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Guest FattyBear

I just can't get this thing to descend right

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Guest FattyBear

Hi. I've gone through the tutorials, I know how to operate the NG, but for some reason, when I let VNAV handle the descent it always gets an overspeed disconnect or a "descent path unachievable". The last one I got was from KLAX-KSFO, I had perfect sid/stars and all that jazz, but somehow VNAV disconnected.

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Hi AlexWell the real VNAV behaviour is not easy to understand either. There are real world pilots who doesn't use it because they don't feel they have control of what is happening so you're not alone.Regarding the OVERSPEED DISCONNECT. With VNAV PATH DESCENT you have to watch the speed. The A/P will only try to keep the path. If you are over any speed restrictions i.e. the standard 240/10000 you will have a SPD DISCONNECT if speed is exceeding vmax-1 KIAS i.e. VNAV will disconnect when you are close to the barber pole speeds.Regarding VNAV PATH UNACHIEVABLE I would suspect you have some altitude constraints on your way down. This can screw the path up if they are not set to keep the path within limits. Once the soft altitude constraints are implemented in the -800/900 this will hopefully change.Hope it helps,

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Guest HepburnUGA

I too often get VNAV PATH UNACHIEVABLE. I almost always get it during a VNAV flown non-precision approach with a glideslope greater than 3 degrees. I program in the altitude constraints and carefully manage my speed, but I haven't figured out the logic for that error mesage yet. I would love to hear some insight as to how I could adjust my flight to get around this.Thanks!

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Jeff simply paste you EXACT route for us to see including all restraints speeds etc and most certainly will fall under the below statement. "DES PATH UNACHIEVABLE After a descent is active, the FMC predictions show the profile contraints at the next wpt cannot be made and the path maintained. Displayed if the required flight path angle to meet the next altitude contraint exceeds 7* for aircraft operation when more than 1,000' above the airport speed restriction (240/10000') altitude; otherwise 6* VNAV will disengage (CWS)." NG Guide page 213 Bill Bulfer. Amend you profile to be within VNAV tolorance.. [h4]Best Wishes,Randy J. Smithhttp://www.precisionmanuals.com/images/forum/betaimg.jpg

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Alex,A *key* to keep in mind, you never let the AUTOS do anything for you but with you. You are responsible for your speed and the monitoring on DES. Above the airport restriction (240/10000') VNAV PTH DES can go as high as 1 knot prior to the POLL (your speed reaching the red waring bricks on the spd tape) before reversion to LVL CH pitch. So if your DES target is say 280 you have much room for speed above this before VNAV will disengage. You can simply IGNORE the FMC message of "DRAG REQUIRED" in this instance and you will not loose VNAV. The issue comes below the airport restriction altitude (240/10000') where VNAV will not stay engaged if you are more than 15 knots faster than DES target. You MUST get out the boards in such cases and take action if needed to help slow the plane down within tolerance of the target SPD. There are many ways one can do this and in that is the challenge of being a pilot ;) (or at least a sim pilot hehe)[h4]Best Wishes,Randy J. Smithhttp://www.precisionmanuals.com/images/forum/betaimg.jpg

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