HISTORY OF FLIGHT On April 7, 1997, at 1209 hours Pacific daylight time, an Aerospatiale AS350D, N317LT, experienced a resonant series of uncommanded takeoffs and landings after landing at the Transamerica heliport in downtown Los Angeles, California. The aircraft sustained substantial damage; however, neither the pilot nor his student pilot rated passenger were injured. There were no ground injuries or damage. The aircraft, operated by Bravo Helicopters of Torrance, California, was awaiting a repositioning flight under 14 CFR Part 91 when the accident occurred. The helicopter had just completed an on-demand air taxi flight under 14 CFR Part 135 and deplaned a non-revenue passenger. The flight originated at the University of California, San Diego, California, at 1100 on the morning of the accident. Visual meteorological conditions prevailed at the time and a company flight plan was filed. After approaching at the Transamerica building on a 250-degree heading, the pilot made a left pedal turn to 180 degrees, set the aircraft down on the deck, and applied the collective lock and control frictions. He then exited the aircraft, opened the right cabin door, and escorted his passenger to the heliport stairway, where they were met by a building security guard. His student pilot rated passenger remained sitting in the left front seat, with instructions not to touch the aircraft controls. The pilot, who was standing with his back toward the aircraft at the time, was just saying goodbye to his passenger, when the security guard exclaimed "Holy Cow!" The security guard stated that, while he was looking, the aircraft began bouncing up and down in a resonant mode until reaching a height of nearly 2 to 3 feet with a frequency that he equated with someone dribbling a basketball. He said he also recalled hearing the engine accelerate about the same time the bouncing began. The pilot, who had glanced back over his shoulder, now also saw what was happening. He ran around the nose of the aircraft and climbed back in the cockpit. The pilot said that after he climbed back into his seat, he immediately pulled the fuel flow control (FFC) lever into the cut off position, effectively shutting down the engine. After bouncing several times with increasing amplitude, the aircraft suddenly yawed to the right, turning about 40 degrees. The aircraft came to rest on a heading of 220 degrees with its stinger resting against the railing on the east side of the heliport. After he shut down the aircraft, both he and his student pilot passenger were able to exit the aircraft without difficulty. The student pilot confirmed that he saw the pilot apply the control frictions and the collective lock. He reported that the aircraft began rocking and yawed to the right after the pilot re-entered the cockpit. He said that when the aircraft movement occurred, his first reaction was to grab his seat in order to brace himself. He denied touching the controls at any time. PERSONNEL INFORMATION The pilot is the owner and chief pilot of Bravo Helicopters, the operator of the aircraft. AIRCRAFT INFORMATION A manufacturer's representative reported that they are aware of instances in the past in which the collective lock was not seated properly and consequently released the collective while the engine was running. The fully down position of the collective is spring loaded to keep tension against the control lock. WRECKAGE AND IMPACT INFORMATION During a postaccident inspection of the aircraft, the Safety Board noticed that the collective was not down and locked. A further inspection of the controls revealed that the frictions were not tightly applied. The No. 2 red Starflex Star arm was separated and that the tailboom was buckled aft of its attachment point to the fuselage. TESTS AND RESEARCH The engine and fractured Starflex Star arm were removed and submitted to their respective manufacturer's facilities for examination. Allied Signal performed an engine inspection and functional test run in an instrumented test cell. During the inspection it was noted that the combustor drain gasket was damaged and a bolt was missing from the customer bleed pad. The engine was not repaired, but rather was installed in a test cell, started, and brought up to 70 percent Ng and then 100 percent Ng where it ran without surging. During the run it met all the points on the power curve specifications. The No. 2 red Starflex Star arm was inspected by Eurocopter. The manufacturer reported that the arm met its specifications regarding its material, curing, part health and dimensional and stiffness checks. They identified the fracture as a static rupture which occurred while the load was in the flap/drag mode. ADDITIONAL INFORMATION The aircraft was recovered by Aircraft Recovery Services and transported to Rotorcraft Support, Inc., in Van Nuys, California. The aircraft was released to a representative of the registered owner on May 5, 1997.