Descent and TOD

[WTE_Pharoah 7]

hi all I've been having a few problems with VNAV in that it seems to be calculating the TOD too late, then starting the descent and giving me an 'Unable to maintain descent profile' or something like that. I've been left high and dry quite a few times now because of this. Is there any way to correct this? my altitudes are usually between FL320 and FL360. I think the problem comes in the STAR requirements eg. above 9000, below 6000 etc. As soon as the bird realises it can't make the descent it levels off. :(

[Mats_J 2]

Flying with winds aloft?What STAR?Let us know the flight plan and I am sure someone will try it out. Basically the VNAV will let you handle the energy management to some extent.There are several ways of doing this. The current version 10.8 shipped with the NGX is quite clever. Please elaborate and we will be able to help,

[Mattias Nordin 23]

Hi! Don't buy the 'unable', most of the times you are able regain your VNAV descent path. I've been high above sometimes but still managed to adjust my descent path to regain. Try to lower your descent speed in descent page in the CDU to e.g. 290 (before reaching TD) to start with. This will give you headroom to get below VNAV path by using the spd intv button to manually set a higher descent speed on the MCP to get below the VNAV descent path.Take a look at Jack Colwill's videos 'Do you really need speedbrakes' videos pinned at the top of the forum.Really great videos to get you manage your energy on descent!

[WTE_Pharoah 7]

Thanks guys I was flying YSSY-YPAD using the following flightplan: YSSY KAT H44 MAXEM Q60 BLACK YPAD. I chose the BLACK9A STAR for a straight in to Rwy 23. Cruised at FL340. I normally don't touch the FMC during descent. I remember having this problem once whilst I was still in 4x speed - I'm assuming this causes a problem as well? oh and I had REX active as my wx program. There was some wild weather at YSSY on dep but not at YPAD from memory.

[oneill875 1]

Don't forget that you can start the descent early by going to the DES page of the CDU and pressing DES NOW > EXE. I often do this with a tail wind, sometimes up to 15 miles early. VNAV should then descend at a slower rate initially until it captures the original profile. This can be a good time to adjust the speed for the descent or to review the constraints and make sure of meeting them. Remember, you are the PIC not the FMC, you need to manage it. JackC's videos are excellent for re-capturing the profile if you are high due to ATC. Dan.

[Alaaar 34]

Hi ThomasThat happened to me before, when I changed the cruise altitude using MCP only and not FMC or if have strong tail wind or bad management of speed / descend rate by me (usually a quick snack or phone call would me the main cause ), please refer to Jack's videos on descend it's amazing, bottom line if you are too high and too close ....why not increase the speed to resolve the issue, who would of thought of that ?Regards