To better understand the MD-11, can someone please enlighten me:1.When I reach the T/D and stay at that altitude, it takes some NM until the vertical deviation indicator starts to move downwards on the scale. When dialing down the altitude and doing a FLCH at that point, it's easy to stay on the vertical path and keep the vdev diamond centered. When an auto descend is made at the T/D instead, the plane dives well below the vertical path and needs much time and distance until it finally intercepts the path again from below. So, the FLCH method looks more "elegant" to me, and I ask why the logic commands a dive when the "real" T/D is still miles ahead and what the reason for that behaviour is.2.Even with no winds at all present, the automatics have some trouble getting descend and speed right: At some point, I can almost forsee the "add drad" message to appear. I now question why the calculations cannot be made so that in extremely calm situations (which don't exist in real life) speed and vertical path are "spot on". I ask because as soon as winds begin to show up, things get even more complicated and it's even more difficult to get speed and descend right.3.During my last approach, everything worked pretty well and the vdev diamond stayed centered - well, until some point below 8000 ft or so when it wandered down relatively quickly, so as if the vertical path was bent down more and steeper than before. There was no warning or advisory, speed was correct and spot on, as were the autoflight modes. Are there significant "vertical curves" in the vertical path?4.Usually, I'd expect aircraft to descend with idle thrust to the first constraint. I know that many don't, so doesn't the MD-11 in all cases. I have no idea why, but maybe someone can give me an explanation how the vertical path can be seen if it were visible in space and if it's a straight line or not and if so, why not.5.The speed target indicators (filled and hollow magenta circles on the speed tape) make me wonder. When exactly is the hollow circle displayed in addition to the filled one? I usually got it when flaps weren't extended enough so the automatics didn't slow down. But on my last flight and only 12NM from the runway, I still was at flaps up at 250, and suddenly the speed target was set to 180 or so with a solid circle only...6.Sorry if that has been asked before: When looking at the vertical deviation parameters on the MCDU, therere are high/low and a short/long values. What does short/long mean?7.To be able to stop a full plane at v1, I found almost no runway sufficiently long to provide for this (they must have more than 10000 ft according to my tests). How is that handled in real life? Can't the plane be full in such cases? Are weight and stop distance tables available somewhere (sorry if they are in the manual, but I have no access to them at the moment)?Andreas

Are you using ECON or POLICY constraints?

Econ.Andreas

I'm not the expert, but here is my stab at your questions:1. When I reach the T/D and stay at that altitude, it takes some NM until the vertical deviation indicator starts to move downwards on the scale. When dialing down the altitude and doing a FLCH at that point, it's easy to stay on the vertical path and keep the vdev diamond centered. When an auto descend is made at the T/D instead, the plane dives well below the vertical path and needs much time and distance until it finally intercepts the path again from below. So, the FLCH method looks more "elegant" to me, and I ask why the logic commands a dive when the "real" T/D is still miles ahead and what the reason for that behaviour is.If by elegant you mean a smooth constant descent, I have found V/S to give the best control over this and FLCH seems to pull the throttle back and fall. True, the PROF does fall below the vertical guidance but it does get on the profile in short order and IMHO follows that profile all the way to the runway better than anything else I've played with.2.Even with no winds at all present, the automatics have some trouble getting descend and speed right: At some point, I can almost forsee the "add drad" message to appear. I now question why the calculations cannot be made so that in extremely calm situations (which don't exist in real life) speed and vertical path are "spot on". I ask because as soon as winds begin to show up, things get even more complicated and it's even more difficult to get speed and descend right.Speed is not the primary variable in the solution, it may vary from the ECON setting while the logic is solving the profile solution. The drag message is advisory and in most cases on a well planned descent I can ignore them and the speeds will fall into the desired range in short order. As far as I am concerned, any IAS within 5-10 kts is probably realistic.3. During my last approach, everything worked pretty well and the vdev diamond stayed centered - well, until some point below 8000 ft or so when it wandered down relatively quickly, so as if the vertical path was bent down more and steeper than before. There was no warning or advisory, speed was correct and spot on, as were the autoflight modes. Are there significant "vertical curves" in the vertical path?Don't know, don't think I've experienced this. Usually below 8,000 you inside of the last STAR altitude constraint, although some STARS like those at KATL might keep you at 12000 until turning downwind. Never noticed a steeper descent for unexplained reasons.4. Usually, I'd expect aircraft to descend with idle thrust to the first constraint. I know that many don't, so doesn't the MD-11 in all cases. I have no idea why, but maybe someone can give me an explanation how the vertical path can be seen if it were visible in space and if it's a straight line or not and if so, why not.The profile is not a straight line for many reasons including the changes in GS at a given IAS as you go lower. My limited understanding of the algorithm is that it is based on minimizing the energy required within altitude/speed constraints. Even if speed were not a constraint, you wouldn't just go to idle thrust and descend at MMO/Vmax because you would be waisting energy (you start the descent with a significant amount of potential energy, which you trade for kinetic energy by descending but the faster you descend the higher your drag which goes up as the square of the speed and the more potential energy you trade off to drag instead of kinetics the less economical your descent.5. The speed target indicators (filled and hollow magenta circles on the speed tape) make me wonder. When exactly is the hollow circle displayed in addition to the filled one? I usually got it when flaps weren't extended enough so the automatics didn't slow down. But on my last flight and only 12NM from the runway, I still was at flaps up at 250, and suddenly the speed target was set to 180 or so with a solid circle only...The hollow circles are the speeds that would be flown if you allowed her to follow her instincts... at 12 nm you should be well below 250 and at 10 nm I consider you within the airport's local traffic. Might help to add a notch of approach flaps a little farther out and she will follow your lead. If you wait to add flaps until that hollow circle appears at 180 KIAS you are probably too fast.6. Sorry if that has been asked before: When looking at the vertical deviation parameters on the MCDU, therere are high/low and a short/long values. What does short/long mean?I'm not looking at a display so I am just guessing you are referring to the distance to intercept the desired profile? Just guessing.7. A V1 RTO is a very scarry event. There's a uTube clip of the 767 test ran by Boeing for certification, as it comes to a stop all the wheels are on fire and the fuses have blown to prevent them from exploding. The key test parameter is that fires don't spread to the airplane within one minute. Whew! I have done a V1 RTO during testing and I thought it was modelled fairly well, much of which depends on the MSFS braking/friction functions not easily varied by add-on designers. Balanced field length tables (accelerate/stop) are used by operators, but since there is a table for every runway this kind of data is not readily available for the hobby.Anyone can jump in and correct, add or refine this, I have no pride of authorship. I'd like to point out Andreas that answering one or two questions in the forum environment is easy but a half-dozen of them takes more time than I will normally spend in any thread.

6. is simply a reference to a fixed waypoint - so high/low will be the altitude at the waypoint and short/long the distance to the waypoint. This only shows in the cdu.As for the descent profile the MD is usually pretty precise if left to it's own resources and even in the tutorial can display an error at the first waypoint but in essence it isn't an error because although trying to hit 15000' the cdu shows 15000+ as the constraint so as long as not below it's ok.John Ellison

I'd like to point out Andreas that answering one or two questions in the forum environment is easy but a half-dozen of them takes more time than I will normally spend in any thread.

Thank you for your patience, Dan, and helping me out!Andreas