S-7OA-9 Black Hawk Helicopter: Internal Panel Cracking Investigation

Abstract : The Australian Army S-7OA-9 Black Hawk fleet is experiencing numerous occurrences of cracking in an internal fuselage panel. The panel is not primary structure, but it is essential. Cracking has occurred only on the right hand side panel, and the standard repair scheme is inadequate. In October 1994, the Australian Army approached DSTO and the Royal Australian Air Force (RAAF) Aircraft Research and Development Unit (ARDU) for assistance in determining the cause of the cracking. To try to minimize the panel cracking, the Army had suspended use of the External Stores Support System (ESSS) which is used to carry external fuel tanks. Since this suspension was causing operational hardships, the Army wanted to know what was causing the cracking to determine whether less severe restrictions might be imposed until a proper repair could be devised for the panel. In February 1995, DSTO and ARDU personnel conducted a flight investigation, at RAAF Base Edinburgh, South Australia, with Black Hawk A25-206. The data gathered were analysed and the results indicated that the ESSS was not responsible for the cracking. The panel strains are largely insensitive to the presence of the ESSS. The cause of the cracking is a structural deficiency in the panel. Beads, pressed into the panel to provide stiffening, are creating a stress concentration factor of approximately 3.0 which couples with the large Ground Air Ground loading cycle to cause the cracking. Once initiated, the high frequency in flight loading to which the panel is subjected causes the cracks to propagate rapidly. There are no operational restrictions which the Army might apply to reduce the frequency or severity of the cracking. The only relief will come when a redesigned panel is installed.