Narrative:

After aircraft leveled at 11000 ft MSL on autoplt, with altitude hold engaged, aircraft drifted down at about 300 FPM. Vertical speed for autoplt showed '00' but 300 FPM down on aircraft's vsi. At 10700 ft the altitude alert signaled an altitude deviation, followed shortly by ZBW asking what was our assigned altitude. We responded '11000 ft,' and had already started a correction at that time. There was no traffic near us. Other pilot duties (briefing of the approach plate, VFR traffic watch, discussion of traffic handling) plus the gradualness of the uncommanded descent, caused our not catching the deviation sooner.

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Original NASA ASRS Text

Title: AN ACR TWIN JET FLC FAILS TO NOTICE THE SLOW LOSS OF ALT FROM THAT ASSIGNED WHILE ON ZBW FREQ 30 MI SE OF BOS, MA.

Narrative: AFTER ACFT LEVELED AT 11000 FT MSL ON AUTOPLT, WITH ALT HOLD ENGAGED, ACFT DRIFTED DOWN AT ABOUT 300 FPM. VERT SPD FOR AUTOPLT SHOWED '00' BUT 300 FPM DOWN ON ACFT'S VSI. AT 10700 FT THE ALT ALERT SIGNALED AN ALTDEV, FOLLOWED SHORTLY BY ZBW ASKING WHAT WAS OUR ASSIGNED ALT. WE RESPONDED '11000 FT,' AND HAD ALREADY STARTED A CORRECTION AT THAT TIME. THERE WAS NO TFC NEAR US. OTHER PLT DUTIES (BRIEFING OF THE APCH PLATE, VFR TFC WATCH, DISCUSSION OF TFC HANDLING) PLUS THE GRADUALNESS OF THE UNCOMMANDED DSCNT, CAUSED OUR NOT CATCHING THE DEV SOONER.

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.