A319
Flight Crew Air Transport Pilot (ATP)
Other / Unknown; Situational Awareness
Deviation - Altitude Crossing Restriction Not Met; Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Flight Crew Became Reoriented; Flight Crew Returned To Clearance
Aircraft; Human Factors
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.
A319 Captain reports descending below 11;000 FT prior to KRANN on the KRANN 3 arrival to BOS.