DESCENT PATH UNACHIEVABLE

[aaronrash 0]

I have been having this come up on the FMC allot lately. The FMC should calculate the time needed to decelerate and stay on path throughout the vertical path. I never started getting this until the hotfix by the way. The only way for me to get back on the vertical path is to adjust the vertical speed and catch back on manually.

[Mats_J 2]

Aaron, Flightplan please? And maybe a few more details on where it happens? It makes it soo much easier to help... Thanks,

[aaronrash 0]

Hi Mats, I was flying this route: SWANN V445 DQO PHLBO2 from KDCA to KEWR. I filled my flight altitude to 17,000 but the FMC automatically changed it to FL200. Should not really matter thought because there was still plenty of room for T/C and T/D. I was using a cost index of 99. This is just one flight and route but it happens on several.

[Mats_J 2]

Aaron,

...this route: SWANN V445 DQO PHLBO2 from KDCA to KEWR.

any SID? PHLBO2 sounds like a STAR? Please excuse me but I am not familiar with the airports.

I filled my flight altitude to 17,000 but the FMC automatically changed it to FL200.

This should not happen. And finally when did you receive the DESCENT PATH UNACHIEVABLE message? Cheers,

[aaronrash 0]

I didn't use a SID on departure but but PHLBO2 was the STAR for KEWR. The aircraft climbs as it should. The DESCENT PATH UNACHIEVABLE comes up shortly after VNAV has put me into descent. Usually the aircraft gets pretty "slippery" and picks up a bit of speed on descent. I usually have to use the speed breaks. Another thing thats strange though is that the auto throttles don't retard and go to idle when it should be on descent. I also noticed that it changes my flight levels by itself on shorter flights. The flight from KDCA to KEWR is around 200 miles.

[Mats_J 2]

Usually the aircraft gets pretty "slippery" and picks up a bit of speed on descent. I usually have to use the speed breaks.

Normal for the 800. If I recall correctly this is the most slippery of the series.

Another thing thats strange though is that the auto throttles don't retard and go to idle when it should be on descent.

Have in mind that flight idle is not the same as ground idle. And there is also an approach idle. I think this all modeled correctly. One thing that comes to mind is winds. What winds do you fly in. And did you put them into the FMS on the FORECAST page in the CDU? Cheers,

[CapnOz 110]

- Yeah I have the same problem, when i DONT put in the forecast winds. Figure out where to find the winds via google, enter them in and enjoy correct VNAV desents BTW- the VNAV and LNAV logic are modelled according to Boeing Documentation. And just in case that wasn't enough, they had RW NG pilots confirm its every detail. Quite obsessive- So if something isnt going " right" its MOST likely user error.

[aaronrash 0]

Yea I love the fact that it's so real that you can mess up your flight without taking real world procedures. I never put the winds into the FMC I just use the live weather from rex. I'll give it a shot. Thanks guys.Does anybody know why it's changing my flight levels though?

[Mats_J 2]

Does anybody know why it's changing my flight levels though?

No, AFAIK it should not do this. Please elaborate on the procedure you use and when you see that the FMC is changing your cruise altitude. Cheers,

[aaronrash 0]

Here's what I did. If you have time you can try it as well and confirm. Started my flight at KDCA parked at a gate.Prepared the aircraft for takeoff. Filled route: SWANN V445 DQO PHLBO2. Set my fuel to 10,000lb set reserves to 4, cost index to 99. set flaps to 5 set departure runway to 1. hit execute and it changed the FL with a message saying that it did. Strangethe altitude I filled was 17000 and it changes it to FL200

[Mats_J 2]

Will check tomorrow. Unless someone beats me to it. Time to go to bed.

[adamant365 28]

Aaron,I think you've touched on some functionality I didn't even know existed. Anyway, if you look at the PHLBO2 STAR, there is a mandatory "above FL200" restriction at DQO VOR. I guess this is why the FMC sets your cruise alt to 200. On a side note, I've always thought the arrivals into KEWR are quite steep. Following the procedure exactly makes it difficult to get down fast enough. Adam

[aaronrash 0]

Now it changed it to FL270 http://imageshack.us/photo/my-images/832/screentest001.jpg/The weird thing is I got the flight plan and altitude from flight aware and not one single flight is above 17000 feet due to it being a short flighthttp://flightaware.com/analysis/route.rvt?origin=kdca&destination=kewr

[mak256 0]

Adam, I think you are correct. The tutorial flight that comes with the documentation mentions similar waypoint altitude restrictions that are imposed during programming.

[adamant365 28]

PHLBO2 has an "at or above FL270" restriction for all transitions except PXT. I didn't see that before. Might you have selected the PXT transthe first time?? I think if you aren't going to make it all the way to the highest alt restriction, you would need to either select all the constraints that fall above your cruise alt or use a different STAR. I don't know how it would be handled in real life, but I would guess the pilot would just request 17000 from ATC since that is the procedure's MEA. Adam