The flight crew of an md-83; distracted by searching for traffic; suffered a momentary excursion fro...

Attributes
ACN	990308
Time
Date	201201
Local Time Of Day	1201-1800
Place
Locale Reference	ZZZ.Airport
State Reference	US
Environment
Flight Conditions	VMC
Aircraft 1
Make Model Name	MD-83
Operating Under FAR Part	Part 121
Flight Phase	Initial Approach
Flight Plan	IFR
Component
Aircraft Component	Altitude Hold/Capture
Person 1
Function	First Officer Pilot Flying
Qualification	Flight Crew Air Transport Pilot (ATP)
Person 2
Function	Pilot Not Flying Captain
Qualification	Flight Crew Air Transport Pilot (ATP)
Events
Anomaly	Aircraft Equipment Problem Less Severe Deviation - Altitude Excursion From Assigned Altitude Deviation - Procedural Clearance

Narrative:

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 altitude 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.

Original NASA ASRS Text

Title: 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.

Narrative: 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.

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.

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.