As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
PHL - FLT 99 - EN ROUTE FROM PHL-SJU, THE NR 1 INERTIAL REFERENCE SYSTEM FAULT LIGHT ILLUMINATED. FLIGHT RETURNED TO FIELD AT PHL AND LANDED WITHOUT FURTHER INCIDENT. NO EMERGENCY WAS DECLARED. MAINTENANCE REMOVED AND REPLACED THE NR 1 INERTIAL REF UNIT. BITE CHECK GOOD. (M)