A PIPER PA31-310 IN CRUISE AT 3500 FT DECLARED AN EMER AND DIVERTED DUE TO THE R ENG COWLING SEPARATING AND WRAPPING AROUND THE R HORIZ STABILIZER. INCIDENT CAUSED BY UNSECURED INBOARD FASTENERS.

Narrative

A TEST FLT WAS BEING CONDUCTED FOLLOWING MAINT ON THE R ENG. DUE TO TIME CONSTRAINTS; A HASTY PREFLT WAS CONDUCTED. AFTER TKOF AND CLB; THE ACFT WAS LEVELED AT 3500 FT; PWR REDUCED AND COWL FLAPS CLOSED. SOON THEREAFTER; THE R ENG COWLING (UPPER) SEPARATED; STRIKING THE HORIZ STABILIZER. THE ACFT RESPONDED WITH A RAPID NOSE DOWN PITCH CHANGE FOLLOWED BY A NOSE UP. THE SEQUENCE REPEATED THROUGH SEVERAL CYCLES INTENSIFYING WITH EACH CYCLE. A MAYDAY CALL WAS MADE. PWR WAS THEN REDUCED AND AS THE AIRSPD SLOWED; ACFT CTL WAS REGAINED ALBEIT SOMEWHAT DIFFICULT. THE ENG COWLING WAS OBSERVED TO BE WRAPPED AROUND THE R HORIZ STABILIZER. THE ACFT WAS THEN RETURNED TO BRACKETT AND A FLAPLESS LNDG MADE WITHOUT FURTHER INCIDENT. CLRLY THE CAUSE OF THE INCIDENT WAS AN INADEQUATE PREFLT INSPECTION AS THE SEPARATION OF THE COWLING OCCURRED DUE TO THE INBOUND FASTENERS NOT BEING LOCKED. THESE ITEMS ARE INCLUDED ON THE PREFLT INSPECTION CHKLIST. CONTRIBUTING TO THE INCIDENT WAS THE FAILURE OF THE MAINT PERSONNEL TO SECURE THE FASTENERS. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE COWLING FASTENERS ARE FLAT HEAD SCREW TYPE FASTENERS AND THEY APPEAR THE SAME LOCKED OR UNLOCKED. THE RPTR SAID THE COWLING FASTENERS WERE THE SUBJECT OF AN AIRWORTHINESS DIRECTIVE TO ADD WITNESS MARKS TO INDICATE THE COWLING FASTENER STATUS; LOCKED OR UNLOCKED. THE RPTR SAID THE PREFLT INSPECTION WAS DONE IN HASTE AND THE FASTENERS WERE OVERLOOKED. THE RPTR STATED THE FAA IS INVESTIGATING THE INCIDENT.