EMB ERJ 170/175 ER/LR
Flight Crew Air Transport Pilot (ATP)
Situational Awareness; Human-Machine Interface
Aircraft Equipment Problem Less Severe; Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Flight Crew Returned To Clearance
Company Policy; Aircraft; Chart Or Publication; Human Factors
ATC cleared us to cross ZIZZI at 16;000 FT. We acknowledged altitude descending through 17;000 FT with altitude alerter aural indication and flashing yellow indication in altitude select box on PFD. I resumed briefing the river visual approach. The aircraft descended through 16;000 FT in VNAV with autopilot engaged. The flying pilot disengaged the autopilot and corrected altitude to 16;000 FT immediately. I notified ATC that we had an autopilot malfunction and that we were correcting altitude.To 'prevent' this type of event; I feel that VNAV operations could be suspended until the software load is updated to prevent the unanticipated occurrence of ALT Select malfunctions as a function of VNAV operations.
The Flight Operations memo regarding VNAV early descents cautions against subsequent altitude restrictions causing such behavior; but no such restriction was in the flight plan. The approach altitude restrictions in place were all 'at or above'.For a few moments as the flight continued; we thought that we might have been affected by the circumstances mentioned in the VNAV memo; but on subsequent review that seems not to apply. The plane simply failed to level off on autopilot as it almost always does successfully; and I happened to be verifying chart information at just the wrong time.
While descending in VNAV to cross ZIZZI at 16;000 FT; the flight crew of an E-145 had to manually return the aircraft to its programmed crossing restriction when the autopilot failed to capture the altitude. A known FMS software flaw; currently under review; may have been a contributing factor.