AN LTT CREW SUFFERED A BLOWN OPEN ENG COWLING DURING A FAULTY STARTING PROC.

Narrative

THE CAPT STARTED R ENG WITH NO PROBLEMS; USING 'HIGH RESIDUAL TEMP' PROCS. DURING L ENG START; THE ENG NEVER GOT ABOVE 20 PERCENT NG DURING THE NORMAL ENG START PROC AND NG ROLLED BACK WHEN HE RELEASED THE START SWITCH. THERE WAS NO LIGHT OFF. I NOTICED THAT THE ENG CONDITION LEVER WAS IN FUEL CUTOFF. THE CAPT THEN SAID THAT HE WAS GOING TO PERFORM A HIGH RESIDUAL START. HE HAD CTL OF THE CONDITION LEVER; START SWITCH AND IGNITION SWITCH. AS THE ENG STARTED; THERE WAS A LOUD POP FROM THE L SIDE OF THE ACFT. THE CAPT PUT THE ENG CONDITION LEVER TO FUEL CUTOFF; MENTIONING THAT THE L ENG COWLING HAD 'POPPED OPEN.' WE THEN DID A NORMAL SHUTDOWN OF THE ACFT AND DEPLANED THE PAX. IN MY OPINION; THE COWL WAS 'BLOWN' OPEN BY AN EXPLOSIVE FUEL AIR MIXTURE THAT ENTERED THE COWL BY AIR CURRENTS OF THE TAILWIND; BLOWING FUEL VAPOR FROM THE ENG TAIL EXHAUST EXTENSION INTO THE BACK OF THE COWLING. THERE WAS NEVER A HIGH EXHAUST GAS TEMP DURING THE START ATTEMPT (198 DEGS C); NOR WERE THERE ANY SIGNS OF FIRE (WARNING BELL OR LIGHTS). I NEVER SAW ANY SMOKE BLOW BY THE COCKPIT WINDOWS. THE DESIGN OF THE ENG EXHAUST EXTENSION FACILITATED FUEL VAPOR COLLECTION IN THE ENG NACELLE DURING TAILWIND CONDITIONS IF LIGHT OFF IS DELAYED. SUPPLEMENTAL INFO FROM ACN 220547: INITIATED A NORMAL L ENG START AND INADVERTENTLY LEFT THE CONDITION LEVER IN FUEL OFF POS. THIS PROCESS ONLY LASTED ABOUT 5 SECONDS WITH NO START UP. THEN DID AN INTERRUPT START MOTORING VENT OPEN INTO A START. WHEN IGNITION AND CONDITION LEVER WERE BROUGHT UP AND ON; A POP WAS HEARD AND THE L COWLING HAD POPPED OPEN. THE WIND AT THE TIME OF ENG START WAS RPTED AT 220 DEGS AT 9 KTS. BUT THE WINDS WERE SWITCHING AROUND AND IT MAY HAVE BEEN A L QUARTERING HEADWIND TO A L QUARTERING TAILWIND AT THE TIME OF ENG START UP.