GW about like yours headed on up. I found that at the FMC's "max" altitude, cruise power was not enough to maintain airspeed. I climbed and stabilized in cruise with CON limit selected. Once I had stabilized at the FMC

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Great reply Sam!I agree, I suppose what I was expecting was that the FMC MAX ALT calculation would be based on whatever power mode was currently selected (so if I were in CRZ then the calculated max alt would be lower than if I was in CON).Thanks,Jason

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HelloIve had the same problem, im not sure what it is, certainly something that has changed either Activesky 6 or the patch as far as i can seeTry doing a power on stall, dip the nose, throttles to idle until you start having the airspeed increase, start pulling up and use full power. This fixed it for me, but the 747 can't seem to handle big winds. Ive spoken to a few people, it could be the fact there is reduced airflow over the wing when activesky is altering the weather causing some problems with maintaining cruise altitude and speed. Mine was maintaining level flight at a 7.5 nose pitch up.Hope this help'sBen

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The manual states (and most 744 crew will verify this) that the 744 is NOT an airplane to get on the bad side of the drag curve with. I did it once on my first visual approach (no ILS for guidance) turning base for YSSY 34L. All of a sudden (very quickly) the aircraft was SERIOUSLY nose up and full power provided little or no relief. AP/AT disconnect, full power, gear up, flaps 20, level the wings and she staggered back up to the Missed App alt in a very gradual fashion.The lesson: Just becuase you can GET the airplane into that corner of the envelope doesn't mean it's sensible to do so. Generally you should always look for an initial cruise level that is 2000 feet above optimum, provided that doesn't put you too close to the MAX alt as well. If it does, (it would be rare) then aim for the OPT alt to start with. As the flight progresses you can sneak up bit by bit.

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It would be really interesting to hear about how the model is setup (I'm coming to trust this darn thing). So, hey Vanglis, Is the Vnav calculated MAX altitude supposed to be maintainable with CRZ power. It seems that it won't . . . but should it?If not, what variables can we look for to help us determine what should be happening? It can be like fuel quantity. If "start" - "fuel used" does not = "remaining," it's time to start looking at something. Similarly, if the airplane will not maintain airspeed at MAX (and it should), it's time for someone to go back, look out the window and check to see if a spoiler (or two) isn't floppin' around out there or a canoe (Flap track faring) didn't depart the premises or a sailboat (pylon access panel) . . . . or something else isn't hanging off the airplane (Yes, it certainly happens IRL! I'm afraid to tell you how many times I've walked around an arrival to find some really big pieces missing.)A thrust v altitude number could be a very important piece of information for a pilot to be aware of.There's no thrust/speed disagree light. We just have to know how the airplane should perform.

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IM new here but I have had similar experiences. basically I find to stay near the optimum crz and stay away from the max. also the calculated step climbs are not always maintainable . IM not sure why but weather is a guess. Thanks for the wealth of infoMike

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