757 v1.3 Dodgy VNAV

[philbrown 9]

Thought I'd start a new post on this bug - the other post was getting a bit off track.Two screenshots of the poor VNAV calculation:http://forums.avsim.net/user_files/177166.jpgThe picture above shows my initial descent. As you can see, FL120 has been entered for AMR01. I am currently in VNAV mode, and you can see from the FMC and the ND it is right on the descent profile. Problem is, MCP is also set to FL120, which means the green arc should be over AMR01, and not 10 miles hort of it. This section of descent is obviously too steep.http://forums.avsim.net/user_files/177167.jpgThe picture above shows what happens on passing RESTU. I am now almost 3000 feet below the descent profile! Yet a mile earlier I was right on it! The aircraft then descends at 500fpm in order to re-establish the profile.Why is there not a steady descent from TOD to EOD? Please answer PSS.

[Guest macflegg]

PhilWhen I flew your approach I got the same results as you did - this does not mean that vnav is wrong.TOD calcultes the distance to touchdown and it did calculate the correct place.However the descent was begun just after the correct place and hence the initial high rate of descent to get to RESTU at 21000' as predicted on the legs page.Your position AMR01 was given a contraint of FL120 and if that restraint were not there the steady descent path would put the aircraft about 3000' below this level hence the FMC computed a less rate of descent to arrive at AMR01 on speed on height as requested.The green arc shows the instantaneous calculated estimate and although you select FL120 in the box it does not mean the arc will immediately go to the next position - if you leave it alone it will will recalculate and adjust the rate of descent to give you what you have asked for and the green arc will come back to the next waypoint.After RESTU you are now high so the initial green arc actually disappears from view until the FMC inceases the rate of descent to arrive at the next waypoint as requested.If you ask the aircraft to do the impossible like descend 6000' with 7 miles to go then the green arc will be beyond that waypoint and you will have to use speedbrake to bring the green arc back - by modulating the speedbrake you can move the arc backward or forwards.In short - vnav works the way I would expect.I have seen that this was modelled on one of the early software versions of the FMC - some of them went blank screen they were so slow and I have had one dump its entire route on me passing the west coast of Ireland so the real thing was not perfect either!Hope this helps.Mac

[kevinh 409]

I noticed a different problem with VNAV. I had entered a short route (EGSS/DVR/KONAN/KOK/EBBR) where TOC and TOD were very close. I added a SID for DVR and a STAR for KOK. Following FS9 ATC (for a change!) I was vectored away from the SID and eventually on a heading towards DVR, which was also close to TOC. I updated the FMC route by selecting DVR as the next waypoint. Somehow the cruise flight level (FL310) got deleted and so VNAV wanted to descend to 7000 ft (an altitude constraint on the STAR). The FMC still showed FL310 as the cruise level, but it the LEGS page showed demanded altitude to be 7000.I could not repeat the problem when I followed the route ignoring ATC vectors, so I believe it may be to do with being vectored a long way from the planned SID by ATC.Kevin

[philbrown 9]

Just loaded up the 777. It's FMC estimates RESTU at FL240, opposed to the 757 which estimates RESTU at FL200. It means a steeper than required descent between TOD and RESTU, causing you to be well below the profile at RESTU.The image below shows what the 757 is doing and what should happen:http://forums.avsim.net/user_files/177180.jpgI appreciate that the real plane can have some wierd bugs in it, but I don't think we should be simulating that here. It should be performing as every other payware flight sim plane I own does. I should be able to descend from TOD to EOD in VNAV mode without this levelling off business at every waypoint in between. I am not asking the plane to fly outside of it's abilities. I have told it what to do, and although it may have calculated TOD correctly, it is not calculating the target altitudes at waypoints between TOD and EOD correctly.

[Guest Phoenix5]

The B777 and B757 are different aircraft, designed with technology from different eras, that of course will have the B777 performing way better than the B757.We have produced the B757 FMC as close to the real thing as we could, because that is what was demanded from us - realism.

[philbrown 9]

This is not realism Steven - I'm absolutely amazed at your reply. The 757 CAN NOT CALCULATE A PROPER DESCENT PROFILE FROM TOD TO EOD. I don't see why it's so difficult to understand. Look at the images above - how can I be descending right on the profile one second, and 3000 feet below the profile the next.Don't try and pass off bugs with this aircraft as features or realism. It is a major bug, and still has not been fixed. The 757 FMC is the same as that modeled in the Level D 767, and that has no problem at all with VNAV descents.

[Guest macflegg]

PhilThe plane did not level off - it recalculted the rate of descent to get to the next waypoint as requested.The software and hardware used at that time as indicated earlier was quite crude but has been updated many times since to reflect what you see in B777.I would not say the plane had bugs in it - merely we were at the time still learning what it's limits were - hence the expression "what is it doing now".Mac

[Guest Phoenix5]

Phil,How many real life B757 pilots do you need to come in here and tell you what I have been telling you? The B757 is not as modern as others, and ours works to the limits of the real ones used by the pilots we asked to test and research our product.You have to fly this plane, you can not rely on the FMC as you can with other aircraft.

[philbrown 9]

Are you telling me that PSS, when writing the code for the FMC, decided to PURPOSEFULLY include this type of error, so that when I enter a level constraint, and have waypoints between TOD and EOD, it causes major errors in the descent path calculation?

[Jim Harnes 13]

Hmmmm... So all the real world 757s with vintage FMC's are also in "error"?Jim Harnes