BEECH KING AIR E90, GALESBURG, ILL., JAN. 8, 2004–During climbout from Galesburg Municipal Airport (GBG) at about 2:27 p.m., King Air N48TA sustained substantial damage when the cabin door opened in flight, became separated and struck the empennage. The nonscheduled domestic passenger flight, headed for Mountain Empire Airport (MKJ) near Marion, Va., was operated by Tidewater Aero under Part 135. Although the weather was VFR, an IFR flight plan was on file. The ATP-rated pilot and his six passengers reported no injuries.