SMA DEPARTS IFR WITH DOOR NOT PROPERLY LATCHED. ALTDEV WHILE TRYING TO SECURE.

Narrative

IN THE HASTE TO DEPART FROM WESTCHESTER COUNTY (HPN); NY; WE FORGOT TO ADEQUATELY EXERCISE THE CHKLIST. THE IFR FLT PLAN WAS FROM HPN TO MFV (ACCOMACK COUNTY; VA). WE WERE CLRED VIA THE WESTCHESTER 9 SID THEN RADAR VECTORS TO DIXIE INTXN; V1 SBY; Y29 SWL; DIRECT MFV. SHORTLY AFTER TKOF WE REALIZED THE TOP DOOR LATCH OF THE SMA WE WERE FLYING HAD NOT BEEN SECURED. SINCE WE WERE BEING CLRED THROUGH THE TCA AND THOUGHT HE COULD SECURE THE DOOR IN FLT WE ELECTED TO CONTINUE. WE CLBED TO 6000 FT WITH NO PROBLEM. UPON REACHING THE SOUTHERN EDGE OF THE TCA WE STILL HAD NOT BEEN ABLE TO FASTEN THE LATCH AND WE DECIDED TO TRY A HARD SLIP TO THE R TO TRY TO RELIEVE THE AIR PRESSURE AND ALLOW US TO FASTEN THE LATCH. DURING THIS MANEUVER WE LOST 300 FT AND DSNDED FROM 6000 FT TO 5700 FT. IMMEDIATELY HE CLBED TO 6000 FT AND CALLED NY APCH AND REQUESTED DSCNT TO ALLAIRE COUNTY (BLM). NY TURNED US OVER TO MCGUIRE AIR FORCE BASE APCH AND TOLD US TO TELL THEM THE REQUEST SINCE WE WERE LEAVING HIS AREA. WE SWITCHED FREQS; CONTACTED MCGUIRE; REQUESTED DSCNT; CANCELLED IFR; REQUESTED ADVISORY FREQ; AND LANDED. AFTER LNDG THE DOOR WAS PROPERLY LATCHED; AND WE TOOK OFF AND COMPLETED OUR FLT VFR. I RESOLVED TO PAY MORE ATTN TO THE CHKLIST AND WHEN IN AN INFLT SITUATION PAY MORE ATTN TO MY FLYING.