Narrative:

On a flight from sfo to las, approximately 50 NM west of bty, headed direct bty for the fuzzy 4 arrival. I stepped back to the cabin for personal needs and upon returning I noticed the first officer had set FL180 in the altitude alerter and had taken a course of direct fuzzy. I was told by the first officer that ATC had assigned us direct fuzzy, cross fuzzy at FL180. In the meantime, I was busy with paperwork when asked us about our altitude. I looked at the electronic attitude director indicator and observed that we were about 15 NM west of fuzzy and still level at FL270. ATC then assigned a 360 degree turn so that we could descend to cross fuzzy at FL180. We complied without any further incident. During my absence from the flight deck the VNAV had disengaged while the first officer was maneuvering the airplane and upon my return I never noticed I was relying on the VNAV to initiate the descent, which never happened. The lesson here, don't depend on the autoflt system to fly the airplane, because, as in this case, it may have disengaged without you knowing.

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Original NASA ASRS Text

Title: MLG ACFT ON STAR ARR FAILED TO COMMENCE DSCNT TO MAKE XING RESTR DUE TO INADVERTENT DISENGAGE OF VNAV.

Narrative: ON A FLT FROM SFO TO LAS, APPROX 50 NM W OF BTY, HEADED DIRECT BTY FOR THE FUZZY 4 ARR. I STEPPED BACK TO THE CABIN FOR PERSONAL NEEDS AND UPON RETURNING I NOTICED THE FO HAD SET FL180 IN THE ALT ALERTER AND HAD TAKEN A COURSE OF DIRECT FUZZY. I WAS TOLD BY THE FO THAT ATC HAD ASSIGNED US DIRECT FUZZY, CROSS FUZZY AT FL180. IN THE MEANTIME, I WAS BUSY WITH PAPERWORK WHEN ASKED US ABOUT OUR ALT. I LOOKED AT THE ELECTRONIC ATTITUDE DIRECTOR INDICATOR AND OBSERVED THAT WE WERE ABOUT 15 NM W OF FUZZY AND STILL LEVEL AT FL270. ATC THEN ASSIGNED A 360 DEG TURN SO THAT WE COULD DSND TO CROSS FUZZY AT FL180. WE COMPLIED WITHOUT ANY FURTHER INCIDENT. DURING MY ABSENCE FROM THE FLT DECK THE VNAV HAD DISENGAGED WHILE THE FO WAS MANEUVERING THE AIRPLANE AND UPON MY RETURN I NEVER NOTICED I WAS RELYING ON THE VNAV TO INITIATE THE DSCNT, WHICH NEVER HAPPENED. THE LESSON HERE, DON'T DEPEND ON THE AUTOFLT SYS TO FLY THE AIRPLANE, BECAUSE, AS IN THIS CASE, IT MAY HAVE DISENGAGED WITHOUT YOU KNOWING.

Data retrieved from NASA's ASRS site as of July 2007 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.