What was found
IAD/BOS - FLT 6418 - DURING CRUISE AT 1125 HOURS ON 6-27-97, THE FLIGHT CREW EXPERIENCED A LT MAIN DUCT FAIL LIGHT INDICATION. THE AIRCRAFT RETURNED TO IAD WHERE THE MAINTENANCE DEPARTMENT WAS NOTIFIED OF THE DISCREPANCY AND BEGAN TROUBLESHOOTING THE SYSTEM AND FOUND THE O-RINGS ON THE LT INBOARD LEADING EDGE HAD FAILED. MAINTENANCE REPLACED THE O-RINGS AND PERFORMED AN OPERATIONAL TEST OF THE SYSTEM IAW THE MAINTENANCE PROCEDURES. AIRCRAFT WAS THEN RELEASED AND RETURNED TO SERVICE. (X)