What was found
OPERATOR NOTED NR 2 ENGINE LOW OIL PRESSURE IN FLIGHT. ENGINE POWER SETTING WAS REDUCED TO IDLE AND CREW ELECTED TO RETURN TO ORIGIN. POST FLIGHT ATTEMPTS TO START NR 2 ENGINE WERE ABORTED DUE TO INABILITY TO OBTAIN PROPER ENGINE ROTATION. ENGINE SN P84313 WAS REMOVED FROM THE AC, A LOANER ENGINE WAS INSTALLED IN ITS PLACE. ENGINE SN P84313 WAS SENT TO MFG FOR FURTHER INVESTIGATION AND REPAIR. TEARDOWN/INVEST OF ENGINE REVEALED FAILURE CAUSED BY NR 4 CARBON SEAL CERAMIC RUNNER, WHICH WAS NOTED TO BE SHATTERED. NR 4 CARBON SEAL CERAMIC RUNNER FAILURE.