As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
SECONDARY ATTITUDE AIR DATA REFERENCE UNIT (SAARU) FAILURE TWO HOURS AFTER DEPARTING ORD EN ROUTE LHR. RETURNED TO ORD. SUPPLEMENTAL DATA UPDATE SAARU WAS REPLACED AND AIRCRAFT RETURNED TO SERVICE. INVESTIGATION BY MANUFACTURER REVEALED A FAILURE OF THE X GYRO IN THE SAARU.