As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
PIT - FLT 1004 - AFTER TAKEOFF FROM PIT, EN ROUTE TO MKE, THE LEFT IRS FAILED IN FLIGHT. DECISION WAS MADE TO RETURN TO FIELD AT PIT. FLIGHT LANDED WITHOUT FURTHER INCIDENT. NO EMERGENCY WAS DECLARED. MAINTENANCE REMOVED AND REPLACED THE LEFT INERTIAL REF UNIT (IRU). FUNCTIONAL CHECK SATISFACTORY. (M)