After installing 4.6 a few times I'm still finding issues with how it calculates vertical path following addition of speed and altitude constraints.

I set up a short flight from EGGP RW27 to EGAC RW 22, FL180, ZFW 70.4 and correctly setup the FMS pre-flight.  FP was WAL L10 IOM NELBO MAGEE.  The last few legs were shown as:

IOM    289/FL170
NELBO 240/FL094
MAGEE 240/3680

I modified MAGEE to 180/3000 as it is the last fix before establishing on the localiser for RW22.

The legs page recalculated as follows:
IOM     289/FL180
NELBO  289/FL180
MAGEE 180/3000
This is clearly not correct and the TD was no longer shown.  This issue is repeatable.

I setup exactly the same flight in 2 other aircraft (B737/767 and B737/800) and did not find an issue. I verified the problem on my Windows 7 FSX installation as well as my WinXP installation.

Additionally I find the Vnav behaviour really inconsistent, in another flight I had a leg speed constraint of 220 set and yet the speed bug was at 250, IAS 250 and VNAV switch illuminated.

I dont recall these problems in 4.4 and while I recognise lots of problems have been fixed in 4.6, I'm left with something I dont trust and pretty unusable for my purposes.  I wonder whether anyone else is finding a similar issue, it may of course be related to something about my setup, but its is not obvious what that might be. I'm at a bit of a loss as to how to proceed.

I tried this and you are right, you can't do what you want to do.

So here is my way of getting it to work.

First. There are two listings of MAGEE, DO NOT remove/delete either of them.

To maintain a T/D and a vertical path, I changed BOTH instances of MAGEE to read 180/3380. If I change it to 180/3000, then I lose the T/D and the vertical path. So to get around this, you can use a FLTCH down to 3000, instead of letting the VNAV only take you down to 3380/3390 feet. Because 3000 feet is below the vertical path, once you arrive at that altitude, the A/P will then use ALTITUDE HOLD and SPEED, but at least it will work.

Hope this helps

Mark

Thanks Mark,

Yes able to work around it and I probably would use FLTCH when getting down into the approach, so thanks for confirmation that I'm not completely bonkers in seeing problems where none exist.  The 757 is such a lovely model and flies nice within its limitations, its just a pain working out what those limitations are :)

Regards
Gerard

So why does the FMC reprogram itself for no apparent reason.  I was doing a flight from KSAN to KSFO and suddenly my speeds where different than what I had programmed.  the speed was 180 in small case even though it kept flying at 292 kts.  But I had no T/D so I had to manually descend until my computer locked up.  Why does it change the settings?  I don't understand yet.