B747 LNDG FRANKFURT; DSNDS BELOW ATC ASSIGNED ALT.

Narrative

I DEVIATED APPROX 700 FT BELOW AN ASSIGNED ALT IN FRANKFURT APCH AIRSPACE WHILE HAND FLYING THE ACFT. THE DEV WAS CORRECTED IMMEDIATELY BUT WAS COMMENTED ON BY THE ATC CTLR. NO ACTION WAS REQUESTED. THE FOLLOWING IS A SYNOPSIS OF THE EVENT. I WAS FLYING THIS LEG OF THE SCHEDULED FLT. THE ACFT WAS CLRED TO DSND AND MAINTAIN FL100 BY FRANKFURT APCH WHILE ON A RADAR VECTOR N OF ETAR. THE WX WAS IMC WITH ICING CONDITIONS AND LIGHT TURB. THE AUTOPLT WAS HAVING DIFFICULTY CAPTURING THE ASSIGNED ALT AS I WAS SLOWING TO 250 KTS FOR AN EXPECTED FURTHER DSCNT. I ALSO NEEDED TO INCREASE DRAG SO I COULD INCREASE PWR FOR ENG ANTI-ICING. I DISENGAGED THE AUTOPLT; SELECTED 'ALT HOLD' AT FL100 AND AT 250 KTS CALLED FOR FLAPS 1 DEG. WHEN I WAS ABOUT TO CALL FOR FLAPS 5 DEGS; I NOTICED THAT THE LEADING EDGE FLAPS AMBER LIGHT WAS ILLUMINATED AND ASKED THE FE TO CHK HIS PANEL. WHILE RESOLVING THE LEADING EDGE FLAP ABNORMAL (SECURING THE WING AND ENG ANTI-ICE FIXED THE PROB) THE FO AND I REALIZED THAT THE ACFT WAS DSNDING FROM THE ASSIGNED ALT. THE 'ALT ALERTER' WAS NOT HEARD BY ANY CREW MEMBER. IMMEDIATE CORRECTIVE ACTION WAS INITIATED AT FL095 AND DURING THE CLB BACK TO FL100 THE APCH CTLR ASKED OUR ALT. THE FO STATED WE WERE RETURNING TO FL100. THE FLT WAS THEN CLRED TO DSND AND MAINTAIN FL060 AND HANDED OFF TO ETAR APCH CTL. FLAPS 5 DEGS WAS SELECTED; ANTI-ICE WAS RE-SELECTED AND THE DSCNT WAS INITIATED. THE ARR AND LNDG AT ETAR WAS NORMAL. I FEEL SEVERAL FACTORS LED TO THIS OCCURRENCE: 1) LACK OF SITUATIONAL AWARENESS BY THE CREW DUE TO THE DISTR OF THE FLT CTL ABNORMAL. 2) CREW FATIGUE DUE TO AN EVENING DEP FOR A 16 HR DUTY DAY WITH MARGINAL WINTER WX AT BOTH DESTS. THE NORTH ATLANTIC XING WAS DIFFICULT DUE TO ALMOST CONTINUOUS TURB AND DIFFICULT TO IMPOSSIBLE HF COMS DUE TO SUN SPOT ACTIVITY. 3) A SLOW TO RESPOND AUTOPLT DURING THE LEVELOFF. 4) A FAILURE OF CREW COORD WHEN THE FLT CTL ABNORMAL OCCURRED. 5) MY FAILURE TO MAINTAIN POSITIVE CTL OF THE ACFT. CORRECTIVE ACTIONS: 1) ADHERENCE TO COMPANY PROCS AND SAFE OPERATING PRACTICES WHILE OPERATING AND WHEN DEALING WITH ABNORMAL SITS. 2) AWARENESS OF CREW FATIGUE AND ITS EFFECTS ON SAFETY OF FLT. THIS WAS ACTUALLY DISCUSSED ON THE APCH BRIEFING. 3) CREW COORD AND CRM DURING ALL PHASES OF FLT. 4) ALWAYS MAINTAINING POSITIVE CTL OF THE ACFT.