A DISTR B727 PIC FOLLOWS HIS ERRANT #1 COMPASS AND TURNS AWAY FROM THE DEP HDG THAT WAS ASSIGNED BY DEP CTLR AT TUS; AZ.

Narrative

AFTER TKOF WE STARTED TO CLEAN UP THE ACFT. WE WERE GIVEN A CLRNC TO 'FLY HDG 120 DEGS AND CLB AND MAINTAIN 17000 FT MSL.' ATC GAVE US A FREQ CHANGE AND THE PF CALLED FOR FLAPS TO 5 DEGS. WHILE I WAS COMPLYING WITH BOTH; THE CAPT WAS FLYING THE ACFT; RESPONDING TO WHAT HE DESCRIBED AS UNUSUAL PERFORMANCE CHARACTERISTICS FOR OUR ADVERTISED WT. WHILE OUR ATTN WAS DIVERTED; THE #1 COMPASS PRECESSED TO ABOUT 70 DEGS L. THE CAPT STARTED A SLOW L TURN TO MAINTAIN THE ASSIGNED HDG (NOT REALIZING THE COMPASS PROB). AS I LOOKED AT MY HSI; I STATED THAT MY COMPASS WAS IN DISAGREEMENT AND WE THEN CHKED THE WHISKEY COMPASS. AT THIS TIME; ATC QUERIED US AND WE ANNOUNCED THAT WE HAD A COMPASS PROB AND RETURNED TO AN APPROX SE HDG. WE CHKED CIRCUIT BREAKERS AND RESET OUR COMPASSES (BOTH). THE PROB WAS SOLVED BUT NOT BEFORE DEVIATING APPROX 60 DEGS FROM HDG. I BELIEVE THAT MINOR CHANGES IN THE NORMAL DEP AND A 'HEADS DOWN' SIT FOR ME (FO) LED TO AN INABILITY TO BACK UP THE PF WITH THE USE OF CORRECT HDG INDICATIONS. THE CAPT'S DIVERSION OF ATTN CAUSED BY BETTER THAN ANTICIPATED PERFORMANCE WAS JUST ENOUGH TO SUBDUE HIS SITUATIONAL AWARENESS ALLOWING THE MALFUNCTIONING COMPASS TO TRICK HIM INTO TURNING SIGNIFICANTLY AWAY FROM THE APPROX RWY HDG. ALSO; A CORRECT ATC CLRNC CAME RIGHT AT A TIME WHERE IT SEEMED TO CORROBORATE A L TURN DIRECTION. AS A SIDE NOTE; BOTH COMPASSES WERE ALIGNED WITH RWY HDG AND BOTH HDG BUGS WERE ON THE R HDG. THE MALFUNCTION OCCURRED SOMETIME AFTER ROTATION.