UNCOMMANDED ROLL DURING STRAIGHT AND LEVEL FLT.

Narrative

DURING CRUISE FLT IN MODERATE TURB ON 'A' AUTOPLT IN 'HEADING COMMAND' ACFT ROLLED ABRUPTLY 45 DEGS R WING DOWN AND WOULD NOT RECOVER ON AUTOPLT. RECOVERY MADE MANUALLY. POST FLT: MECHS FOUND THAT A RATE GYROSCOPE HAD FAILED. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATED THE ROLL INTO THE 45 DEG BANK WAS SMOOTH AND NOT ABRUPT. WHEN IT ACHIEVED ITS MAX BANK IT SIMPLY STAYED THERE AND DID NOT MAKE ANY ATTEMPT TO ROLL BACK TO LEVEL WHERE IT WAS COMMANDED TO BE. RPTR WAS PF AND STATED THAT AS THE ACFT ROLLED INTO THE BANK THE OTHER FLC MEMBERS WERE SAYING; 'DO SOMETHING ABOUT IT.' BECAUSE OF THE TURB AND IMC CONDITIONS NO ONE ELSE BESIDES THE FLC EVEN KNEW IT EVER HAPPENED. RPTR RECOVERED FROM THE BANK BY DISCONNECTING AUTOPLT. AFTER RETURNING TO STRAIGHT AND LEVEL FLT AND REGAINING THE 500 FT LOST DURING THE ROLLING BANK; HE ENGAGED THE AUTOPLT AGAIN AND IT ROLLED SLOWLY INTO A 20 DEG BANK. AT THIS TIME HE DISCONNECTED THE 'A' AUTOPLT AGAIN; RETURNED THE ACFT TO STRAIGHT FLT MANUALLY; THEN ENGAGED THE 'B' AUTOPLT. AFTER LNDG AND MAINT MADE CORRECTION; HE WAS INFORMED THE RATE GYRO HAD FAILED. THIS IS WHAT CAUSED THE UNCOMMANDED ROLL.