On a date on or about July 3, 2003, about 1100 mountain daylight time, a North American P-51D, N2869D, registered to and operated by the pilot as a 14 CFR Part 91 personal flight, experienced a loss of engine power shortly after takeoff from Jerome County Airport, Jerome, Idaho. During the return for landing, the aircraft landed hard and slid off the side of the runway. Visual meteorological conditions prevailed at the time and no flight plan was filed. The aircraft was substantially damaged and the commercial pilot and his passenger were not injured. During a telephone interview with the pilot on December 4, 2003, the pilot reported that shortly after takeoff, the engine overheated and then lost coolant. The coolant covered the windshield and he had a hard time seeing in front of him for the return landing to runway 8. During the touchdown, the aircraft landed hard and slid off the side of the runway. The pilot could not recall the actual date of the accident. He further stated that he was not aware that he needed to report this accident to the NTSB. The accident was reported to the NTSB on December 3, 2003. A telephone conversation with the mechanic who was accomplishing the repairs to the aircraft reported that the left wing spar was bent. As a result of this damage, which adversely affected the structural strength of the spar, it was being replaced. The mechanic also reported that he believed that the accident occurred just before the fourth of July weekend. The mechanic reported that the relief valve to the aftercooling system header tank failed, subsequently releasing the coolant. Section IV of the P-51D, Maintenance Manual states, "The system is of the low pressure type, incorporating a relief valve which relieves at approximately 18 to 20 pounds per square inch." The Mustang Flight Handbook indicates for engine starting and for all ground operations, oil and coolant flaps should be fully opened to prevent overheating. A caution is noted indicating that if coolant temperature exceeds 100 degrees C, the coolant radiator air control switch should be placed in the open position until airborne. Restricted ground operations during hot-weather is recommended to prevent overheating, and to maintain a constant check on carburetor air and coolant temperature. The Handbook instructions further state that "If the engine overheats in flight (indicated by coolant relief valve pop-off, maximum coolant temperature being exceeded, or white smoke coming from exhaust stacks), move coolant radiator air control switch to OPEN and hold." Two letters with the National Transportation Safety Board Pilot/Operator Aircraft Accident Report Form 6120.1/2 were sent to the pilot on December 4, 2003, and then on January 15, 2004, with instructions to complete the form and return to the NTSB. The pilot's response to the first letter was that he needed more time to complete the form. When the pilot did not return the 6120.1/2, the second letter was mailed with another 6120.1/2 form attached. The pilot's response to the second letter stated that he just had a hard landing and did not consider this an accident because there were no injuries and no damage on the airport. To date, the pilot has not returned Form 6120.1/2.