MD-11
Flight Crew Air Transport Pilot (ATP)
Situational Awareness; Fatigue; Human-Machine Interface
Deviation - Altitude Overshoot
Flight Crew Became Reoriented
Human Factors
We were descending on the arrival. The FO (PF) decided to hand fly; announced it; and disconnected the autopilot. As we were approaching level off; the aircraft started a slight left turn away from course. He said 'What's it doing?' and went up to the FCP to select a heading back to course; then he reselected NAV. The aircraft stayed in a slight left turn and as we approached our level off he made no effort to raise the nose. I said 'There's your altitude' as we passed through it; and he raised the nose to level off. We descended 200 - 300 FT below our assigned altitude. As he was correcting he responded; 'I forgot the motion was on!' meaning he forgot he was hand flying. Nothing was heard from ATC about the deviation. He managed to remember he was hand flying for the rest of the descent and approach.For some reason the FO decided to hand fly the arrival then promptly forgot he was doing it! This was our second leg of the night; and we were definitely on the back side of our body clocks. When you even remotely feel tired; let the automation be your friend.
MD11 First Officer forgot that he was hand flying the arrival and questioned why the aircraft started to turn and did not level off.