A FRACTIONAL C525 CAPT LEFT A KEY IN THE ACFT'S DOOR PREFLT. DURING POSTFLT; DAMAGE WAS DISCOVERED ON THE L ENG. CREW SUSPECTS KEY WAS INGESTED.

Narrative

WHILE OPENING THE MAIN CABIN DOOR FROM THE OUTSIDE; THE KEY WAS LEFT IN THE DOOR LOCK. OUR 1 PAX BOARDED THE ACFT WITH MY FO. I WALKED AROUND THE ACFT ONE LAST TIME PRIOR TO BOARDING AND DID NOT NOTICE THE KEY. AT ABOUT THE SAME TIME AS GEAR RETRACTION ON DEP; A FLUTTERING SOUND COULD BE HEARD IN THE CABIN AND COCKPIT FOR APPROX 8 SECONDS. NOT KNOWING WHAT THE SOUND WAS; WE GOT THE ACFT TO A SAFE ALT; THEN I ATTEMPTED TO TROUBLESHOOT AND REASSURE OUR PAX. NOT ABLE TO FIND THE CAUSE; WITH GOOD WX; A CALM PAX AND HEALTHY ENG INSTS; WE PRESSED ON TO OUR DEST OF ZZZ. 10 MINS AFTER DEP; I REALIZED IT MIGHT BE A KEY WHEN I PROMPTED MY PARTNER TO CHK HIS POCKETS. AN ACFT KEY WAS INDEED MISSING. WE PREPARED FOR A POSSIBLE SINGLE ENG SCENARIO AND EVENTUALLY LANDED SAFELY AT ZZZ. ONCE OUR PAX WAS SAFELY ON HER WAY; WE CAREFULLY POSTFLT'ED THE ACFT. THE KEY HAD BEEN INGESTED BY THE L ENG; CREATING EXTENSIVE DAMAGE TO THE FIRST COMPRESSOR AND THE FAN CASE. I HAVE LEARNED THAT GOOD KEY SECURITY/ETIQUETTE IS NOT SOMETHING TO TAKE FOR GRANTED. I HAVE ALSO LEARNED THAT CHKING EVERY INCH OF AN ACFT'S SKIN IS NECESSARY ON A PREFLT WALKAROUND IN ORDER TO MINIMIZE THE CHANCES OF PREVENTABLE FOD.