FLC OF A BE1900 EXPERIENCE A BUMP SENSATION DURING CLB. UPON FINDING NO INDICATIONS INSIDE THE COCKPIT THEY CHK OUTSIDE TO DISCOVER AN ENG NACELLE PANEL HAS LEFT THE ACFT.

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AFTER COMPLETING THE MANUFACTURERS CHKLIST FOR THE PREFLT AND RUN-UP; I TAXIED THE ACFT FROM THE COMPANY MAINT HANGAR TO THE TERMINAL. I THEN WENT INSIDE TO CHK THE DISPATCH RELEASE; WX AND COMPANY MEMOS WHILE THE FO ASSISTED IN PAX BOARDING. I THEN CAME OUT OF THE BUILDING; PERFORMED A WALKAROUND INSPECTION; AND THEN PROCEEDED WITH START AND TAXI OPS. AFTER DEP; AND AT ABOUT 1500 FT AGL THE FO AND I FELT AN UNUSUAL POSTURAL SENSATION. WE DETERMINED THAT ALL SYS WERE FUNCTIONING NORMALLY AND PROCEEDED TO INSPECT THE EXTERIOR OF THE ACFT VISUALLY FROM THE COCKPIT. IT WAS AT THAT TIME I NOTICED WE HAD LOST A NACELLE PANEL. WE TOLD ATC THAT WE WANTED TO RETURN TO THE ARPT; AND LANDED WITHOUT FURTHER INCIDENT. UPON INSPECTION OF THE FRAME OF THE NACELLE; A PORTION OF THE FASTENERS WERE STILL ATTACHED; WHILE THE OTHERS WERE GONE WITH THE PANEL. ON THE CHKLIST IT STATES TO MAKE SURE THE PANELS ARE SECURE; WHICH IT WAS. HOWEVER; IT IS UNKNOWN AS TO WHETHER SOME OF THE FASTENERS WERE ATTACHED PROPERLY; OR SOME METAL FATIGUE (IE; THE PANEL WAS PULLED THROUGH THE FASTENERS). CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATES THAT THE PANEL WAS NEVER FOUND SO IT IS DIFFICULT TO DETERMINE WHAT ACTUALLY OCCURRED. HE HAS HEARD OF THIS HAPPENING PREVIOUSLY BUT NOT RECENTLY; NOTHING THAT WOULD INDICATE A PATTERN. THE ONLY SENSATION THEY HAD WAS LIKE THE ACFT PASSING OVER A SPD BUMP. THERE WAS NO DAMAGE TO THE ACFT ITSELF.