A md-80 failed to capture the digital flight guidance panel selected altitude nor did the pilots mon...

Narrative:

On descent; our clearance was to level at FL250 but for whatever reason; the autopilot went into 'capture' mode but never leveled the aircraft and stop the descent. We descended approximately 300 ft below our assigned altitude before the first officer (his leg) disengaged the autopilot and leveled the aircraft at FL250. The controller did query us about our altitude but we were already level by the time he asked and I accurately told him that we were level at FL250. Both the first officer and I observed the capture mode but took our heads away from the FMA just long enough to miss the fact that I did not level and hold the altitude. Neither one of us was distracted by anything in particular nor were we focusing our attention on something we should not have been. Unfortunately; we both assumed the autopilot would do what it was supposed to and; at that moment; while the first officer turned his head to look over at the charts; I just happened to reach down to pick up the handset so I could make a PA. I'm not sure why the autopilot didn't level the aircraft. It could have been operator error but both the first officer and I believe we saw the autopilot capture the altitude. My hands were nowhere near the digital flight guidance panel and I never saw the first officer touch anything that would disengage the altitude capture/hold mode. If it was operator error; neither of us recognized it. The autopilot behaved normally before and after that so if it was an anomaly in the system; it only presented itself at that particular moment. Regardless of how it happened; I realize that the real issue is the fact that we both took our eyes away from the instruments at the most inopportune time. I've been flying the MD80 for 16 years now and have heard the mantra about how important it is to monitor the moment of level-off more times than I can count; yet here I am submitting this report. Will this event cause me to be even more vigilant during level-off? Sure. Will it be enough to counter the 'human element' that will eventually come back to bite me in spite of my diligence? I wish.

Original NASA ASRS Text

Title: A MD-80 failed to capture the digital flight guidance panel selected altitude nor did the pilots monitor it; the aircraft descended 300 FT below the assigned altitude.

Narrative: On descent; our clearance was to level at FL250 but for whatever reason; the autopilot went into 'capture' mode but never leveled the aircraft and stop the descent. We descended approximately 300 FT below our assigned altitude before the First Officer (his leg) disengaged the autopilot and leveled the aircraft at FL250. The Controller did query us about our altitude but we were already level by the time he asked and I accurately told him that we were level at FL250. Both the First Officer and I observed the capture mode but took our heads away from the FMA just long enough to miss the fact that I did not level and hold the altitude. Neither one of us was distracted by anything in particular nor were we focusing our attention on something we should not have been. Unfortunately; we both assumed the autopilot would do what it was supposed to and; at that moment; while the First Officer turned his head to look over at the charts; I just happened to reach down to pick up the handset so I could make a PA. I'm not sure why the autopilot didn't level the aircraft. It could have been operator error but both the First Officer and I believe we saw the autopilot capture the altitude. My hands were nowhere near the digital flight guidance panel and I never saw the First Officer touch anything that would disengage the altitude capture/hold mode. If it was operator error; neither of us recognized it. The autopilot behaved normally before and after that so if it was an anomaly in the system; it only presented itself at that particular moment. Regardless of how it happened; I realize that the real issue is the fact that we both took our eyes away from the instruments at the most inopportune time. I've been flying the MD80 for 16 years now and have heard the mantra about how important it is to monitor the moment of level-off more times than I can count; yet here I am submitting this report. Will this event cause me to be even more vigilant during level-off? Sure. Will it be enough to counter the 'human element' that will eventually come back to bite me in spite of my diligence? I wish.

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.

Attributes
ACN	840712
Time
Date	200906
Local Time Of Day	0601-1200
Place
Locale Reference	ZZZ.ARTCC
State Reference	US
Environment
Flight Conditions	VMC
Aircraft 1
Make Model Name	MD-80 Series (DC-9-80) Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Flight Phase	Descent
Flight Plan	IFR
Component
Aircraft Component	Altitude Hold/Capture
Person 1
Function	Captain Pilot Not Flying
Person 2
Function	First Officer Pilot Flying
Events
Anomaly	Aircraft Equipment Problem Less Severe Deviation - Altitude Overshoot Deviation - Procedural Clearance

A md-80 failed to capture the digital flight guidance panel selected altitude nor did the pilots monitor it; the aircraft descended 300 ft below the assigned altitude.