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saltypete

Strange Vnav behavior

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Hi everyone, just encountered a wierd situation on approach to VHHH. I took the simulator off pause at the TOD and for about 5000 feet the descent went fine, then all of a sudden the plane levelled out above the descent path and the airspeed rocketed up and would have overspeeded if i hadn't intervened. I took it off V nav and used V/S , speed select and speedbrakes until the aircraft got back on track, when i reconnected V Nav. This proceeded to happen again and i lost the autothrottle ( no thrust annucniation on the left top the PFD) i rearmed the autothrottle but to no avail. I had to use manual throttle control all the way to the runway. I had no constraints entered into the FMC. this has never happened to me before i have usually been able to use V nav all the way down a descent to final. has anyone else encountered this problem? i did a search but was presented with 51 pages of results.RegardsPete

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If autothrottle is unable to control speed then VNAV will trip off. Maybe this was your case. I always use V/S and MCP SPD on my descents and never require speedbrakes. I only use VNAV for climbs.

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Did you happen to notice what the wind were? From the description it would seem that you had a significant taiwind component that was causing difficulty staying on the calculated descent path. CheersSteve

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Not sure about the tailwind , i was flying using the clear skies weather conditions and the winds aloft are never more than about 25 knots. i will try and replicate this behavior again today, it may just be a one off (touch wood!!)RegardsPete (YBBN)

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