THE FLT CREW OF AN E120 EXPERIENCE DEGRADED OIL PRESSURE ON THE R (#2) ENGINE. FOLLOWING QRH PROCS; POWER ON THE AFFECTED ENGINE IS REDUCED; BUT THE ENGINE IS NOT SHUT DOWN. THEY ATTEMPT TO RETURN TO THE DEP ARPT; BUT ELECT A DIVERSION TO A CLOSER ARPT UPON ENCOUNTERING UNANTICIPATED DRAG FROM THE R ENGINE.

Narrative

AT TOP OF CLB; LOW OIL PRESSURE ENGINE #2 ALARM WENT OFF. CREW CONSULTED QRH WHICH CALLED FOR POWER REDUCTION TO BELOW 75% NH. I AS CAPT; ASKED FO IF QRH CALLED FOR USE OF SINGLE ENGINE-CHKLIST. IT DOES NOT. WE CALLED ATC TO RETURN TO DEP ARPT. AFTER UNEVENTFUL DESCENT; WE WERE CLRED FOR VISUAL APCH. ACCORDING TO NORMAL CHKLIST; CREW ADVANCED BOTH CONDITION LEVERS TO 'MAXIMUM.' CREW WAS UNPREPARED FOR DRAG INCREASE ON ENG #2. WE DESCENDED BELOW SAFE APCH PROFILE. IN ATTEMPT TO RECTIFY PROB; WE ABANDONED APCH; AND OPTED FOR STRAIGHT-IN LNDG TO ADJACENT MIL BASE. AT THIS POINT WE DECLARED AN EMER; RECEIVED LNDG CLRNC TO ZZZ1 AND MADE UNEVENTFUL LNDG. HAD THE QRH CALLED FOR FEATHERING PROP AND/OR REFERRED TO USING SINGLE-ENGINE LNDG CHK; THE (TIRED) CREW WOULD NEVER HAVE FALLEN INTO THIS (OTHERWISE) OBVIOUS TRAP. OUR TRAINING HAD NEVER ADDRESSED THIS SCENARIO; WHICH IS A CONTRIBUTING FACTOR.CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR VERIFIED THAT THE OIL PRESSURE AT THE TIME OF THE WARNING WAS IN THE RANGE BETWEEN 40 AND 55 PSI WHICH TRIGGERED THE USE OF ONLY THE FIRST FEW LINES ON THE QRH PROC. HE EXPANDED ON THE SHORTCOMINGS OF THE ACFT CHKLIST DESCRIBED IN THE NARRATIVE; POINTING OUT THAT ALL SUBSEQUENT STEPS OF THE PROC APPEARED CLRLY RELATED SOLELY TO A CONDITION IN WHICH THE OIL PRESSURE WAS BELOW 40 PSI. IN ADDITION; THE RPTR HAD REVIEWED 'GENERIC' E120 CHKLISTS OBTAINED FROM A LARGE INDEPENDENT TRAINING SCHOOL THAT; LIKE THE COMPANY CHKLISTS; FAILED TO DIRECT THE FLT CREWS TO TREAT THE LNDG PHASE AS A SINGLE ENGINE OPERATION; INCLUDING THE DIRECTIVE TO FEATHER THE PROPELLER ON THE MALFUNCTIONING ENGINE. FINALLY; THE RPTR ADVISED THAT A SIMILAR INCIDENT HAPPENED ON THE SAME ACFT AT A LATER DATE. MAINT DETERMINED THE OIL CAP -- AS WELL AS A ONE WAY CHK VALVE INTENDED TO PREVENT LOSS OF PRESSURE AND/OR ENGINE OIL -- WAS FAULTY.