On December 13, 1998, approximately 1745 central standard time, an Aerospatiale AS-350-BA twin engine helicopter, N911MV, was substantially damaged when it rolled over during takeoff from a grass covered area located on the San Angelo Airport, San Angelo, Texas. Both of the commercial helicopter pilots, the instructor pilot-in-command and the pilot receiving new hire instruction/check-out in the AS-350, were not injured. Visual meteorological conditions prevailed and a flight plan was not filed for the 14 Code of Federal Regulations Part 91 instructional flight. The local training flight originated from the San Angelo Airport about 3 hours prior to the accident. The helicopter was owned and operated by Southwest Helicopters Inc., of Tucson, Arizona, and was being leased as a air evacuation helicopter by the Shannon Hospital, San Angelo, Texas. The pilot-in-command reported that he had been giving the newly hired pilot a check out and instruction in the helicopter for the 3 hours prior to the accident. The new hire pilot had just completed a successful "hydraulic system off" landing. The pilot-in-command then took the controls to perform a normal takeoff (with hydraulics "ON"). The pilot picked the aircraft up to a hover, made a clearing turn to the left and initiated a clearing turn to the right when the helicopter rolled over to the left, and the main rotor blades impacted the ground. The pilot was able to roll the engine throttles "off" and right the helicopter on its skids. Also, the new hire pilot reported that he thought that he saw the hydraulic warning light "flicker" during the event. After the accident, the pilot-in-command stated that he thought that a "hydraulic hardover" caused the helicopter to roll over. He also stated that the helicopter operated normally in all flight regimes during the entire 3 hours of flight prior to the accident. Examination of the helicopter by an FAA inspector at the accident site revealed that all of the main rotor blades were substantially damaged, the tail boom was partially severed, and three of the four main transmission mounts were fractured. The helicopter was transported to Air Salvage of Dallas, Lancaster, Texas, for further examination of the hydraulic system. The sight glass on the hydraulic reservoir was found broken and no fluid was visible. The manufacturer stated that this would be consistent with the hydraulic system operating at the time of impact, pumping fluid out of the broken sight glass. No metal chips were found on the magnetic chip detector. The hydraulic pump drive belt was in good condition but slightly loose. The hydraulic system had three accumulators installed, one for each hydraulic servo. The charges on the three servo accumulators were found to be as follows: Forward and Aft: 90 PSI Right Hand Lateral: 85 PSI Left Hand Lateral: 110 PSI All three servo accumulators, by manufacturer's specifications, should have been charged to 200 PSI, which is enough pressure for hydraulic assisted control for about 10 seconds if hydraulic pressure is lost. After the post-accident inspection of the hydraulic system, a serviceable hydraulic fluid tank was installed and hooked up to an external pressure source to test the hydraulic system for functionality. With normal operating hydraulic pressure applied, the lights, warning horn logic, and control function boost all operated within normal parameters. After the test, the hydraulic pump was removed for examination. The pump splines were found normal (backlash was within parameters as per manufacturer's specifications). The pump shaft turned without binding.