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FreightDawg

Des Path Unachievable

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My question is i was doing a flight at Fl360 and the descent started at only 94 miles away from the airport and because the plane couldn'treach its waypoints as required i get this message Des Path Unachievableis there a way to program the fmc to start a descent farther out?? thx


John C

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I always start down about 30 miles before the TD, the FMC always seems to compute the TD too close. I usually fly around FL320, so at your altitude I would start down even earlier, use the 3-1 rule for a ball park figure.


Jay

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I have had the same issue. I thought I had made some bad programming between UN623 and the STAR for ENBR RWY17. For the occation, a lot of the altitudes was AT or Above (A). Maybe I should have removed the A on a couple of them to force it. I noticed that the plane could have descended a lot faster than it did at the moment Des Path U was announced.


Andy Hagen

ENGM

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This is an advisory message as opposed to an alerting message !When in a path descent and above the path, the FMC predictions show the profile restrictions at the next waypoint cannot be achieved (LNAV remains engaged)The solution is to modify the restrictions !Fred.


Frederic Steiner.

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you could hit descend now in the FMC good question though
I'll try that next time, seems to me the fmc would compute the correct descent range and vnav would take it all the way in to glidescope

John C

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I always let the VNAV commence the descent and I let it start at T/D. Never had an issue, except pretty much all the times the FMC bothers me with " drag required ".

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have raised this as an issue before. never had this problem with the 747X or LevelD 767 or even the pmdg 737NG. most of the time I've already plugged in a STAR and the route is well mapped out so I don't understand why this should occur because I fly the same routes with the above a/c. Surely in a bird as advanced as the NGX, it should be able to figure this out even if I don't put in winds aloft, etc. Still don't have any answer and its my one gripe with the NGX.


J Thomas YBBN

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I always use 80-90 as cost index ( I wanna be fast ) . But I have only drag required " issues ".

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If you want an accurate descent profile, then you will need to use forecast winds at various altitudes. That way, and only that way does the FMS have any real chance of computing the profile accurately. I remember this with the MD11X. It is you might say a legacy of the accuracy of the simulation we have the opportunity of using. Tail winds on descent really stuff the descent profile up if not accounted for in forecast. Andrew


Andrew Entwistle

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Another mate in my thread gave me the same suggestion, I'll will that for sure.Thanks and it's a pleasure to learn each day a new thing anyway in my drag required thread I have placed a question regarding why the forecast does not auto update the FMC VNAV f.i. 30nm before T/D.

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Just finished a flight from London Luton rw26 to Amsterdam Schiphol rw06.The descent went fine till 14.000 ft and then the plane was descending to slow . The approach was the same thing.The aircraft did not slow down enough to use the flaps.So I lowered the gear way to soon and set the flaps to max to descent faster and slow down.In this way I could make a good landing.The AT kept the speed continuesly 20-30knots above the speed the CDU showed.The moment I used the airbrakes to slow down the speed , the throttles even came up a bit.Two days ago I had the same issue with the same flight.Never had it before SP1.


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I always use 80-90 as cost index ( I wanna be fast ) . But I have only drag required " issues ".
Try use 30!! According real pilots form this Forum this would be the standard CI! GOL in Brazil have used 27!

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The 'drag required' is why I start early, I dont like having to 'pop the boards' just to stay on profile. I use 100 CI cause Im not buying the fuel, and start down at 290kts and VS of 1500 FPM, about 30-35 miles before the computed TD, initially. At that descent rate the plane will respond to any speed changes just fine. By the time I am below FL200 or so, I start adjusting the VS to keep the 'banana' where I want it.Not only does everything work fine this way, I get to actually do something while flying instead of just monitoring a 200K computer :-) It may not be 'real world' correct, but it works for me, I have never had a single 'descent unable' or 'drag required' message yet.


Jay

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