Narrative:

First officer was pilot flying. On climbout; the autopilot leveled off at intermediate altitude by going directly to altitude hold without passing through altitude cap mode. I discussed this with the captain -- that it was odd and that we should be vigilant. The autopilot performed normally for the remainder of the flight until attempting to level off at 6000' MSL on approach. The captain had already made the 'prepare for landing' PA by this time. With slats extended/flaps retracted; autopilot on and 6000' MSL set and armed; the aircraft descended through 6000' MSL without announcing altitude cap or altitude hld. Captain (pilot monitoring) and I (pilot flying) recognized the problem at the same time. The following items happened in one quick flow of events. Thinking that there might be a trim problem I look for the ap trim light which was not illuminated (a later bulb test was good) -- while at the same time clicking off the autopilot in order to intervene and stop at the descent. Having experienced the flight control 'bump' that occasionally accompanies an autopilot disconnect; I got a good grip on the yoke before I clicked off the autopilot. As the autopilot disconnected; the yoke 'pulled' backwards into my hands. I would not characterize that control yoke movement as an uncompounded flight control movement. A better description would be that the flight controls appeared to be trimmed nose up -- definitely more than might be expected with ap trim light illuminated -- and required prompt application of nose down trim which seemed to correct the problem completely. The nose pitched up more rapidly than I would have manually commanded but I was able to control the nose up pitch motion and leveled the aircraft at 6000' MSL per the clearance without having to 'push over.' our excursion from the clearance altitude was 200 ft low. I finished trimming the aircraft and continued the approach. I felt nothing unusual in the aircraft flight controls or trim system thereafter. Shortly after the autopilot disconnect; we were notified by the lead flight attendant that a flight attendant had fallen at the aft flight attendant position and hurt her head. The captain immediately notified ATC that we would require paramedic response to our gate.

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Original NASA ASRS Text

Title: A MD80 autopilot altitude hold system malfunctioned preventing the MCP altitude set from being captured. When First Officer disconnected the autopilot to halt the descent the autopilot had trimmed nose up. As the aircraft pitched up an aft Flight Attendant fell and injured her head.

Narrative: First Officer was pilot flying. On climbout; the autopilot leveled off at intermediate altitude by going directly to ALT HOLD without passing through ALT CAP mode. I discussed this with the Captain -- that it was odd and that we should be vigilant. The autopilot performed normally for the remainder of the flight until attempting to level off at 6000' MSL on approach. The Captain had already made the 'Prepare for landing' PA by this time. With slats extended/flaps retracted; autopilot on and 6000' MSL set and armed; the aircraft descended through 6000' MSL without announcing ALT CAP or ALT HLD. Captain (pilot monitoring) and I (pilot flying) recognized the problem at the same time. The following items happened in one quick flow of events. Thinking that there might be a trim problem I look for the AP TRIM light which was NOT illuminated (a later bulb test was good) -- while at the same time clicking off the autopilot in order to intervene and stop at the descent. Having experienced the flight control 'bump' that occasionally accompanies an autopilot disconnect; I got a good grip on the yoke before I clicked off the autopilot. As the autopilot disconnected; the yoke 'pulled' backwards into my hands. I would not characterize that control yoke movement as an uncompounded flight control movement. A better description would be that the flight controls appeared to be trimmed nose up -- definitely more than might be expected with AP TRIM light illuminated -- and required prompt application of nose down trim which seemed to correct the problem completely. The nose pitched up more rapidly than I would have manually commanded but I was able to control the nose up pitch motion and leveled the aircraft at 6000' MSL per the clearance without having to 'push over.' Our excursion from the clearance altitude was 200 FT low. I finished trimming the aircraft and continued the approach. I felt nothing unusual in the aircraft flight controls or trim system thereafter. Shortly after the autopilot disconnect; we were notified by the Lead Flight Attendant that a Flight Attendant had fallen at the aft Flight Attendant position and hurt her head. The Captain immediately notified ATC that we would require paramedic response to our gate.

Data retrieved from NASA's ASRS site as of April 2012 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.