What was found
(CAN) DURING A FLT ON FINAL DESCENT LDG GEAR WAS SELECTED "DOWN". NLG FAILED TO EXTEND WITH BOTH DOOR & UNSAFE LIGHT ILLUMINATED. A MISSED APPROACH WAS CARRIED OUT BY FLT CREW WITH GEAR SELECTION NOT CHANGED. DURING MISSED APROACH, APPROX ONE MIN AFTER GEAR WAS SELECTED DOWN NLG EXTENDED & LOCKED DOWN WITH NO INPUT FROM FLT CREW.FROM GO-AROUND AIRCRAF T MADE ANOTHER APPROACH FOLLOWED BY A LDG W/O INCID.. SUBSEQUENT ENGINEERING INVEST. REVEALED THAT NLG ACTUATOR WAS UNSERVICEABLE DUE TO A CONTAMINANT BEING FOUND INTERNALLY IN ACTUATOR. ACTUATOR WAS REPLACED, GROUND RETRACTIONS CARRIED OUT SATISFACTORILY, & A/C WAS RT-SRV.