ALTDEV.

Narrative

I WAS HAND FLYING THE AIRPLANE DUE TO INOP AUTOPLT BECAUSE OF FAILURE OF CAPT'S PITOT SOURCE WHICH DRIVES/PROVIDES INFO TO THE AIR DATA COMPUTER WHICH PROVIDES SIGNALS TO THE AUTOPLT. DURING THIS TIME VMO HORN SOUNDED DUE TO MALFUNCTION OF CAPT'S AIRSPD INDICATOR; TRUE AIRSPD AND STATIC AIR TEMP DATA DROPPED OFF OUR EFIS SCREENS AND WE WERE TRYING TO TROUBLE-SHOOT THE VARIOUS MALFUNCTIONS. APCH THEN CLRED US FROM 11000 FT MSL TO 10000 FT MSL. WHILE HAND FLYING THE AIRPLANE AND BEING DISTRACTED BY THE AFOREMENTIONED PROBS; I DSNDED TO 9700 FT MSL AND WAS APPRISED OF THIS BY OUR ALT ALERTER WHICH SOUNDS OFF AT 300 FT LOW AND BY CAPT WHO NOTICED MY ALT SIMULTANEOUSLY. I IMMEDIATELY CLBED BACK UP TO 10000 FT MSL. WE SAID NOTHING TO THE CTLR. THE CTLR CALLED US OUT AS TFC TO ANOTHER AIRPLANE AS 'LEVEL AT 10000 FT' (I BELIEVE HE WAS BEING CHARITABLE) AND APPARENTLY THERE WAS NO CONFLICT WITH ANY OTHER ACFT.WHY DID THIS HAPPEN? 1) PF DISTRACTED BY OVERSPD HORN. TRUE AIRSPD AND STATIC AIR TEMP DATA LOSS; AND HELPING PNF TROUBLE-SHOOT. HOW COULD THIS HAVE BEEN PREVENTED? 1) PF FLIES AIRPLANE; LETS PNF TROUBLE-SHOOT. 2) USE OF STANDARD ALT CALLOUTS (1000 FT AND 500 FT ABOVE; LEVELING).