Descent not achievable

[WTE_Pharoah 7]

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

[Jeff_B 8]

That is odd. And im very familiar with that arrival - so from that alt your TOD should be approx 110nm from SY VOR - is that where it was calculated by FMC?Also did the star have 6000A for the altitude restriction in the FMC? Just trying to get a starting point for this behaviour. Sometimes the stars/sids may be programmed with an error.

[XCLTM3 112]

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

Roger that. I have experienced this issue not only on that particular run (NZAA YSSY) but on some domestic runs in Australia. I put it down to the NAVDATA for a particular run. It doesn't always occur (many other runs work out AOK) therefore I deduced it has something to do with NAVDATA and/or NGX FMC interpretation of the NAVDATA.Oh, and another thing: if you can input winds aloft data this also assists the FMC with making essential/correct descent paths.Just waiting on SP1 to see if that clears this issue. Also an issue with NAVDATA causing crashes/freezes at certain intersections/waypoints. I'm told HOTFIX 4 addresses that issue but thought I'd wait until SP1 as some users have experienced worsening probs since HF4. Rgds,

[WTE_Pharoah 7]

hey mate yep all the issues I've experienced have been flying in or into Australia (I've posted previously re this but got no answer). If there is a fix, I might just wait for it, however it could be that I'm not inputting winds aloft during cruise. Having said that though....I thought the FMC could just deal with it (maybe not?). Oh and yes the approach had 240/6000A in the leg page. The actual STAR is correct per the printed sheet I have (I normally run through the STAR and match against my approach plates to famil myself before or during descent).

[b0gey_dead_six 11]

Maybe Cost Index is too high and you're descending at a too high of a speed?

[WTE_Pharoah 7]

Maybe Cost Index is too high and you're descending at a too high of a speed?

I normally use a cost index of 50...don't ask me why...I just use it. :) as for descent, thats autocalculated by the FMC - I 'up' the spoilers on request but otherwise I leave it up to the FMC - maybe I'm too reliant on the FMC but its done an excellent job in the past (descending at idle at around 2000 fpm +/- 500fpm then levelling off at FL100 before bleeding off speed to 240kts) - just now and then (and it happens a lot), I get the above error.

[RYR738 9]

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

" Descent Path Unachieveable " is an advisory message !When in a path descent and above the path, the FMC predictions show the profile restrictions at the next waypoint cannot be achieved.The restriction needs to be modified. Fred.

[WTE_Pharoah 7]

yeah but this is what I don't get....what does the FMC need to calculate the descent profile? obviously starting height (eg. FL360), speed (mach .74), and the way points (altitude/speed) from the nominated STAR which is usually from a certain navpoint. its the profile between TOD and the start of the STAR navpoint which is what is falling over...within the STAR, there isn't an issue. Possibly due to winds aloft not being factored, etc.

[RYR738 9]

yeah but this is what I don't get....what does the FMC need to calculate the descent profile? obviously starting height (eg. FL360), speed (mach .74), and the way points (altitude/speed) from the nominated STAR which is usually from a certain navpoint. its the profile between TOD and the start of the STAR navpoint which is what is falling over...within the STAR, there isn't an issue. Possibly due to winds aloft not being factored, etc.

The descent path is actually calculated backwards starting from the E/D all the way back to the T/D.The reason it does it that way is much too complicated to go into it here.Something as simple as innacurate NAV DATA can cause lots of route problems especially for VNAV calculations. I will try your flight tomorrow and see can I spot anything unusual. Fred.

[WTE_Pharoah 7]

thanks mate. as I said...I let the FMC do everything but then it cracked the $hits when it couldn't achieve the descent profile and just said 'sorry, can't do it' :) (you know what I mean). if you do fly it and you don't get any issues, please let me know what you've done...it could just be something I'm missing in my preparations eg. not inputting winds aloft, etc.

[fstulm 4]

Hello This is probably caused by a significant tailwind that is not taken into account. The computations are executed in still air unless winds are also put into FMC. Usually, it should be an FMC message specifically requesting drag "DRAG REQUIRED". Good airmanship and experience will give you the feel and need for an early decend with the FMC option DESCEND NOW; that should allow the FMC to recompute the data and eventually provide a better descent path. Also, what kind of descent are you executing? PATH or SPEED? I am guessing PATH. Don't forget: letting the FMC know ahead of time what winds await for your decescent, the better the computations. You can experiment with this; while still in cruise, put different winds in FMC and watch the TD change. This is neat trig! Hope this helps:) John Gabriel

[MarkW 1,491]

I just had this problem flying into ESGG yesterday. "No decent path after waypoint" and "decent path not achievable". I ended up just switching to FLCH mode and added some drag and it was not a problem.

[Jude Bradley 659]

Does exactly what it says on the tin. Just use the speedbrakes. (it could be due to tailwind,or adding flaps), as Fredric says, it's an advisory.

[emvaos 13]

I just had this problem flying into ESGG yesterday. "No decent path after waypoint" and "decent path not achievable". I ended up just switching to FLCH mode and added some drag and it was not a problem.

The phrase "..and added some drag" answers the issue. The FMC warns you that it is impossible to descend and decelerate (usually) on idle thrust within the particular flight plan constraints and possible weather conditions (e.g. unforecasted high ISA or tailwind). Mathematically the required descend path angle towards a particular restriction exceeds the one that corresponds to VMO - 5 (above first deceleration) or the speed restriction altitude value +10 (usually 240 at 10000 hence 240 + 210 = 250). In other words the FMC tells you " I am going down but bare in mind that I cannot maintain the speed target requested because the angle is too high.." It is just a friendly reminder.

[Guest]

I've fixed this problem by checking the the speed/altitude settings in the STAR against the approach plates as supplied by Airservices Australia (free download). I found that sometimes the FMS calculations as made by 737NGX are out.