Narrative:

I was pilot monitoring during a krann 3 arrival into boston. The aircraft was operating in the FMS mode; managed altitude and airspeed; with number 2 autopilot selected. Prior to commencing descent from cruise altitude; the first officer and I had completed a thorough verification of the planned descent profile on the krann 3 arrival with no discrepancies noted on nd or mcdu. The first constraint was to cross krann at 11;000 ft and 270 KIAS; which was confirmed. However; prior to reaching krann; the pilot flying noticed that the aircraft was descending below 11;000 ft. He disconnected the autopilot and initiated an immediate climb at approximately 10;300 ft. The automation was reset at 11;000 ft and the arrival continued. I did not notify ATC of the deviation; nor were we advised by them of any altitude deviation or traffic conflict. At that point; the first officer and I tried to assess exactly why the aircraft had descended below the previously set and confirmed altitude restriction; but could not determine any procedural error or equipment malfunction. The aircraft was programmed properly to fly the krann 3 arrival in the FMS mode and was verified to do so. I did not notice any impending indication on the nd or pdf that the aircraft would not level off at 11;000 ft as planned. However; I did not closely monitor the expected level off as the aircraft actually reached 11;000 ft. As a result; an altitude deviation of about 700 ft occurred before corrective action was taken by the pilot flying. In the future; I will monitor all automated altitude level off's closely to ensure that the aircraft does; in fact; level off at the planned and programmed altitude.

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Original NASA ASRS Text

Title: A319 Captain reports descending below 11;000 FT prior to KRANN on the KRANN 3 arrival to BOS.

Narrative: I was pilot monitoring during a KRANN 3 arrival into Boston. The aircraft was operating in the FMS mode; managed altitude and airspeed; with Number 2 autopilot selected. Prior to commencing descent from cruise altitude; the First Officer and I had completed a thorough verification of the planned descent profile on the KRANN 3 arrival with no discrepancies noted on ND or MCDU. The first constraint was to cross KRANN at 11;000 FT and 270 KIAS; which was confirmed. However; prior to reaching KRANN; the pilot flying noticed that the aircraft was descending below 11;000 FT. He disconnected the autopilot and initiated an immediate climb at approximately 10;300 FT. The automation was reset at 11;000 FT and the arrival continued. I did not notify ATC of the deviation; nor were we advised by them of any altitude deviation or traffic conflict. At that point; the First Officer and I tried to assess exactly why the aircraft had descended below the previously set and confirmed altitude restriction; but could not determine any procedural error or equipment malfunction. The aircraft was programmed properly to fly the KRANN 3 arrival in the FMS mode and was verified to do so. I did not notice any impending indication on the ND or PDF that the aircraft would not level off at 11;000 FT as planned. However; I did not closely monitor the expected level off as the aircraft actually reached 11;000 FT. As a result; an altitude deviation of about 700 FT occurred before corrective action was taken by the pilot flying. In the future; I will monitor all automated altitude level off's closely to ensure that the aircraft does; in fact; level off at the planned and programmed altitude.

Data retrieved from NASA's ASRS site as of July 2013 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.