PLT OF AN AERO COMMANDER 560 INADVERTENTLY RAISED THE MAIN LNDG GEAR DURING TAXIING OFF THE RWY AFTER LNDG CAUSING MINOR DAMAGE TO THE BOTTOM FUSELAGE SKIN. THE RPTR ADMITTEDLY FAILED TO HAVE THE LNDG HANDLE IN THE LOCKED POS AND MOVED IT INSTEAD OF THE FLAP HANDLE WHEN ATTEMPTING TO RAISE THE FLAPS AFTER LNDG.

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WHILE RETURNING FROM A BUSINESS TRIP AFTER HAVING WORKED 4 DAYS STRAIGHT. VISUAL APCH INTO PALOMAR ARPT; WENT THROUGH CHKLIST 3 TIMES. MADE NORMAL LNDG ON CTRLINE OF RWY. SLOWED TO ABOUT 20 MPH; AS I STARTED TO TURN TOWARDS THE TXWY; I REACHED TO RAISE THE FLAPS AND EITHER BUMPED OR INADVERTENTLY RAISED THE LNDG GEAR (MAIN GEAR ONLY) CAUSING THE BELLY OF THE PLANE TO SCRAPE ON THE RWY FOR A DISTANCE OF ABOUT 40 FT. THERE WAS NOT ANY PROP STRIKE AS TWIN COMMANDER'S PROPS SPIN HIGHER THAN THE BOTTOM OF THE PLANE BY ABOUT 10 INCHES. ALSO; AERO COMMANDERS DO NOT HAVE SQUAT SWITCHES. VERY SURPRISED; I PROCEEDED TO SHUT DOWN THE ENGS; FUEL SHUTOFFS; RADIOS AND MAIN MASTER AND MAGNETOS. I EXITED THE PLANE AND DISCONNECTED THE BATTERY FOR SAFETY. SOON AFTER THE PLANE WAS JACKED USING STANDARD ACFT JACKS AND THE MAIN GEAR WAS JACKED DOWN USING THE HAND PUMP. AT THIS TIME THE PLANE WAS TOWED OFF THE RWY AND PARKED. I BELIEVE THE INCIDENT COULD HAVE BEEN AVOIDED IF THE GEAR KNOB HAD BEEN LOCKED. (I HAD NEVER LOCKED THE GEAR UNTIL AFTER LNDG; IN CASE I HAD TO GAR; I COULD CLEAN UP FASTER. NEEDLESS TO SAY; MY NEW PRACTICE IS GEAR DOWN AND LOCKED!) I ALSO FEEL FATIGUE PLAYED A MINOR ROLE; BUT THE BOTTOM LINE IS; THE PROC OF NOT LOCKING THE LNDG GEAR WAS WRONG. I'M SURE I WON'T DO IT AGAIN.