On July 29, 2004, about 1000 Alaska daylight time, a Cessna U206G airplane, N9178G, sustained substantial damage when the aft cargo door came open during the takeoff roll, and damaged the fuselage, at the Yakutat Airport, Yakutat, Alaska. The airplane was being operated by the US Department of the Interior as a visual flight rules (VFR) cross-country transportation flight under Title 14, CFR Part 91 when the accident occurred. The commercial pilot and sole passenger were not injured. Visual meteorological conditions prevailed, and a VFR flight plan was filed. During a telephone conversation with the National Transportation Safety Board (NTSB) investigator-in-charge (IIC) on August 26, the director of maintenance for the operator said during an inspection of the accident airplane he discovered unreported substantial damage to the airplane's fuselage. He said he traced the damage to an incident where the aft cargo door opened during a takeoff roll, and baggage fell onto the runway. He said the pilot told him he checked the door for damage, but did not see significant damage to the fuselage. The director of maintenance said several fuselage stringers aft of the cargo door will have to be replaced. In a written statement from a US Department of the Interior investigator dated September 1, the investigator reported no mechanical deficiencies with the cargo door latching mechanism, and that he could not duplicate a way for the cargo door to inadvertently open when it was properly secured.