DES PATH UNACHIEVABLE: FMC Message when descending

[mackalex 0]

Hi my name is Alex Makoviecki. I recently got the PMDG 737NGX and I really... really love it! The only problem I have with it is descending. I am about thirty miles back from the TOD (top of descent) and I set in the altitudes in the FMC that I should be at for each waypoint and VOR for the OMN.HILEY2 STAR at Miami Intl. I was using a chart that shows you the altitudes for each waypoint and VOR. I then set the altitude for descent to 5000ft which was the altitude for one of the waypoints that I would descend to and stay at for a while until descending to the glideslope intercept altitude. When I reached the top of descent the engines went to idle and the aircraft started descending but then leveled out at 37000ft (my cruising altitude was 38000ft). I noticed that the FMC had a message that said DES PATH UNACHIEVABLE, that was weird because I set the right altitudes for each waypoint and VOR for exactly what they should have been. I got the aircraft to descend a little more by copying of the speed and altitude of the first descent waypoint and pasting it again on the same waypoint. But I only got the aircraft to descend to 32000ft. I also noticed that when I am climbing above 25000ft the aircraft slows down the climb to 1000fpm, compared to 1800fpm in other payware aircraft. I would appreciate some help here!Thanks,Alex.

[RYR738 9]

Hi my name is Alex Makoviecki. I recently got the PMDG 737NGX and I really... really love it! The only problem I have with it is descending. I am about thirty miles back from the TOD (top of descent) and I set in the altitudes in the FMC that I should be at for each waypoint and VOR for the OMN.HILEY2 STAR at Miami Intl. I was using a chart that shows you the altitudes for each waypoint and VOR. I then set the altitude for descent to 5000ft which was the altitude for one of the waypoints that I would descend to and stay at for a while until descending to the glideslope intercept altitude. When I reached the top of descent the engines went to idle and the aircraft started descending but then leveled out at 37000ft (my cruising altitude was 38000ft). I noticed that the FMC had a message that said DES PATH UNACHIEVABLE, that was weird because I set the right altitudes for each waypoint and VOR for exactly what they should have been. I got the aircraft to descend a little more by copying of the speed and altitude of the first descent waypoint and pasting it again on the same waypoint. But I only got the aircraft to descend to 32000ft. I also noticed that when I am climbing above 25000ft the aircraft slows down the climb to 1000fpm, compared to 1800fpm in other payware aircraft. I would appreciate some help here!Thanks,Alex.

The CDU message DRAG REQUIRED is displayed if an unexpected tailwindresults in a significant increase in airspeed to maintain path. The CDU messageDES PATH UNACHIEVEABLE is displayed if the FMC determines that theplanned descent profile cannot be accomplished. VNAV disengages if a limitspeed will be exceeded.A path descent must be initiated while within the allowable cross–track error forLNAV, however LNAV may be disengaged during descent while remaining in thepath mode. VNAV will remain in path regardless of cross–track.

[patrickbc 33]

I had this 1 time from EKCH to ESSA. I thought i was because i first used VS and then after u had descented 10000 feet tried to engage VNAV

[Thomas_Kenobi 0]

I had this when trying to land in 25R, Madrid - Barajas. The IAP had a steep descent just before G/S intercept from 9500' @IAF to 5000' @G/S Intercept. The plane was lightly loaded that day so it had difficulty coping. I had to make judicious use of in-flight speedbrakes and even then it was tricky. Well they said the 738 is slippery on descent and it is :)

[mackalex 0]

Well, it's still not working, and since I'm flying STARS I don't want to manage the VS (vertical speed) or FL CH. On every flight I tried it happened, I got all the way up to the descent and I failed... or the plane failed. I tried every other aircraft on each STAR that I flew with the NGX and they could descend normally. Something must be up with me configuring the LEGS for the descent with the STAR, but I'm not sure because each waypoint's speed and altitude look fine with the chart compared. I think it might be the altitude between one waypoint and another.(made up waypoints): EXAMPLEBOAGA - 240/6000A ----> Speed and altitude given on the STAR chart.SANTO - 240/4543 ----> Speed and altitude calculated by FMC.SVT - 220/3354 ----> Speed and altitude calculated by FMC.CLANT - 200/2500A ----> Speed and altitude given on the STAR chart. This kind of gives you the idea of what I mean by the waypoints in between that have speeds and altitudes calculated by the FMC. What I think is maybe they have to be changed? Thanks,Alex.

[JaneRachel 515]

I also noticed that when I am climbing above 25000ft the aircraft slows down the climb to 1000fpm, compared to 1800fpm in other payware aircraft. I would appreciate some help here!Thanks,Alex.

Hi Alex, this climb rate is absolutely correct, dependent on conditions and aircraft weight. PMDG are the only developer who accurately model VNAV. all the bestJR

[Nixon 30]

Hi AlexI asked a similar question and Ryan of PMDG answered that it was my throttles not being set manually to idle and to change the throttle setting in the PMDG set up page to never. Have a look at this thread " VNAV issues (solved-not understanding AT overide modes) and read Ryan's post.Hope that helps

[Tabs 2,790]

These FMC messages "NO DES PATH AFTER XXX", "DES PATH UNACHIEVABLE", DRAG REQ AFTER XXX, STEEP DES AFTER XXX etc are warnings to the crew that under normal circumstanes (ie without speedbrakes, flaps, gear etc) the descent prediction algorithm thinks the aircraft is going to be outside the VNAV limits for the target speed and path angle limit. Basically the FMC is telling you that you're probably going to need to take some kind of corrective action like extending drag devices or, if that still fails to keep you within limits, informing ATC that you're unable to comply with the restrictions - that does happen in real life, especially with an extremely slippery aircraft like the 738. Try flying the 900, it actually gets better descent performance despite being heavier just due to aerodynamic reasons with the fuselage length and shape. The 700 is the easiest model to descend with, you'll see that when it's released.

[chapstick 2,754]

I get NO DES PATH AFTER XXX always when XXX is a vector. At that point I'm not really sure what to do other than HDG SEL and LVL CHG. I wonder how real pilots handle that.

[Tabs 2,790]

(VECTOR) means in real life you're awaiting ATC instructions. If (VECTOR) is the last waypoint in your flightplan, it makes sense that there's no descent path - you have an indefinite end to the leg. The plane has no idea where it will be and thus there's no end of descent waypoint for it to calculate a path with.

[scandinavian13 5,192]

Also, make sure to take a look at the stickies about the use of speedbrakes at the top of the subforum here. They're really good at explaining how to "get back in the game." Another big help is to tell the system what the descent is going to look like. Winds aren't the same through the descent all the time, so setting up the winds at the various levels on the Descent Forecast page (along with the ISA DEV / QNH ) will help accuracy.