What was found
OAK - UPON ARRIVAL AT BILLINGS, LEFT MAIN GEAR GREEN LIGHT DID NOT ILLUMINATE. GEAR UNSAFE RED LIGHT DID NOT ILLUMINATE UPON THROTTLE BACK AT TOP OF DESCENT. PERFORMED LOW APPROACH AT BILLINGS FOR MX AND TOWER VERIFICATION OF GEAR EXTENSION. GEAR APPEARED DOWN, NO HORN SOUNDED. LANDING WAS UNEVENTFUL. (M)