VNAV behaviour question

[Guest mikealpha]

Hi all,I'm scratching my head a bit about a certain VNAV behaviour. Not meant as complaint, because at that point I have really no idea what the real McCoy does. Might be correct, might be not.Anyway, here goes : It's the PSA25 procedure into EDDF Runway 25L. Clear weather, no wind. In the first pic you see the legs page from PSA to DF036. 8000 ft at DF036 is in the Procedure text and the 250/11000 restriction at PSA I have entered myself. What you see is a linear descent gradient from PSA to DF036 in the legs page including a deceleration from 250 to 240 kt.Now the next pictures show the plane does not follow it. After PSA it descends to FL107 and then stops descent due to an internal recalculation. After CHA it starts descending again, but only with around 100 fpm. At every waypoint passed the Speeds and Altitudes got recalculated. But as every DF0XX waypoints stay above 10000 ft due to the low descent rate, their speeds automatically got changed from 240 kt to 250 kt (which was not intended). The end situation shows an upcoming rather steep descent from DF035/10100ft to DF036/8000ft including a deceleration 248->240 that was intended much earlier. Much in contrast to the initial linear and smooth descent all over the route.Now the question : Is this real world behaviour ? To get a linear descent rate, would I have to "hardcode" ALL the waypoints in the route exactly like initially calculated in the first pic ?Mikehttp://forums.avsim.net/user_files/54658.jpghttp://forums.avsim.net/user_files/54659.jpghttp://forums.avsim.net/user_files/54660.jpghttp://forums.avsim.net/user_files/54661.jpg

[Guest mikealpha]

No opinions regarding that ? Randy ???Mike

[Guest ha5mvo]

I have no solution for you Mike, just wanted to confirm that the FMC tends to go for steep approaches (that is - in the sim, maybe this is how its real world counterpart would do - I don't know).What I do is very much what you suggested - review the last few legs and insert manual restrictions. steep approaches make it a bit hard to maitain desired speed without having to slam the spoilers.I too would like to hear comments.Thanks//Mike

[Guest amerton]

Hi Mike,Some explanations :- VNAV builds the path bottom up. In your case, it does as follows from DF036 to PSA : -- first build an idle segment from DF036 to 10000 (as in DES page), about 3.5 degrees subject to environment and aircraft. -- then build the decel segment from 10000 up to what is required, on a about 1.5 degree slope -- then build the segment fron there to your PSA constraint- the prediction system is not too elaborate on segments including the airport speed limit decel, this is why you see it rather linear, whereas the aircraft will fly as explained above.Hope this helpsanthonyAnthony MertonPrecision Manuals Developmenthttp://www.precisionmanuals.com

[Guest lbb]

Guys,Yesterday I did some flights on VNAV. Usually the MSFS ATC will throw you off course so I decided to ignore and simply fly a VNAV ECON PATH approach into EGKK twice.Once I had set the wind to 0 and had left some constraints in the TIM2E arrival, to see how it would handle those (nothing TOO steep, since you can't blame the aircraft for that). And second, with the wind set to 080/20 and putting that into the DES forecast as well, but with all constraints removed, to see how well it would know when to start the descent. Since there are some turns involved the aircraft had to deal with both head and tail winds.I can say only one thing: SPOT ON! It was really a joy to see, especially the second one, from FL240 to 3,000 feet, all the way on idle, never deviating more than a few knots from the target speed.I even tried a VNAV approach, coupled with VOR/LOC to the runway. Again, spot on, but for the absence of a VNAV FLARE mode :-)I really have to say that for MSFS this has to be the most impressive FMC yet. My compliments!!!Leo Bakker