COWLING CAME LOOSE IN FLT AND CAME OFF ON THE SUBSEQUENT RETURN FOR LNDG AND WENT THROUGH THE PROP.

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THE MECHS FINISHED WORKING ON THE ACFT 30 MIN BEFORE DEP. THE R ENG FIRE LOOP TESTED INTERMITTENTLY AND R MAIN WHEEL BRAKES LEAKED FLUID. ACFT UNCOWLED AND PANELS REMOVED TO FIND A LOOSE CONNECTION IN THE FIRE LOOP. WATCHED MECH PUT TOP ENG COWLING IN PLACE. PREFLT COMPLETED BY CAPT AND FO PRIOR TO FLT SHOWING COWLING SECURE. APPROX 2 MIN AFTER DEP A PAX BRINGS TO OUR ATTN A LOOSE R ENG PANEL. FO LOOKS OUT TO SEE TOP R ENG COWLING NOT SECURED. CAPT ASKED FO HOW HIGH COWLING IS SEPARATED. FO SAYS IT'S NOT QUITE R AND IT CRACKED A LITTLE. AFTER CONSIDERING WT OF ACFT (600-700 LBS OVER LNDG WT) WE RETURNED FOR LNDG. AT TOUCH DOWN OR SHORTLY AFTER MAIN WHEELS TOUCH THERE'S A LOUD BANG AND SHAKING AND VIBRATION FROM R SIDE OF ACFT. CAPT SAYS I THINK WE BLEW TIRES. FO LOOKS OUT AND SAYS WE LOST THE ENG COWLING. WE CLR THE RWY AND CAPT ANNOUNCES TO TWR WE THINK WE LOST THE ENG COWLING ON THE RWY. WE TAXI TO GATE AND DEPLANE PAX. UPON FURTHER INSPECTION WE NOTICE 2 PROP BLADES ARE BROKEN OFF AT THE BASE AS WELL AS THE TOP ENG COWLING IS GONE. CREW WAS TOLD SECOND HAND THAT UPON TOUCHDOWN THE COWLING POPPED UP IN THE AIR. AT NO TIME DID WE CONSIDER THAT THE COWLING MIGHT GO INTO (FORWARD) THE PROP. THE THINGS WE CONSIDERED WERE 1) LAND BEFORE WE LOSE THE COWLING AIRBORNE. 2) OVER GROSS LNDG WT (SOFT LNDG NEEDED 300 FPM DSCNT RATE) 3) THE RISK OF BLOWING TIRES AT LNDG DUE TO WT AND SPD. WHAT I THINK HAPPENED: AS SOON AS WE TOUCHED DOWN THE COWLING POPPED UP AT THE SAME TIME AS THE PROPS WERE FLATTENED (GND FINE BUT NOT REVERSE) OUT AND IT SUCKED THE COWLING INTO THE PROP. THINGS NOT CONSIDERED: 1) THAT GND FINE (PROP POS) COULD SUCK ANYTHING FORWARD AT 100 KTS AIRSPD ON THE GND. THINGS I MIGHT DO DIFFERENTLY IN SAME SIT: 1) NOT USE GND FINE (PROP AERODYNAMIC BRAKING) TO SLOW ACFT ON RWY. 2) MORE THOROUGH PREFLT AND INSPECTION OF AREAS KNOWN TO BE WORKED ON BY MECH PRIOR TO FLT. 3) LOOK AT COWL LOCK ARROWS MORE CLOSELY. OTHER PROBS AND CONSIDERATIONS: 1) COWL LOCKING ARROWS ON LOCKING NUT DOES NOT ALWAYS SHOW MECHANISM LOCKED. 2) ARROWS WORN OFF; COVER WITH EXHAUST SOOT; NOT IN PROPER POS WHEN LOCKED. 3) SHUTTING DOWN OR FEATHERING ENG ON LOOSE COWLING SIDE COULD HAVE PREVENTED COWL PROP PROB BUT COULD HAVE CAUSED INFLT PROBS AT NEAR GROSS WT. DIDN'T WANT TO SHUT DOWN AN ENG PRODUCING THRUST.