R ENG WAS SHUT DOWN ON A C551 WHEN THE FLC NOTICED OIL PRESSURE INDICATIONS BELOW OPERATING LIMITATIONS. APCH CTLR WAS INFORMED OF ACFT CONDITION AND FLT DIVERTED. ACFT EQUIP PROB CRITICAL LOW ENG OIL PRESSURE.

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I HAD RECEIVED FLT ASSIGNMENT TO COVER FLT FOR FO (ON CHK RIDE). THIS REQUIRED ME TO GO FROM ARRIVING FLT (FROM 2 DAY TRIP) TO DEPARTING FLT AND RELY ON CAPT TO FILE FLT PLAN AND PREFLT ACFT. I WAS TO FLY TO DEST (35-40 MIN FLT) WITH CAPT TO FLY RETURN TRIP TO BASE. PRIOR TO START; CAPT INFORMED ME THAT THIS ACFT HAD MAINT PERFORMED ON R ENG AFTER HIS TRIP PREVIOUS DAY; FOR ENG BLADE PROB. NORMAL START; NO ABNORMAL LIGHTS; TKOF AND CLB NORMAL; ONLY 1-2 INCH THROTTLE POS DIFFERENTIAL FOR IDENTICAL FAN SETTINGS. UPON REACHING CRUISE ALT OF 17000 FT; CAPT AND I NOTED THE VIRTUALLY IDENTICAL SETTINGS (FAN; TURBINE; INTER-STAGE TURBINE TEMP; OIL TEMP AND PRESSURE) FOR THE L AND R ENGS. APCH WAS BUSY; WITH MODERATE TURB IN DSCNT. PALM SPRINGS APCH WAS ASSIGNING VECTORS TO OTHER CORP ACFT INTO UDD. PERFORMED L TFC ENTRY INTO DOWNWIND FOR RWY 10; CANCELED IFR; RECEIVED FREQ CHANGE TO UNICOM AND CTAF. AT 1500 FT; RECEIVED MASTER WARNING; LOW R ENG OIL PRESSURE ANNUNCIATOR LIGHT. CONFIRMED WITH CAPT OIL PRESSURE GAUGE READING OF 10-20 PSI (MINIMUM IS 40 PSI) AND WE AGREED FOR IMMEDIATE ENG SHUTDOWN (PRECAUTIONARY) EVEN THOUGH ENG WAS PRODUCING SOME THRUST. AS ACFT WAS HEADING DIRECTLY TOWARDS PSP; 5- 10 MI AWAY; WE AGREED TO DIVERT BECAUSE OF LONGER RWY; CTL TWR; PROX OF OTHER CORP ACFT AND ENTRY POS; AND EMER SVCS AVAILABLE. PERFORMED LOW OIL PRESSURE AND ENG SHUTDOWN CHKLIST. CAPT CONTACTED PALM SPRINGS APCH; INFORMED THEM OF PRECAUTIONARY SHUTDOWN OF R ENG; DID NOT DECLARE EMER NOR REQUEST PRIORITY. I CLBED ACFT TO 3000 FT AGL; AND WE WERE CLRED DIRECTLY TO ARPT AND RWY 13 FOR DOWNWIND ENTRY AND HANDED OFF TO PALM SPRINGS TWR. TWR CLRED US TO LAND. ACFT WAS IN GEAR DOWN AND FLAPS APCH CONFIGN WHEN SHUTDOWN WAS INITIATED; AND LEFT THAT WAY DUE TO RESPONSIVENESS OF CTLS AND L (GOOD) ENG. ACFT WAS MANEUVERED TO UNEVENTFUL LNDG AND TAXIED TO FBO. MET BY EMER FIRE AND POLICE VEHICLES. POLICE REQUIRED MY NAME; HOME ADDRESS; HOME PHONE; COMPANY NAME; COMPANY PHONE; DRIVER'S LICENSE; PLT'S LICENSE; AND REASON FOR ENG SHUTDOWN. MY RECENT UPGRADE TRAINING TO CAPT APPROX 3 MONTHS EARLIER; FAMILIARITY WITH CAPT; NUMBER OF HRS IN TYPE; AND CAPT AND MY ADHERENCE TO COMPANY OPS SPECS; FARS; AND MANUFACTURER AND TRAINING PROCS; MADE THE SIT UNEVENTFUL AND 'JUST LIKE TRAINING.' CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR DOES NOT KNOW WHAT CAUSED THE LOW OIL PRESSURE INDICATION. THE C551 WAS PARKED AT PSP AND PRATT AND WHITNEY MECHS PERFORMED THE MAINT. THE RPTR HAS NOT RECEIVED ANY FURTHER INQUIRY REGARDING THE INFLT PRECAUTIONARY ENG SHUTDOWN. RPTR WAS ASKED WHY HE DIDN'T DECLARE AN EMER. HE STATED THAT HE DID NOT WANT TO CALL ATTN TO HIMSELF. ANALYST STATED WHY DECLARING AN EMER WAS A GOOD IDEA.