HISTORY OF FLIGHTOn January 4, 2020, about 1009, Pacific standard time, an experimental amateur-built TM-1 Thunder Mustang airplane, N151WR, was destroyed when it was involved in an accident near Santa Clarita, California. The pilot was fatally injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. The airplane was cleared for takeoff and instructed to maintain at or below 2,500 ft mean sea level (msl) until north of the airport. On departure, the airplane was approved a frequency change. However, the airplane turned back towards the airport and the pilot re-established communication with the tower controller. The pilot reported that he “had smoke out of the right stack,” and then advised the controller that he was “going to have to put her down.” The controller asked for his position and the pilot responded, “over Newhall.” The pilot further reported that he could not see anything and that he lost power to his engine. The controller acknowledged and then the pilot stated that “I am going to have to put it down on,” when the transmission was cut off. A witness observed the airplane maneuvering at a low altitude with smoke trailing from the airplane just before impact. He stated the airplane appeared to be under control. The airplane struck a tall tree and subsequently came to rest in a grassy field located in the median of an interstate’s off-ramp cloverleaf roadway. AIRCRAFT INFORMATIONThe low-wing airplane was powered by a Falconer V12 liquid cooled engine. The engine was equipped with a four-bladed, variable pitch propeller. The airframe and engine logbooks were not located during the investigation. AIRPORT INFORMATIONThe low-wing airplane was powered by a Falconer V12 liquid cooled engine. The engine was equipped with a four-bladed, variable pitch propeller. The airframe and engine logbooks were not located during the investigation. WRECKAGE AND IMPACT INFORMATIONExamination of the accident site revealed that the airplane impacted level terrain in a median on a freeway exit ramp. The terrain consisted of grass and dirt. On the east side of the median several tall trees were present. Near the top of about a 60 ft tall tree, several branches were observed broken with fresh breaks. A section of the airplane was embedded in the tree. Additionally, parts of the propeller blades were located at the bottom of the tree. The right-wing tip and parts of the wing were also located near the initial impact with the tree. The airplane wreckage came to rest upright on about a 140º magnetic heading about 75 ft past the initial impact. All major components of the airplane were contained within the main wreckage site. Most of the wreckage debris, consisted of small fragments of the canopy and of the landing gear that had sheared off. There was a postimpact fire that thermally damaged the majority of the airplane and engine. Flight control continuity could not be verified due to thermal and impact damage. An examination of the airframe and engine was accomplished at a secure facility. All engine components and accessories were present and remained attached to the engine. The engine sustained thermal and impact damage. Damage was noted to the engine mount support tubes. The engine would not rotate. A borescope inspection of the cylinders revealed evidence that the 6 cylinders on the right side of the engine had normal operating conditions, however, several cylinders on the left side of the engine, cylinders Nos.1, 3, and 5 were nearly black in color. The intake manifold, valve cover, and intake plenum were removed from the engine. The exhaust valve push rod for the No. 5 cylinder was not secured and observed lying inside the engine. The left side head assembly was removed and revealed that the Nos. 1, 3, and 5 cylinder head attachment areas each had material deformation consistent with an internal combustion gas leak. The No. 1 cylinder had material deformation through a coolant passageway and the crankcase, and about a 1 cm wide by 3 ml deep hole was observed. Additionally, discoloration and material deformation near the hole was consistent with extreme hot combustion gasses. On the Nos. 3 and 5 cylinders, the hot combustion gasses had only penetrated into the coolant passageway and had not yet burnt through the crankcase. All the sparkplugs were removed, and the spark plug electrodes were dark in color and exhibited normal wear signatures when compared to the Champion Check-A-Plug comparison chart, with the exception of those for cylinders Nos. 1, 3, and 5, (left side), which were very black in color, oily, and carbon fouled. The No. 1, cylinder spark plug electrode was significantly darker in color that the others. The No. 3 and No. 5 cylinder’s spark plug electrodes were dark in color but not as dark as the No. 1 cylinder. The engine was disassembled. The cylinders’ piston tops and cylinder heads were absent of any carbon build up and exhibited a sandblasted appearance consistent with detonation. Further, the cylinders exhibited damage consistent with detonation at the cylinder head and piston face edges. Traces of oil were present, and no coolant was observed in the engine. The fuel manifold was removed, and each fuel injector was examined. Except for the thermal damage no anomalies or obstructions to flow were noted. Postaccident examination of the airframe and engine revealed no additional preimpact mechanical anomalies or malfunctions that would have prevented normal operation. ADDITIONAL INFORMATIONThe FAA Handbook of Aeronautical Knowledge defines detonation as “an uncontrolled, explosive ignition of the fuel-air mixture within the cylinder’s combustion chamber. It causes excessive temperatures, which if not corrected, can lead to failure of the piston, cylinder, or valves.” The FAA Handbook also states: Detonation is characterized by high cylinder head temperatures and is most likely to occur when operating at high power settings. Common operational causes of detonation are: Use of a lower grade fuel than that specified by the aircraft manufacturer Operation of the engine with extremely high manifold pressures in conjunction with low rpm Operation of the engine at high power settings with an excessively lean mixture MEDICAL AND PATHOLOGICAL INFORMATIONThe Department of Medical Examiner Office, Los Angeles, California, conducted the autopsy on the pilot. The medical examiner determined that the cause of death for the pilot was attributed to multiple blunt force injuries. The FAA's Forensic Sciences Research Laboratory, Oklahoma City, Oklahoma, performed toxicological testing on the pilot. The pilot’s results for the testing were negative except for positive results for Glucose and Tamsulosin. The pilot had Glucose in the vitreous and urine but had no reported history of diabetes mellitus. Tamsulosin is a prescription medication used to treat the symptoms of an enlarged prostate. Tamsulosin is in a class of medications called alpha blockers. This medication is generally acceptable for use by airmen.