HISTORY OF FLIGHT: On June 9, 1993, at 1646 central daylight time, a Convair 580, N580HH, was substantially damaged during a precautionary landing on runway 17 at Majors Airport, Greenville, Texas. Neither the airline transport rated pilot, the commercial rated copilot, nor the thirteen passengers were injured. Visual meteorological conditions prevailed at the time of the accident. The intent of the flight was to flight test onboard electronic equipment. According to the crew and test personnel onboard the airplane, several electrical spikes occurred while in flight to the alternating current (AC) system. The captain stated that the left engine's fire warning light illuminated several times along with other fluctuating instruments. He secured the left engine and used the fire extinguishing system which resulted in the warning light going out and a loss of AC power to the airplane. A successful single engine landing was accomplished. During the landing roll, aircraft control was lost. Interviews with the crew revealed that the crew did not activate the emergency direct current (DC) hydraulic power switch to provide nose wheel steering capabilities, normally an AC powered system. PERSONNEL INFORMATION: According to interviews conducted by the investigator in charge of the pilot, copilot, and other company personnel, the following information was obtained. The pilot in command and the copilot had both logged very little time recently in this airplane. As corporate pilots they both had primary flight duties in either the company owned Douglas DC3 or a Learjet. This airplane was flown only for test and evaluation of the onboard electronic equipment installed in the passenger compartment. FLIGHT RECORDERS: A Sundstrand model CVR-80 cockpit voice recorder was sent to the National Transportation Safety Board laboratory for analysis. The tape contained no pertinent information and no transcript was produced. WRECKAGE AND IMPACT INFORMATION: All of the following distances were measured by the investigator in charge and are approximate. The aircraft landed 2,000 feet from the landing threshold. Physical evidence, ground scars, indicated that the aircraft ran off the west side of the runway 5,500 feet from the threshold and crossed a drainage ditch to a parallel taxiway and came to a stop 300 feet from the northern most end of the taxiway on a heading of 172 degrees. When the aircraft exited the drainage ditch to the parallel taxiway the nose gear assembly broke at the mounting yoke, the right propeller assembly contacted the ground and separated from the aircraft at the gear reduction assembly, and the fuselage underside, aft of the aft cargo door area, contacted the ground. TESTS AND RESEARCH: The electrical systems (AC and DC) were evaluated for continuity and functional operation, no discrepancies were found. The left engine was examined and no anomalies were found. ADDITIONAL DATA: The airplane was released to the operator for repair or disposition.