On March 1, 2015, at 0940 Pacific standard time, a Piper PA-46-310P, N9133G, executed a forced landing into Roberts Field Airport, Redmond, Oregon, after a loss of engine power. The airplane was registered to, and operated by, the private pilot under the provisions of 14 Code of Federal Regulations, Part 91. The pilot was not injured, and the airplane sustained substantial damage to both wings. Visual meteorological conditions prevailed, and an instrument flight plan had been filed. The flight originated from Oak Harbor, Washington, at 0800.The pilot reported that he was cruising at 23,000 feet mean sea level (msl) when he noticed the engine running a bit rough as he passed Portland, Oregon. He noticed that the number 5 cylinder head temperature (cht) was lower than normal. The pilot cycled the magnetos and noticed that the engine ran rougher than normal when the left magneto was selected. The engine smoothed out when he enriched the mixture. He continued to monitor the engine and adjusting the mixture. About 30 minutes later engine power completely dropped off. The pilot diverted to Roberts Field, Redmond, Oregon, using his GPS for navigation. About 2 miles out he acquired the airport visually, and attempted to line up for a runway but did not have enough altitude. He performed a forced landing into the airport infield. The landing resulted in a collapsed nose landing gear, and wing spar damage at both main landing gear mounts. On March 3, 2015, an airframe and power plants (A&P) mechanic examined the airplane under the supervision of a Federal Aviation Administration (FAA) inspector. He reported on the airframe structural damage, and that he found no obvious reason for the loss of engine power. The airplane was then relocated to a facility in Greeley, Colorado, for further examination. The airplane was equipped with a EDM 930 engine data monitoring system. On April 14 the data from the EDM 930 was downloaded and sent to the NTSB Investigator-in-Charge (IIC). The data showed that about 25 minutes before the loss of engine power the number 5 cylinder head temperature (cht) started to trend about 50°F below its previous steady temperature (300°F), and then fluctuate twice between 300°F – 250°F during the last 12 minutes. The exhaust gas temperature (egt) remained steady for cylinder number 5 throughout the record. All the remaining cylinders exhibited steady egt and cht values. On May 13, 2015, the NTSB IIC, and a technical representative from the engine manufacturer examined the airplane. A fuel sample was taken from the fuel strainer. The fuel sample was light green in color and tested negative for water or kerosene/jet fuel contamination. Results of a chemical analysis confirmed that the sample was chemically consistent with 100LL avgas. The engine was examined externally, the cylinders bore scoped, sparkplugs inspected, internal magneto timing verified, and internal continuity confirmed by rotating the propeller/crankshaft. The number 5 cylinder fuel injection nozzle was removed, examined, and found to be in good condition with no blockages. An external fuel tank was plumbed into the right-wing fuel outlet and an external priming pump placed inline. The engine started on the first attempt, ran smoothly at idle, magneto checked performed, and the engine was run up to red line producing full power. No anomalies were noted during the engine run other than that the number 5 cylinder head temperature was notably 50°-70°F cooler than all the other cylinders.