THE FLT WAS FOR CREW COORDINATION TRAINING & WAS ADMINISTERED BY FLT SAFETY INTL FOR AER LINGUS, A FOREIGN AIRLINE. AN INSTRUCTOR (CFI) & 2 FOREIGN PLTS WERE ABOARD (STUDENTS HAD PREVIOUSLY GOTTEN U.S. CERTS WITH SINGLE/MULTI-ENG RATINGS).BEFORE THE ACDNT, THE ACFT WAS HDG NE. RADAR DATA SHOWED THAT IT TURNED EAST WHILE MAINTAINING A CONSTANT ALT OF 3500'. AFTER TURNING EAST, ITS GND SPD SLOWED TO 53 KTS. THE ACFT THEN BEGAN A RGT TURN AT A RATE OF 125 DEG/MIN. AFTER TURNINGSOUTH, THE ACFT'S GND SPD SLOWED TO 37 KTS. THE ACFT THEN ENTERED A DSCNT, WHICH INCREASED TO 8000'/MIN. WITNESSES SAW THE ACFT SPINNING LEFT, IN A NOSE DWN ATTITUDE, BEFORE IT IMPACTED IN THE OCEAN. NO PREIMPACT PART FAILURE/MALFUNCTION WAS FND. THERE WAS EVIDENCE THAT DRG IMPACT, NEITHER ENG WAS OPERG, FLAPS WERE EXTNDD 10 DEG, MAIN GEAR WAS EXTNDD & THENOSE GEAR WAS IN TRANSIT. THE ACFT MANUFACTURER RPRTD THAT A SPIN MAY CAUSE AN INTERRUPTION OF FUEL TO THE ENGS BY EXCEEDING THE CARB LIMITATIONS. INTENTIONAL SPINS WERE PROHIBITED. VMC & STALL SPDS WERE 56 & 54 KTS, RESPECTIVELY.