HISTORY OF FLIGHTOn July 11, 2015, about 0745 Pacific daylight time, a Piper PA-22-160, N9712D, was destroyed when it impacted the ridge of Twin Sisters Mountain, about 12 miles north of Hamilton, Washington. The airplane was registered to the private pilot who was operating the airplane under the provisions of 14 Code of Federal Regulations Part 91, as a visual flight rules flight. The pilot and single passenger were fatally injured. Visual meteorological conditions prevailed for the flight, and no flight plan had been filed. The flight originated from Stehekin, Washington, about 0700. The airplane was subject of an ALNOT (alert notification) missing aircraft issued on July 14. Initial radar data depicts an airplane track starting at 8,300 feet mean sea level (msl) 16 miles northwest of Stehekin, at 0718 on July 11. The track continues in a northwesterly direction at an altitude about 10,500 feet mean sea level (msl). At 0738, the airplane's track altitude starts to decrease; by 0744 the altitude was 7,500 feet, and continued in the northwesterly direction. The last radar return occurred at 0745, at an altitude of 6,000 feet msl. The wreckage was located on the morning of July 15, at the 6,000-foot level of the Twin Sisters Mountain ridge line in the vicinity of the final radar return data point. PERSONNEL INFORMATIONThe pilot, age 62, held a private pilot certificate with a single-engine land rating, issued May 26, 2010, and a third-class airman medical certificate issued February 3, 2014, with the limitation that the holder shall possess glasses for near and intermediate vision. The pilot's log book was not recovered for examination. On the pilot's February 3, 2014, application for his medical certificate he reported 440 total flight hours, with 47 hours within the previous six months. AIRCRAFT INFORMATIONThe four-seat, high-wing, fixed-gear airplane, serial number 22-6624, was manufactured in 1959. It was powered by a Lycoming O-360 series normally aspirated engine. It was equipped with a fixed pitch propeller. Aircraft records were not located or obtained for examination. METEOROLOGICAL INFORMATIONAn Automated Weather Observing System (AWOS) was located at Skagit Regional Airport (BVS) in Burlington/Mount Vernon, Washington, which was located about 22 miles southwest of the accident location at an elevation of approximately 145 feet. At 0735, BVS reported a variable wind at 5 knots, visibility of 9 statute miles, ceiling broken at 4,200 feet above ground level (agl), overcast cloud base at 5,000 feet agl, temperature of 17° Celsius (C), a dew point temperature of 14°C, and an altimeter setting of 29.88 inches of mercury. WSR-88D Level-II weather radar base reflectivity imagery from Seattle/Tacoma, Washington (KATX), is located approximately 36 miles southwest of the accident site at an elevation of about 500 feet. Assuming standard refraction and considering the 0.95° beam width for the WSR-88D radar beam, the KATX 1.435° tilt would have "seen" altitudes between about 5,000 and 8,650 feet above msl at the accident location, the KATX 2.404° tilt would have "seen" altitudes between about 8,700 and 12,350 feet above msl at the accident location, the KATX 3.364° tilt would have "seen" altitudes between about 12,400 and 16,000 feet above msl at the accident location, and the KATX 4.292° tilt would have "seen" altitudes between about 15,900 and 19,500 feet above msl at the accident location. The KATX base reflectivity imagery identified light values of reflectivity coincident with the accident location at the accident time, in a pattern consistent with light rain showers being present above/at the accident site. A review of a loop of KATX base reflectivity imagery indicated that these reflectivity features were moving from the south/southeast. An Area Forecast that included the Cascade Mountains westward within the state of Washington was issued at 0345. The portion of the Area Forecast directed toward the Cascades forecasted for the accident time: broken clouds at 8,000 feet, cloud tops to FL200 (flight level 20,000 feet msl), scattered light rain showers, isolated thunderstorms and light rain, cumulonimbus cloud tops to FL350. An Airmen's Meteorological Information (AIRMET) advisory for mountain obscuration issued at 0745 PDT was active for the accident location. Prior to this AIRMET, an AIRMET for mountain obscuration was issued at 0145 PDT and was active for the accident location. AIRPORT INFORMATIONThe four-seat, high-wing, fixed-gear airplane, serial number 22-6624, was manufactured in 1959. It was powered by a Lycoming O-360 series normally aspirated engine. It was equipped with a fixed pitch propeller. Aircraft records were not located or obtained for examination. WRECKAGE AND IMPACT INFORMATIONThe wreckage was located near the summit of Twin Sisters Mountain at 6,000 ft mean sea level (msl). The Whatcom County Sheriff Search and Rescue team provided photos of the accident site and wreckage. The wreckage consisted of two distinct sections within close proximity of one another on the rock face of steep terrain. The area was void of vegetation. The first section contained the cockpit, fuselage, left wing and tail. The second portion of the wreckage consisted of the right wing. The engine had separated from the engine mount and was located in the debris field. The propeller had separated from the engine crankshaft flange, one blade had about 4 inches of the tip missing and the trailing edge was deformed in an elongated s-shape, the other blade was bent aft. The wreckage was not recovered from the accident site and no follow-up examination was accomplished. MEDICAL & PATHOLOGICAL INFORMATION An autopsy was performed on the pilot on July 17, 2015, by the Whatcom County Chief Medical Examiner, Bellingham, Washington. The cause of death was listed as multiple blunt trauma injuries. The FAA's Civil Aerospace Medical Institute (CAMI) Forensic Toxicology Research Team performed forensic toxicology on specimens from the pilot with negative results for ethanol or listed drugs. Tests for carbon monoxide and cyanide were not performed.