Help! cannot do a DES NOW in the 744 properly

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I am en route to Langkawi (again!) in the Queen, at FL250 and IAS 313 (CI=99)in LNAV+VNAV. At 40nm from the ToD, I hit DES NOW having entered an MCP ALT of 15000. The Queen descends promptly in VNAV PTH no more than 1000 vertical feet at the FMC mandated descent speed of IAS 290 and then levels off at FL240. Speed then falls to IAS 280...then 270...then N1 kicks in to accelerate me back to IAS 290, but no further descent occurs. The speed bleeds off again down to 280...and 270 and the cycle recurs. No descent at all.What am I doing wrong? I wondered if I should be in VNAV SPD rather than VNAV PTH but can't figure out how to change. I don't really know what I am doing!I've tried this a dozen times from different FL and it's always the same or a similar result.Appreciate any help, thanks!Jonathan

[Guest D17S]

DES NOW is broken with the old PAX airplane (You are describing the exact symptoms of the old buggy descend now routine). You have to buy the freighter addon. It's fixed there.

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Aha! OK, thanks, Sam. Guess I'll live without DES NOW; can't keep up with all the upgrades of this stuff. Jonathan