Hello Folks,I'm having a problem (both fs9 and fsX versions) in winter flights at cruise altitude (especially over the North Atlantic to Europe) where the MCDU commands a speed (cost index of 80) that causes an overspeed condition. Also, I get a "fuel temp low" warning. I get to a lower alt (lower then optimum) to "warm up" a bit, but this causes higher fuel burn as you would understand. The only way to alleviate the situation of "overspeed" on continuously, I have to set the cost index to 30 - this brings the cruise to around M .80 and turns off the nuisance overspeed warning. I'm using Active Sky and have a registered FSUIPC module. Is there possibly a glitch in the MD-11 flight dynamics here, or must I change something in the weather to (unrealistically) warm up the skies above FL370?? This doesn't ever happen in the PMDG 744 or 737. Any advice appreciated.Regards,Bill Alexander

The fuel temp issue is a real world issue, but the sim's recognition of the cold fuel is not modeled in the 744 or 737 series (which is why you only notice it in the MD-11. The OPT altitude only takes into consideration aircraft weight versus fuel burn. If the OPT altitude results in fuel getting too cold, I must change altitude for warmer temperatures. Just like water, if the fuel becomes too cold it will freeze (the fuel actually turns more gelatinous than anything), which will cut off fuel flow. I don't know about you, but I'd rather have higher fuel burn and running engines than no fuel burn and a glider.The MD-11 actually has a fuel circulation scheme to move the fuel around between the tanks to allow it to warm up in tanks that are warmer than others on average, but this can only have so much of an effect. If it's too cold, it's too cold.The overspeed issue I'm not sure of, but what you are describing I have never seen before. Then again, I've only flown the MD-11 in FSX.

The overspeed condition might be due to MSFS wind shifts. Try it without any weather, then if it goes away get a registered version of FSUIPC and use wind smoothing.

The overspeed condition might be due to MSFS wind shifts. Try it without any weather, then if it goes away get a registered version of FSUIPC and use wind smoothing.

Hello again folks,Thanks for the replies. I understand the cold fuel issue and simply descend to a lower altitude (I wonder if the R/W MD11 has fuel heaters??). I cannot understand the overspeed issue where the MCDU is actually forcing the aircraft to fly (CI of 80 max) into the red zone - above M .824 (sometimes higher). What I believe is happening here is that because the cold air is dense and the MCDU is not recognizing that the speed tape red zone (as low as M .799) is much lower than the speed it's commanding. This is also occuring at high altitudes (FL390 and above), where the "cold fuel" issue is happening simultaneously. I have changed a few habits I got into using the 744/73X where I always obeyed the FMC "Opt Cruise Alt". I now tried the same MD11 flight from CYYZ to EGLL at FL370, rather than obeying the MCDU "OPT Alt" telling me I should be at FL390 (the high speed/cold fuel happens in winter typically at or above latitude 59N using either a NAT or great circle, than later on it says FL420 is optimum as I near Iceland (fuel load here was around 45Klbs, with prediction of arriving EGLL with around 30Klbs (IF I flew at the Optimum computed FL).So, I did 4 things differently which have alleviated this "problem": 1) I disobeyed the calculated optimum cruise and flew no higher than FL370. 2) I left CYYZ with more fuel to burn. 3) I changed Active Sky options to "force constant temperature aloft" (must have a flight plan processed - something I always do). 4) enabled wind smoothing in FSUIPC (pretty sure I said they're fully registered in original post).I'll be happy to duplicate conditions/settings per my original post, plus attach a screen shot of the speed tape, "fuel temp low", and the MCDU's commanded over-speed/Opt Alt using a CI of 80 so you can see more clearly what I tried to describe, if anyone's interested. I should have said that the fuel is only too cold in the horizontal stab...which makes sense after understanding the circulation system Kyle just described (I also gather there are probably no fuel heaters).Regards,Bill Alexander

You might also want to try changing the fuel type to JET B on the FUEL INIT page which lowers the FEEZE TEMP to -50. I fly northern latitudes in winter with JET B and have never had a fuel freeze problem.Here's the description from Aviation Jet Fuel Information Jet B is a distillate covering the naphtha and kerosene fractions. It can be used as an alternative to Jet A-1 but because it is more difficult to handle (higher flammability), there is only significant demand in very cold climates where its better cold weather performance is important. In Canada it is supplied against the Canadian Specification CAN/CGSB 3.23

I also gather there are probably no fuel heaters.

You would be correct - just circulation.As far as the overspeed goes, check your units of weight. It sounds almost like a weight mis-match (the PMDG Options menu shows LBS selected, but you're entering KG weights in the FMS). This could also be pushing you higher in the OPT ALTs, too, because if it is set to LBS and you're entering KGs, the numbers are smaller as KGs than LBS, so it is being tricked into thinking it's lighter than it really is. It's a really random thought, but it might be the problem.

As far as the overspeed actually being commanded by the FMS, I remember something similar happening to me a while back but i cant remember what exactly happened. I do remember that the Autopilot did begin to pull the speed back automatically though (i.e. the speed in the FMA changed from magenta (FMS commanded) to white (other than FMS commanded like human or protection commanded)). It could be a result of the air being so cold that the FMS sees .824 as the ECON speed but because the air is much much denser, the aerodynamic forces are too high at that speed.WARNING: COMPLEX EXPLANATION OF WHAT'S GOING ON IN MY HEAD! I'm not sure about the ECON speeds in jet and/or swept wing aircraft but its probably the point where the total drag is lowest or a compromise with speed; the compromise between speed and drag being controlled by the Cost Index. As the aircraft moves through the air, 2 types of drag are affecting the aircraft. One is parasite drag generated by antennae, gear, bolts, dents, wings, etc. This increases with speed. The other is induced drag, a by-product of the production of lift and the angle of attack. This decreases with speed. The diagram at the bottom of THIS page will help visualize what I'm saying. The total of these two types of drag is total drag. My guess is that CI0 corresponds with the bottom of the total drag curve. As the number increases, you increase speed but at the same time, parasite, and thus total drag increases causing a greater fuel burn. In the case of CI80, its a reasonable compromise between a higher speed and lower drag. Also, if you notice on the performance page there is a second slower speed, about .78 or something. This may be the CI0 speed and thus the bottom of the total drag curve.Now, with respect to speed and air density, the colder the air is, the denser it is. Basically, the colder it is, the closer the molecules are to each other; think of it as air molecules cuddling to conserve heat. Keep in mind that the speed of sound changes with air density and thus is affected by temperature. The denser it is, the less compressibility there is, and the sooner a shockwave will form. On swept wing jet aircraft, certain areas will begin breaking the sound barrier before others. Mmo, or redline is the safest highest speed the aircraft that can fly at WITHOUT having ANY portion of it breaking the sound barrier.This is completely separate from the total drag curve and it is possible that the bottom of the curve corresponds with BOTH Vs (Stall speed in the clean configuration) and Mmo. In fact, if you go high enough in the MD11 (assuming it's possible) you could actually get the bottom of the red line to touch the top of the yellow line. This region is know as coffin corner because any faster, the aircraft will tear itself apart; any slower and it will fall out of the sky and probably tear itself apart on the way down.Now, back to the original topic. CI80 with your current SAT (not TAT as this is taking friction into account) may be a speed that now just happens to fall above Mmo. You also, said that you were cruising at a pretty high altitude. I usually don't get that altitude on most of my flights unless the aircraft is empty or close to it. My last flight I flew today, which just happened to be from JFK to LHR in the MD11 was modestly loaded with a ZFW of 178 metric tonnes (just under 400,000lbs) and 59 tonnes (130,000lbs) of fuel, but I never went higher than FL370. Also, it was cold enough to trigger the fuel circulation but not a low fuel temp warning. Remember, the higher you go, the colder it gets (to a certain point but lets not get even more complicated here) and if it starts getting even colder than standard, you might not be able to reach .80. I remember on one flight if flew in Concorde from HNL to NRT, the SAT actually hit -72C and I couldn't even fly at 2.00 because of the Mmo limit.This extremely complex explanation may explain what's going on.

Great stuff Ryan,Considering all the variables mentioned, this bird is simulated to amazing levels. I tried to reproduce the same situation to no avail, however, I did get close to "coffin range" (very familiar territory here) in trying. I guess the experience I had with the MCDU pushing the speed past the red line was a remote one where conditions were just right.Being fairly new to the MD-11 with only 10 flights under my belt, I have learned an awful lot and still have a way to go before I master her. I get immense satisfaction flying this aircraft as I'm always looking to come as close to real life flight as possible. When I fired up the PMDG 744 for the very first time many years ago, I was hugely impressed (I never could enjoy flying a default jet again), but the MD-11 simulation is absolutely mind boggling. Unlike the 744, I am keeping this one lower and am now inputting Jet B (hard to believe that's also modelled here). As far as weights (in the options menu), I have it set to Lbs (my preference) and the MCDU weights are reflecting same as far as fuel weight is concerned. For ZFW, I will do a double or triple take next flight to make absolutely sure it's correct. I certainly don't want it thinking I'm lighter than I am. Thanks to all for your input here.Bill A.

<br />Hello Folks,<br /><br />I'm having a problem (both fs9 and fsX versions) in winter flights at cruise altitude (especially over the North Atlantic to Europe) where the MCDU commands a speed (cost index of 80) that causes an overspeed condition. Also, I get a "fuel temp low" warning. I get to a lower alt (lower then optimum) to "warm up" a bit, but this causes higher fuel burn as you would understand. The only way to alleviate the situation of "overspeed" on continuously, I have to set the cost index to 30 - this brings the cruise to around M .80 and turns off the nuisance overspeed warning. <br /><br />I'm using Active Sky and have a registered FSUIPC module. Is there possibly a glitch in the MD-11 flight dynamics here, or must I change something in the weather to (unrealistically) warm up the skies above FL370?? This doesn't ever happen in the PMDG 744 or 737. <br /><br />Any advice appreciated.<br /><br />Regards,<br />Bill Alexander<br />

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Hi Bill,I cannot agree with you more regarding the MD11, I cannot get enough of it. I am glad you started this thread, as I too discovered the realisim of freezing fuel whilst flying over Siberia enroute to Japan from EDDF. The number 1 and 3 engine flamed out and had to descend to 10000ft to re light the engines.....and so if I had changed to JET B I would have been fine. I am always learning new things about this plane.Also may I suggest something ? I have been using a fantastic utility called "Topcat" and quite honestly, it configures the MD superbly. Read all about it here http://www.topcatsim.com/I use this on all my planes including the MD11 and 747X and swear by it. Does all the calculations for Weight and balance,fuel and take off and landing. It can be used on a network computer via wide fs or preflight on a FSX machine. It sends the data direct to the MD11 via aircraft cfg or direct to the MD if you have it running.CheersJulian

Perhaps a little late, but I stumbled upon a passage in the manual concerning speed limitations. The maximum speed also depends on your fuel load. The fuel load in your tip tanks, to be more precise. If you have less than 90% of fuel in those tanks your vne decreases significantly making it much more likely to encounter an overspeed condition.