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MD-11 Overspeed

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I had the same thing happen to me. I was crusing with the FMC ECON speed at .82, then the wind changed a little and I was getting overspeed warnings at .821-.823. This went on for a couple of minutes and then stopped. I found this strange because earlier in the flight I was cruising at .844 and there was no issue.Dave S

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Have experienced this issue twice now. It has happened both times within 100nm of TOD. Cruising at M0.83 below the red tape when it jumps down and puts me in overspeed (and get yelled at by the lady). The FMC says ECON of M0.831, but now the plane has slowed to M0.825 to stay in the clear. (M0.825 ennunciated in magenta as well even though M0.831 should be commanded, guess its still close enough.)No sudden change in wind or temperature. Can't find a good reason for it. What drives the speed limits other than temperature, weight and altitude? It doesn't seem like this should change so rapidly without apparant reason.Eager to hear if anyone has found a cause (or even better a solution!). :( Thanks,Kelly

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Have experienced this issue twice now. It has happened both times within 100nm of TOD. Cruising at M0.83 below the red tape when it jumps down and puts me in overspeed (and get yelled at by the lady). The FMC says ECON of M0.831, but now the plane has slowed to M0.825 to stay in the clear. (M0.825 ennunciated in magenta as well even though M0.831 should be commanded, guess its still close enough.)No sudden change in wind or temperature. Can't find a good reason for it. What drives the speed limits other than temperature, weight and altitude? It doesn't seem like this should change so rapidly without apparant reason.Eager to hear if anyone has found a cause (or even better a solution!). :( Thanks,Kelly
Addon weather programs tend to make the MD11 unstable with unrealistic wind changes. Use the FSX weather and see what happens.

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Addon weather programs tend to make the MD11 unstable with unrealistic wind changes. Use the FSX weather and see what happens.
There were no shifts in the wind at the time of the overspeed, but will try a flight or two with default (FS9) weather anyway to see if the issue is present then. Hope that there isn't a major conflict between AS weather and the MD11. Would hate to give up that realism after using it all this time.To Marcelo & Dave: Either of you using external weather when the overspeed issue occured?Thanks,Kelly

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There were no shifts in the wind at the time of the overspeed, but will try a flight or two with default (FS9) weather anyway to see if the issue is present then. Hope that there isn't a major conflict between AS weather and the MD11. Would hate to give up that realism after using it all this time.To Marcelo & Dave: Either of you using external weather when the overspeed issue occured?Thanks,Kelly
I was using AS6.5 for the whole flight. I can

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When I looked at Marcelo's screenshot I thought "what the heck is he doing to his system?". The overspeed zone is pretty close to his not so exorbitant high cruising speed. Poor guy...That's what I was thinking.Well, yesterday I ran into an overspeed warning as well at Mach 0.819. FS9 together with AS 6.5, MD-11 updated. Flying between Calgary and Winnipeg at FL 370 with a cost index of 80 and a weight of about 180 tons it was pretty cold up there. I had an OAT of -65C and the FMC was suggesting something around Mach 0.829. At this speed there should have been a considerable margin to the overspeed zone.But it wasn't at all. Mach 0.829 was already well within the red area. The system reduced speed to 0.818 to stay within legal bounds. Of course, a wind gust of a mere 1 knot would be enough to trigger the overspeed warning. And sure, you could switch off wind gusts. But that leaves you with Mach 0.818 (in my case) with 469 knots TAS during a CI 80 cruise. Sounds almost like a 767.I think it has nothing to do with the winds or the windshifts.The same happens when you compare climb with descent phase. I climbed with about 345 kts IAS. Overspeed area started around 360 kts, I think. During descent I was happy to have a 290 kts policy descent because the red area started at 320 kts already. At 13000 feet.It would be interesting to see what exactly pushes the red area that low. Is it the temperature? No idea. Winnipeg had bone-chilling -30C yesterday...

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OK, here is an update:Flying from CYWG Winnipeg to EDDP Leipzig, CI 80 again, ZFW of 171.5 tons, 63 tons of fuel on board after a flying time of 25 minutes, FL 330, FMC suggests M0.825. Perfectly in order.TAS is 471 kts, headwind component 40 kts, only slight turbulence. IAS is 301 kts at a temperature of IAT -29C, OAT -58C. The red line is at about 312 IAS, so I have a margin of 11 kts. Pretty comfortable even if windgusts should appear.Problems seem to arise when I get higher because the safety margin shrinks to zero until I'm even in the red area at M0.825.Don't get me wrong, I can live with FL 330. It's just that I want to understand what exactly happens when I increase the flight level.

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The colder than standard air is denser than normal. This would be an interesting calcuation to run, I think MSFS is providing an incorrect Mach number calculation. If it were not compensated for temperature then the indicated Mach would be too low, so the simulation would fly at a higher TAS to achieve the Mach setpoint.To verify this, convert TAT to OAT at M0.823, -33C is equivalent to -62CNow use the TAT to find the Mach number for a TAS of 454 indicated on the ND using the OAT derived above comes up with M1.25.I think this is the problem, feel free to check my calculations (TAT and Mach formulae can be found in Wikipedia).MSFS is coming up with a M0.823 instead of M1.25 at a TAS of 454 with an OAT of -62C (211K).The next step in confirming my suspicions that "as real as it gets" has simplified the Mach calcuation is to find the corresponding TAS that you should have in these cold conditions and then check that TAS in the simulation against the speed tape. The corresponding TAS (kts) should end up being about 10 kts below MMO. I'll leave that to the interested student.

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The colder than standard air is denser than normal. This would be an interesting calcuation to run, I think MSFS is providing an incorrect Mach number calculation. If it were not compensated for temperature then the indicated Mach would be too low, so the simulation would fly at a higher TAS to achieve the Mach setpoint.To verify this, convert TAT to OAT at M0.823, -33C is equivalent to -62CNow use the TAT to find the Mach number for a TAS of 454 indicated on the ND using the OAT derived above comes up with M1.25.I think this is the problem, feel free to check my calculations (TAT and Mach formulae can be found in Wikipedia).MSFS is coming up with a M0.823 instead of M1.25 at a TAS of 454 with an OAT of -62C (211K).The next step in confirming my suspicions that "as real as it gets" has simplified the Mach calcuation is to find the corresponding TAS that you should have in these cold conditions and then check that TAS in the simulation against the speed tape. The corresponding TAS (kts) should end up being about 10 kts below MMO. I'll leave that to the interested student.
I have never seen this, but looks like you all found a bug. The plane flies 15knots +- below the overspeed tape with a 150 ci at fl390/fl400. I think the problem may be at higher altitudes.

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According to my Jeppeson CR-3 M0.825 with an IAT of -29C gives me a TAS of about 478 kts. So it's not that much out of wack.Edit:If I perform this calculation with calibrated airspeed, pressure altitude and TAT, I still get around 478 kts, although it gives me more than Mach 0.83. But nothing in the supersonic area.

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Well, still having this issue. Noticed something strage last time around. I was in another room when I heard the lady yelling "OVERSPEED!" at me so I ran in and hit pause so I could evaluate (and maybe take some screen shots). Missed the screen shots but was surprised to see that when pause was engaged, my IAS was lower than when the simulation was running. Same mach number, different IAS, lower by 6-8 knots. Now why in the world would that be?Certainly seems that calculation of IAS is strange. I don't want to point fingers at the MD-11, but never saw this problem on any other aircraft. Is this an FS9 bug or something in the MD? Sure would like to find out. I'd hate to be stuck at M0.82 in this bird for life...Thanks,

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Hi, I don't have the problem. I use Active Sky Advanced. The Overspeed warning comes up at .861 speedtapes3aq.jpgJan-Paul

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1) Not an issue organic to MD-11--I've had the same overspeed with Level D in several recent flights (FL370/M.85 etc)2) Will not be an issue if you deselect winds aloft on AS6.5 options setting--for many, not a realistic fix--but it singles out AS as the culprit3) More likely to appear in winter months when jetstream is howling and temperatures are extremely cold4) Occurs when a station is reporting radically different windspeed than current--AS team should be able to explain inputs/outputs to wind model5) Hand entering cruise winds via MCDU does not seem to significantly alter the equation6) Seems to occur with very strong head/tail component--then that component dramatically falls/raises--power setting cannot react in timeRecommendation: PMDG Dev Team work with Active Sky Dev Team and develop a patch that establishes a set of wind rate of change do not exceed values/criteria--if a delta between current cruise winds exceeds x degrees/second than that will be discounted and the model will default to an averaging methodology. I'm not a programmer, but if you can translate the above into 0s and 1s--that would be awesome :( Thanks for a superb product.Col (ret) Rich Perry, USAF

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Just to piggy back on the Rich's answer, I use FSUIPC to dampen wx shifts for both FS9 & FSX. I set the smoothing to 1 deg/kt change per 8 seconds and also turn off turbulence and smooth the pressure/temp values to half the default rate provided by FSUIPC. I like the HiFiSim products but so far they still don't seem to get into the MSFS internals and force it to settle down.

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Hi, I don't have the problem. I use Active Sky Advanced. The Overspeed warning comes up at .861 Jan-Paul
Jan-Paul,You don't have the problem yet and it's also not related to any WX simulation used.Try to lower your total fuel on board via the PMDG Option Menu to a total of 12000 kgs.Then see what happens under the same conditions as in your screenshot.Why this happens? (Kelly already mentioned it in this thread)Tanks 1&3 have two compartments. The most outboard compartments, refered to as WingTipFuelTank are kept full as long as possible to have an Mmo of .87When this WingTipFuel is transfering into the inner compartments (starting around 14000Kgs total fuel and below) Vmo/Mmo are decreasing.Refer to: FCOM Limitations page L00.6 and page L00.9Systems manual, Fuel System, page Fuel 10.10, Gravity and Jet Pump Transfer.Problem in the simulation is that Mmo is coming down below the real limit of M.85/320.Hope the development team will correct this, or maybe some beta tester is able to put it on their list.Regards,Harry

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Recommendation: PMDG Dev Team work with Active Sky Dev Team and develop a patch that establishes a set of wind rate of change do not exceed values/criteria--if a delta between current cruise winds exceeds x degrees/second than that will be discounted and the model will default to an averaging methodology. I'm not a programmer, but if you can translate the above into 0s and 1s--that would be awesome :(
You mean the PFD is influenced by cross- and headwinds and decreases the red line to M0.82?

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Jan-Paul,You don't have the problem yet and it's also not related to any WX simulation used.Try to lower your total fuel on board via the PMDG Option Menu to a total of 12000 kgs.Then see what happens under the same conditions as in your screenshot.Why this happens? (Kelly already mentioned it in this thread)Tanks 1&3 have two compartments. The most outboard compartments, refered to as WingTipFuelTank are kept full as long as possible to have an Mmo of .87When this WingTipFuel is transfering into the inner compartments (starting around 14000Kgs total fuel and below) Vmo/Mmo are decreasing.Refer to: FCOM Limitations page L00.6 and page L00.9Systems manual, Fuel System, page Fuel 10.10, Gravity and Jet Pump Transfer.Problem in the simulation is that Mmo is coming down below the real limit of M.85/320.Hope the development team will correct this, or maybe some beta tester is able to put it on their list.Regards,Harry
Hi Harry, I have lowered the fuel to empty 6.800 kgs. The Speedtape comes down a little bit, but not below M0.85 where the Overspeed starts. Jan-Paul

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Here are some screen shots for consideration...Please let me know if I am interpreting incorrectly.Image #1 - Crusing at FL310 with lots of fuel. Note where overspeed and protection is at. (Noted the first amber number, M0.861)Image #2 - Same spot with only 21K lbs. of fuel. I assume this is after the outboard to inboard transfer. Note overspeed and protection here. Much lower! It would not be possible to cruise at calculated M0.835 here!These tests wre conducted with no external weather. Please see the temps and wind on the MCDU.This shows me that the Mmo is dropping to well below M0.85. Whether this is the aircraft simulation, or the way that FS9 is calculating I do not know. But it is clearly happening.Open to all comments!EDIT: One more question while we're all chatting...what is the storage capacity of outboard and inboard sections of tanks 1&3?Thanks,

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Development team and others reading this thread.[[Problem in the simulation is that Mmo is coming down below the real limit of M.85/320.]][[Hope the development team will correct this, or maybe some beta tester is able to put it on their list.]]Quoting myself here, puts total shame on me.Based on the first screenshot in this topic I thought the indication being wrong.Checking both version FS9 and FSX more intense, show the MaxSpeedTape indicating fully correct.My apologies,Harry

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Hi, Harry, you are right, now I was able to reproduce the problem, it depends on the total weight of the aircraft. When you select 2/3 Pax load, the problem doesn't occour at low fuel loads. When you select 1/3 Pax the problem appears (Mmo is coming down below the real limit of M.85/320). So it must have something to do with the weight calculation. I don't have the time now to figure out the critical GW. I will try it tomorrow. Jan-Paul

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Hi, Harry, you are right, now I was able to reproduce the problem, it depends on the total weight of the aircraft. When you select 2/3 Pax load, the problem doesn't occour at low fuel loads. When you select 1/3 Pax the problem appears (Mmo is coming down below the real limit of M.85/320). So it must have something to do with the weight calculation. I don't have the time now to figure out the critical GW. I will try it tomorrow. Jan-Paul
Jan-Paul,Thanks for this "possible new evidence". Will do some more testing as well.Harry

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