Hi all I fired up the NGX on a flight from Auckland (NZAA) to Sydney (YSSY) last night at FL360. I even dialled in the MARLN 8 STAR SY transition for Rwy 34L ie. you approach Syd from SE, overfly Syd airport at 6000ft, hang a left for downwind and then you're set up for 34L. All went to plan and at TOD I reset the ALT to 6000 and the bird started descending...at about FL250 the FMC gave me the 'descent not achieveable' and just left me there. This has happened quite a few times. is there an issue with how the descent is calculated, particularly TOD? from the times I've had this issue (not always), it seems that the TOD is too late and descending at 2000 fpm isn't sufficient to achieve the correct descent profile. I've ended up having to manually override and inc descent to approx 3000fpm or thereabouts until FL100 on full spoiler before I try to bleed off airspeed. never had this issue with the 747 or the leveld 767