What was found
ENGINE C-BOX CHIP LIGHT CAME ON IN-FLIGHT. AIRCRAFT WAS ON FINAL TO WORK SITE WHEN C-BOX OIL PRESSURE DROPPED TO ZERO. MAINTENANCE INSPECTED CHIP PLUG AND OIL FILTER. C-BOX WAS REMOVED FROM SERVICE AND SENT FOR REPAIR. POSSIBLE OIL PUMP SHAFT DRIVE FAILURE OR GEAR FAILURE. OIL SAMPLES SHOWED SLIGHT INCREASE IN IRON OVER PAST 100 HOURS. OIL SAMPLES WERE TAKEN EVERY 25 HOURS. (OVERHAUL IS 4,000 HRS).