PA31 CHIEFTAIN HAS ENG COWLING BECOME LOOSE AND TEAR AWAY FROM THE ACFT. OIL FILTER WAS CRACKED AND CAUSED OIL TO SPRAY FROM THE ENG.

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SCHEDULED CARGO FLT FROM SCOTTSBLUFF; NE; TO COS. IN CRUISE FLT APPROX 30 MI N OF GREELEY; I NOTICED A RAISED CORNER ON THE LEFT ENG COWLING (L; FRONT; TOP CORNER). I CALLED ZDV AND REQUESTED A DIVERT TO GREELEY ARPT (REASON; LOOSE COWL). DURING DIVERT TO GREELEY; I CAUGHT MOVEMENT ON MY L AND TURNED TO SEE COWLING OF L ENG SEPARATE FROM ACFT. THE EVENT CRACKED THE OIL FILLER NECK CAUSING OIL TO SPRAY OUT OF ENG. FLT CTLS AND ENG PERFORMANCE SEEMED UNAFFECTED. I CONTINUED VISUAL APCH TO GREELEY; CANCELED IFR; AND MADE A NORMAL AND UNEVENTFUL APCH AND LNDG. DING IN VERT STABILIZER NOTICED ON GND. NOTHING ODD NOTICED DURING NORMAL PREFLT INSPECTION. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATES THE ACFT IS A PA31; PIPER CHIEFTAIN. HE DID NOT HAVE TO SHUT DOWN THE ENG AND HAS NOT BEEN ABLE TO DETERMINE THE REASON FOR THE COWLING TO SEPARATE. RPTR HAS SPOKEN WITH BOTH THE NTSB AND THE FAA BUT HAS HEARD NOTHING FURTHER FROM EITHER. THE NTSB INSPECTOR WAS QUITE ANNOYED TO HAVE ANOTHER SUCH INCIDENT OCCUR SINCE HE HAD MADE RECOMMENDATIONS TO THE FAA AND TO THE MANUFACTURER SOME YRS AGO TO HOPEFULLY PREVENT THIS FROM OCCURRING AGAIN. THERE WAS AN INCIDENT IN 1991 WITH SIMILAR COWLING PROBS AND STILL ANOTHER AT A LATER DATE IN WHICH THE COWLING OPENED UP LIKE A CLAM SHELL BUT REMAINED ATTACHED. THIS CAUSED SO MUCH DRAG THE ACFT BECAME UNCTLABLE AND RESULTED IN A FATALITY. THE INSPECTOR HAD SUGGESTED THAT THE POH INDICATE A CLOSE INSPECTION OF THE COWLING DURING THE PREFLT. RPTR COULD NOT EVEN FIND THE WORD COWLING MENTIONED IN THE POH REGARDING PREFLT PROCS. THE SECOND RECOMMENDATION WHICH HAS BEEN ADOPTED BY THE RPTR'S COMPANY IS TO PLACE MARKINGS ON THE ACFT WHERE THE SLOT OF THE FLAT HEAD SCREW SHOULD BE ALIGNED WHEN IT IS PROPERLY SEATED TO HOLD THE COWLING. THE INSPECTOR'S SUGGESTIONS HAVE NOT BEEN ENDORSED BY EITHER PARTY.