As part of my flight into KSFO today, I found that VNAV didn't behave nicely on approach from down to OSI (Woddside VOR). After passing DUXBY at 240/8000, VNAV did not descend me properly toward OSI; it dawdled in this segment of the descent, so to speak, and maintained a RoD of a mere -350 fpm instead of proceeding down smartly to 200/5000. I received no alerts anywhere on the FMC, or the PFD during this period. I could not figure it out. I finally hit FLCH and descended at an excessive -2300 fpm toward OSI and got a DRAG REQUIRED annunciation that I followed faithfully, and with judicious flap management as well, all went fine. Did I do something wrong with my waypoint management or was VNAV just being naughty for the evening?Jonathan

Jonathan,There may be other reasons, but there is an issue with the AFDS under certain circumstances stops commanding correct pitch. Recycling of VNAV will help in these situations.Take a look at this thread and see if this applies to your scenario:http://forums.avsim.net/dcboard.php?az=sho...64686&mode=full

Per:Thanks for your post and link. I shall study it later today.Jonathan

OK, I looked at the linked thread. VNAV has worked well for me on most occasions, especially descent (!), and I am puzzled by the latest experience flying into KSFO.This evening, I repeated my approach into KSFO via PYE, Lozit, Duxby, OSI, etc. and at every waypoint from PYE (the first one from ToD and located at 14,000'), the descent in VNAV "stalled" and I had to "re-cycle" VNAV by clicking it OFF and then ON again a couple of moments later. This caused VNAV to work normally for each successive leg between restrictions. This has never happened before. I am sure it is a bug.Jonathan

P.S. I should point out that VNAV has NOT disconnected at any time during these "stalled" descent legs, nor has it said DES PATH UNACHIEVABLE. Jonathan