PLT ABORTED TKOF AT 20-40 FT AGL WHEN DOOR OPENED.

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THE FLT WAS INTENDED AS A CHKOUT FOR ME. I HAVE NEVER FLOWN AN AERO COMMANDER BEFORE. WINDS WERE STRONG AND GUSTY; A SLIGHT L XWIND VARYING IN HEADING. PIC WAS A CFI EMPLOYED BY THE FBO WHICH OPERATED THE LEASEBACK ACFT. HE WAS IN THE R SEAT; I IN THE L. TAXI AND RUN-UP/PRE-TKOF CHKS WERE NORMAL EXCEPT THAT I HAD SOME DIFFICULTY CLOSING THE PLT'S (L SIDE) DOOR SECURELY SINCE THE FIRST ATTEMPT INADVERTENTLY WAS MADE WITH THE UPPER LATCH IN THE 'CLOSED' POS. AFTER THE THIRD ATTEMPT; HOWEVER; THE DOOR SEEMED SECURELY FASTENED. I REMEMBER PUSHING OR THUMPING THE LOWER PORTION OF THE DOOR; AND THE UPPER REAR CORNER; BOTH OF WHICH SEEMED SOLIDLY CLOSED. THE FBO CO-OWNER; A PAX IN THE REAR SEAT; OBSERVED THIS AND MADE SOME REMARK LIKE 'SEEMS TIGHT;' OR WORDS TO THAT EFFECT. I CALLED FOR TKOF AND WAS CLRED. AFTER NORMAL ACCELERATION; ROTATION AND LIFT-OFF AS THE ACFT ENTERED FIRST SEGMENT CLB; THE L DOOR BLEW OPEN; AT AN ALT OF ABOUT 20-40 FT AGL; AS I RECALL. I IMMEDIATELY REDUCED PWR AND CALLED FOR AN ABORT. I RECALL THE CHK PLT/CFI CALLING 'WATCH YOUR SPD.' I LOWERED THE NOSE; THEN RAISED IT AGAIN TO AVOID DIVING INTO THE RWY. I FLARED HARD AND WAS IN THE PROCESS OF STARTING TO ADD PWR WHEN THE ACFT TOUCHED DOWN; UNDER CTL BUT IN A TAIL-LOW ALT. THE TAIL CONE CONTACTED THE GND JUST BEFORE THE MAIN WHEELS; WITH SUFFICIENT FORCE THAT THE METAL TAILSKID-TIEDOWN FITTING WAS FOUND; ON SUBSEQUENT EXAMINATION; TO HAVE BEEN PUSHED UP FLUSH WITH THE FIBERGLASS UNDERSURFACE; WHICH SHOWED SIGNS OF ABRASION BUT DID NOT SEEM SUBSTANTIALLY DAMAGED.