MD-83
Flight Crew Air Transport Pilot (ATP)
Situational Awareness; Human-Machine Interface; Distraction
Aircraft Equipment Problem Less Severe; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance
Flight Crew Returned To Clearance; Flight Crew Regained Aircraft Control; Flight Crew FLC Overrode Automation
Aircraft; Human Factors
Level at 11;000 FT and searching for assigned crossing traffic as we reached an intersection on the STAR. While configuring the aircraft and simultaneously turning to the outbound heading with the autopilot engaged; the aircraft exited ALT HOLD and began an uninitiated descent. We descended no more than 250 FT when I detected the descent; disengaged the autopilot and quickly returned to our assigned altitude.While we were very busy on the approach and both searching for assigned traffic we asked this aircraft to do too much; i.e. slow; configure; and turn all at the same time. This aircraft; in particular; has a tenancy to switch into uncommanded Vertical Speed mode and did so this time while we were distracted. I will be more vigilant next time I'm asking the auto flight system to multi-task.
[No substantive additional information was contained in the secondary report narrative.]
The flight crew of an MD-83; distracted by searching for traffic; suffered a momentary excursion from their assigned altitude when the autopilot exited altitude hold mode and descended until the pilot flying disconnected the autopilot and returned to their assigned altitude.