Narrative:

[I] (captain) hand flew aircraft to FL390. Weather [was] no factor. I leveled the aircraft at fl 390. Aircraft was in trim and stable. Engaged autopilot a shortly after level off. Autopilot was maintaining speed; course and altitude with no problems. While reviewing the flight plan; within forty seconds of engaging the autopilot; the aircraft dove from FL390 for no apparent reason. At the time of the altitude excursion; the FMA on the pfd indicated FMC speed; LNAV; and VNAV path. Neither I nor the first officer were manipulating the MCP. By the time we noticed the aircraft was descending; we had lost 200 ft. I disconnected the autopilot; arrested the rate of descent and began a climb to the assigned altitude of FL390. I noticed the aircraft bottomed out at 38;640 ft before I was able to start the climb. While returning to FL390; center cleared us to fl 400. I elected to hand fly the jet to the new altitude. In trim; we re-engaged autopilot a without incident. There were no further issues with the aircraft. ATC made no mention of our altitude deviation and neither did we. We elected to continue and did so without incident. Cause of the event was completely unknown. The event was completely out of left field. Neither I nor the first officer had encountered a previous occurrence of this nature while in VNAV path with no inputs to the MCP. Automation can lead to complacency; although that was not the case in this event. Frequent scans of the pfd; FMA; nd; and aircraft systems are a vital part of maintaining situational awareness and staying ahead of the aircraft; especially on long flights. Vigilance is the key word; you've got to stay on your toes. When things happen unexpectedly; you must maintain aircraft control; analyze the situation and take corrective action. This may take a few moments; but; in the final analysis when the automation goes south; kick off the magic; gee-whiz stuff and fly the jet. Hand flying the aircraft in a permissive environment insures you have a feel for how the aircraft responds in different phases of flight and configurations and gives you the confidence to do just that...fly the aircraft; even without the automation.

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Original NASA ASRS Text

Title: A B737-700 had an uncommanded descent shortly after Autopilot 'A' was engaged at FL390 while in VNAV PATH. The Captain disconnected the autopilot and returned to assigned altitude after aircraft descended 360 FT.

Narrative: [I] (Captain) hand flew aircraft to FL390. Weather [was] no factor. I leveled the aircraft at FL 390. Aircraft was in trim and stable. Engaged Autopilot A shortly after level off. Autopilot was maintaining speed; course and altitude with no problems. While reviewing the flight plan; within forty seconds of engaging the autopilot; the aircraft dove from FL390 for no apparent reason. At the time of the altitude excursion; the FMA on the PFD indicated FMC SPD; LNAV; and VNAV PATH. Neither I nor the First Officer were manipulating the MCP. By the time we noticed the aircraft was descending; we had lost 200 FT. I disconnected the autopilot; arrested the rate of descent and began a climb to the assigned altitude of FL390. I noticed the aircraft bottomed out at 38;640 FT before I was able to start the climb. While returning to FL390; Center cleared us to FL 400. I elected to hand fly the jet to the new altitude. In trim; we re-engaged Autopilot A without incident. There were no further issues with the aircraft. ATC made no mention of our altitude deviation and neither did we. We elected to continue and did so without incident. Cause of the event was completely unknown. The event was completely out of left field. Neither I nor the First Officer had encountered a previous occurrence of this nature while in VNAV PATH with no inputs to the MCP. Automation can lead to complacency; although that was not the case in this event. Frequent scans of the PFD; FMA; ND; and aircraft systems are a vital part of maintaining situational awareness and staying ahead of the aircraft; especially on long flights. Vigilance is the key word; you've got to stay on your toes. When things happen unexpectedly; you must maintain aircraft control; analyze the situation and take corrective action. This may take a few moments; but; in the final analysis when the automation goes south; kick off the magic; gee-whiz stuff and fly the jet. Hand flying the aircraft in a permissive environment insures you have a feel for how the aircraft responds in different phases of flight and configurations and gives you the confidence to do just that...fly the aircraft; even without the automation.

Data retrieved from NASA's ASRS site as of April 2012 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.