Can anyone explain this?

[Guest dirkvanderlaan]

I just made a perfect RNav approach at EHAM 18c. From 2000 ft to DA right on GS and Loc. That works great and restores a little bit of my trust in the VNAV mode!I still have problems though, with the VNAV descents. Whatever I do with the winds (real wind or big tailwind) I usually end up getting the dreadful 'drag required' message (often more than once). F.e. today, on a NORKU1A arrival into EHAM from FL310, I passed the T/D and immediately the message came on. How is this possible? Shouldn't the computer at least begin with reducing at the right moment to 284 kts in order to stay on the path? Now the speed remained 320 kts and I was above the path up until passing ARTIP which should have been at FL100.And although I had put a huge tailwind (60 kts) instead of the real wind (cross 25 kts), still the speed was moving quickly to the 294 kts mark.Why is the VNAV PATH descent of this B737NG so inaccurate?

[Mats_J 2]

Dirk,As Dan was mentioning in an earlier reply. Do you enter the predicted winds for your descent into the FMS through the FORECAST page? This will tell the FMS what to expect and it will adjust the predicted path accordingly. VNAV PATH will only guide the A/C to keep the path. Speed control is up to the PF.Hope it helps,

[michal 44]

>I>passed the T/D and immediately the message came on. How is>this possible? Why is it so strange?FMC is smart enough to know based on the flight plan restrictions it is supposed to meet that the requested path angle is such that riding it down will cause the overspeed conditions, hence the message. And this is not such a 'dreadful' message - real pilots deal with it every day.I can see you need to step back, draw some diagrams and try to understand fundamentals behind VNAV, there is a fair amount of physics and aircraft performance involved here. There is a also a great deal of VNAV knowledge on this forum - just do the search. Michael J.http://img142.imageshack.us/img142/9320/apollo17vf7.jpg

[Guest dirkvanderlaan]

>I>>passed the T/D and immediately the message came on. How is>>this possible? >>Why is it so strange?>FMC is smart enough to know based on the flight plan>restrictions it is supposed to meet that the requested path>angle is such that riding it down will cause the overspeed>conditions, hence the message. And this is not such a>'dreadful' message - real pilots deal with it every day.>I might be wrong here, but isn't the FMC supposed to find the most economical descent path/path angle from the T/D till the first restriction? And isn't that path computed in such a way that it should be flown at flight idle, without applying any throttle or speed brakes? And, if (and only if) forecasted conditions prevail, shouldn't that path really be flown at the computed speed? Doesn't applying speedbrakes mean that the T/D has been too late? And doesn't applying throttle mean that the T/D has been too early? If this is true then, at T/D (where no conditions have altered), shouldn't the FMC ever give that message?Tell me where I'm wrong.Btw I have studied the VNAV extensively. I have had years of training in ATPL theory as part of my CPL. I have studied the many contributions in this forum and other forums. I do know the principles behind VNAV descents and I really think the flight model used needs some finetuning.

[Qavion 19]

"Tell me where I'm wrong."Never come across something called Drag Factor (where the FMC is concerned)?The real world FMC assumes all 767's are equal... apart from things like engine type, fuel flow and DRAG FACTOR.There are no two 767's with an identical drag factor. SOME airlines go to the effort of fine tuning the FMC to match the drag factor of each airplane in their fleet, others perhaps think it is a waste of time (as already explained.. you're not going to get a perfect descent in the real world anyway, no matter how much fine tuning you do)The FMC assumes a standard atmosphere unless told otherwise. Who is to say that FS's standard atmosphere agrees with Honeywell's idea of a standard atmosphere? PMDG's sims are created with lots of real world numbers... If they disagree with FS's numbers.... Who is at fault here?Although engines have electronic control of idle speed, in the real world, there can be large differences in rpms (between engines on the same aircraft) during descent. One engine can be spinning faster that the other one (creating different levels of drag/thrust).Perhaps the real FMC (and PMDG's FMC) doesn't take into account aircraft CG during descent? The aircraft is bound to fly differently if the CG is different.The fact that PMDG makes you use spoilers and thrust only adds to the realism. Otherwise you may as well be flying an airplane on a wire.Random variations are part of life.. Otherwise it would be very predictable... and boring. PMDG is, in a way, preparing amateur pilots for reality.My 2c's worth...Q> イアン

[zubart 5]

I was wondering if you use the MSFS acceleration 2X, or more at TOD and descent? This feature will cause IAS to increase. Don

[Guest dirkvanderlaan]

>If this is true then, at T/D (where no conditions have>altered), shouldn't the FMC ever give that message?>>Not sure what your asking.>I might be thinking too simple here, but if I manually compute a T/D, as I have done many times (real and sim), incorporating everything I know about the aircraft characteristics and (wind)conditions, I would never have to correct it immediately after passing T/D. I would have to correct it of course after a while, because of all kinds of imperfections. But what amazed me here was the 'drag required' message IMMEDIATELY after passing the FMC computed T/D.

[Mats_J 2]

Dirk,Have you altered the speeds on the CRZ and DES pages? If the former is higher than the latter the NG will have a slight problem slowing down on TOD.Hope it helps,

[Guest dirkvanderlaan]

>Have you altered the speeds on the CRZ and DES pages? If the>former is higher than the latter the NG will have a slight>problem slowing down on TOD.Don't think so. The only speed I consequently change is the 240kt/FL100 restriction. The CRZ and DES speeds I leave unaltered.

[Guest bombayduck260992]

What i do know about it, is that you need to intercept the glideslope from beneith, otherwise it just doesn't want to land.I have it myself, and thats the solution. Try it out.

[Guest dirkvanderlaan]

>What i do know about it, is that you need to intercept the>glideslope from beneith, otherwise it just doesn't want to>land.>I have it myself, and thats the solution. Try it out.First read what we're discussing, read well, my friend.