What was found
SHORTLY AFTER TAKEOFF, DURING CLIMB 1400' CREW HEARD A LOUD BANG FOLLOWED BY A LOUD RUSH OF AIR. MAIN CABIN DOOR & DOOR SEAL ANNUNCIATORS ILLUMINATED. MOMENTS LATER MAIN CABIN DOOR OPENED IN FLIGHT. AN EMER WAS DECLARED. THE ACFT RETURNED & LANDED WITH THE MAIN CABIN DOOR OPEN. ACFT SUSTAINED MAIN CABIN DOOR & LT ENG DAMAGE DURING LANDING. NEW MED INSTALLED, MED OPENING REPAIRS MADE, REPAIRED ORIGINAL ENG INSTALLED & SUPPLEMENTAL CABIN DOOR WIRING SYS MOD INSTALLED. SUCCESSFUL OPERATIONAL CHECK FLIGHT PERFORMED. ACFT RTS.