On February 11, 1997, at 1335 central standard time, a Cessna 210-5 airplane, N1866Z, owned and operated by the pilot under Title 14 CFR Part 91, was substantially damaged following a forced landing near Lea County/Hobbs Municipal Airport, Hobbs, New Mexico. The private pilot received serious injuries and the sole passenger received minor injuries. Visual meteorological conditions prevailed and no flight plan was filed for the personal cross country flight which originated from Corsicana, Texas, at 1049. In an interview with a FAA inspector, the pilot reported that two hours and forty minutes of flight time had elapsed from departure at Corsicana, Texas, to the forced landing. He stated that, after becoming airborne from Corsicana, he could not get the cabin heater to work and that there was no odor of fuel in the airplane during the flight. He wondered if there was a fuel leak from a line that was not properly secured during maintenance. After approximately one hour and ten minutes of flight time, the engine quit and the left fuel gauge indicated empty. He reported that after the engine quit, he "switched to the right tank, [restarted the engine] then called Flight Service to find the nearest place to land and not have to do an instrument landing." The pilot ascertained from flight watch that Hobbs, New Mexico, was "clear", so he proceeded to Hobbs. After about 1 hour and 30 minutes of uneventful flight to Hobbs (fuel selector on the right tank), the pilot commenced a VFR approach to land on runway 12. Upon turning onto a left base to final for runway 12, the engine quit and the airplane impacted the ground. Several witnesses heard the engine "sputter" just prior to the forced landing. After the accident, the pilot reported that the airplane's oil sump pan was replaced on the day prior to the accident. In a written statement, the airframe and powerplant mechanic that did the work on the oil sump pan, stated that he "disconnected the fuel supply line and return line at the engine driven pump, on the left hand side of the accessory section of the engine, to prevent unnecessary strain on the lines." After the job was complete, he "reconnected the lines." He further stated that he "did not disconnect any lines at the firewall." He also stated that, he "was baffled as to why the pilot did not land the airplane when the left fuel tank exhausted prematurely." An examination of the aircraft at the accident site by a FAA inspector revealed that the right wing fuel tank was empty. The left wing tank was found to contain approximately 9 gallons of fuel. Both right and left fuel tank structural integrity was not compromised during the impact and there were no noticeable fuel or stains around the engine compartment or on the belly skin of the airplane. The fuel selector valve was found in the "right" tank position. The airplane sustained structural damage to the firewall, engine mount, and the left wing. The airplane had an IO-520 engine-conversion from the standard IO-470-S engine. The optimum cruise performance engine fuel burn according to the IO-520 engine manufacturer's specifications is 14.5 to 15.5 gallons per hour. The total fuel capacity of the long range fuel tanks installed in the airplane was 84 U.S. gallons. The pilot stated that normal fuel consumption for his airplane was about 14 gallons per hour during cruise. According to the above information, the airplane would have burned approximately 38 gallons of fuel for the two hours and forty minutes of flight time, leaving a total of 46 gallons of fuel remaining in the fuel tanks. According to aeronautical charts, the total distance from Corsicana, Texas to Hobbs, New Mexico is 348 nautical miles. According to a refueling agent at Corsicana Municipal Airport near Corsicana, Texas, the airplane was "topped off", at the pilot's request the day prior to the accident, and visually verified (by the refueler) by checking each tank, confirming fuel attainment of 1/4 to 1/2 inch from top of the filler neck. A fuel receipt showed that 35.9 gallons of fuel was delivered to the airplane on February 10, 1997. The refueling agent further reported that he did not notice any fuel stains or leaks during the time he was under both wings and around the engine cowling. According to an interview with an FAA inspector, the pilot stated that he did not visually verify the fuel in the tanks by looking in the tanks prior to departure. The pilot further conveyed in a statement, that when the fuel gauge shows empty "he still has nine gallons of fuel remaining". According to the airplane manufacturer's Pilot Operating Handbook (Exterior Inspection, item 5b.), it recommends that prior to flight, the pilot should "remove fuel tank cap and check fuel level for agreement with gauge reading. Secure cap". Examination of the aircraft at a recovery facility in Phoenix, Arizona, revealed that the fuel return line fitting was found separated from the engine firewall. Further examination by the NTSB Investigator-in-Charge revealed deformation of several threads on both the male and female connection point of fuel fitting. Due to inconclusive physical evidence, it was not possible to determine if the fuel return line fitting was properly, improperly, or not connected prior to flight, or became separated due to impact forces. Additionally, conflicting reports from the pilot, the mechanic, and the refueler, made it difficult to ascertain the state (fuel and mechanical) of the aircraft when it originally departed from Corsicana, Texas.