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.
Synopsis
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.
Second reporter narrative
Descending on the arrival we were given a clearance FL250. I; as the flying pilot; monitored to descent to assure altitude restriction and level off. The autopilot failed to capture 250 and the preselect altitude. I immediately disconnected the autopilot and leveled the aircraft. During this process the aircraft went below the cleared altitude by about 250-300 FT before being corrected back to 25;000 FT.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.