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Guest dirkvanderlaan

VNAV PATH descent question

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Guest dirkvanderlaan

VNAV PATH descents and FMC actions keep amazing me. This time I was hit by the FMC on a flight from EHAM to EGLL. I was flying on a LAM3A arrival towards EGLL, descending inbound LAM from FL240. At FL100 I was, as my VNAV PATH had indicated at LAM12 ie about 12 NM before LAM decelerating from 284 kts to 250 kts. Then, for no apparent reason reaching 250 kts the VNAV disconnected itself, leveling off at FL100. My mcp alt was set at 7000 (to be level at LAM) and the target was ILL82 at 2500 ft, so I couldn't discover why the disconnection came about. I was exactly at the path as planned, flying the speed as planned, flying the heading as planned ...Anyone?

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VNAV will disconnect if it senses loss of control over path or speed, even if you think everything is as it should be. There's lot's of reasons I don't use VNAV for descents, especially during arrival, and this is one of them.I don't see a clue as to why you had a VNAV Disconnect, but neither can I see much in the way of detail either.I do not like the method that VNAV uses to deaccelerate, and this might be what happened. Hard to tell. Regardless, I like to "stay in the loop" and prefer to manually manage my arrival with the MCP controls (usually SPD and VS). I have learned to slow down during descent to the transition altitude so there's no need to raise the nose to continue below 10,000. Besides, in the real world you have ATC giving you speed for separation so that eliminates the use of VNAV in most cases.

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It's hard to read any data from the screenshot you attached. My guess why VNAV disconnected is some altitude constraint ahead giving an excessive descent angle but that should on the other hand result in a different disconnect message on the CDU. Cheers,

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