What was found
AIRCRAFT DEPARTED OFFSHORE RIG BOUND TO A SHORE BASE, THE LANDING GEAR FAILED TO RETRACT. LOWERED THE LANDING GEAR HANDLE, GEAR HAD AN UNSAFE INDICATION. USED EMERGENCY BLOWDOWN HANDLE, GEAR SHOWED SAFE INDICATION. AIRCRAFT LANDED AT DESTINATION WITH NO FURTHER INCIDENT. EMERGENCY CHECKLIST WAS USED DURING ALL PHASES OF THE MALFUNCTION. POST FLIGHT INSPECTION REVEALED THAT THE EMERGENCY RELEASE PIN ON THE LT MAIN LANDING GEAR HAD VIBRATED LOOSE FROM THE RETAINING SPRING CLIP, THUS DISABLING THE UPLOCK MECHANISM. THE PIN AND THE RETAINING CLIP WERE REPLACED AND A FUNCTION TEST OF LANDING GEAR RETRACTION/EXTENSION SYSTEM WAS CONDUCTED SATISFACTORILY. THE AIRCRAFT WAS RETURNED TO SERVICE.