On April 26, 2018, about 1620 Pacific daylight time, a Yakolev Yak 52, N2YK, sustained substantial damage during an off-airport landing following a loss of engine power about 3 miles south of the Portland-Hillsboro Airport (HIO), Hillsboro, Oregon. The airline transport pilot and pilot rated passenger sustained serious injuries. The airplane was registered to and operated by Classic Aircraft Aviation Museum Inc. as a Title 14 Code of Federal Regulations Part 91 positioning/ferry flight. Visual meteorological conditions prevailed, and no flight plan was filed for the cross-country flight, which originated from Hobby Field (77S), Creswell, Oregon, about 1530, with an intended destination of Scappoose Industrial Airpark (SPB), Scappoose, Oregon. Representatives of the pilot reported that he did not have any recollection of the accident flight. According to a pilot (witness), who was flying in formation with the accident airplane, the flight was abeam the Corvallis Airport, at 4,500 ft mean sea level (msl), when the pilot of the accident airplane informed him that the engine was "starting to miss." The witness asked the accident pilot of his intentions, to which the accident pilot replied he would continue to Hillsboro. As the flight was south of McMinnville Airport (MMV), McMinnville, Oregon, the accident pilot informed the witness that he might need to declare an emergency; however, did not specify the nature of the problem. The witness stated that the flight continued north, despite his repeated attempts to communicate with the accident pilot and tell him to land at MMV with no response. The witness further stated that at that time, the accident airplane started a moderate descent from 4,500 ft msl, passed his airplane on the right side, and continued a heading toward HIO. The witness then transmitted a position report of nearby airports, with no response from the accident pilot. Shortly after, the accident pilot asked the witness about his location, in which the witness responded that he was in the 5 o'clock position, high, and one mile in trail. The witness stated that he was convinced that the accident airplane was going to crash into a mountain ridge, as it was still descending. As the flight was east of Yamhill, Oregon, the accident airplane turned left, paralleling the ridge, on a northwesterly heading; however, it had descended to what appeared to be about 150-200 ft agl. The witness again asked the accident pilot of his intentions, with no response. The witness continued to observe the airplane on a northwesterly heading, at a low altitude, before the accident airplane turned to a northeasterly heading. A short time later, the witness saw the accident airplane turn to the right, go under his wing, and impacted trees about 3 miles southwest of HIO. The witness added that throughout the flight from when the pilot reported the engine "starting to miss" to the time of the accident, they had overflown 6 airports that were "a suitable emergency airport." The passenger, who was also a rated pilot and mechanic, reported that he recalled arriving at 77S, and the accident airplane was already moved from the hangar. Following a preflight with the pilot, he had signed the special flight permit, so he could add it to the airframe's logbook upon their arrival to SPB. The passenger stated that the engine start and taxi out was normal, and they took off, and climbed to about 3,000 or 4,000 ft, in loose formation with another airplane. The passenger flew the airplane for a while, before turning the airplane control over to the pilot. The passenger stated that sometime after giving control of the airplane to the pilot, the engine began to sputter, followed by the pilot saying they weren't going to SPB, but instead they were headed to Hillsboro. The passenger further stated that he recalled the impact with trees. Examination of the airplane by a Federal Aviation Administration (FAA) inspector revealed that the fuselage and left wing were structurally damaged. The airplane was recovered to a secure location for further examination. Examination of the recovered wreckage by the National Transportation Safety Board (NTSB) investigator-in-charge (IIC) and representatives of the FAA revealed that both wings had been separated from the airframe to facilitate wreckage transport. Flight control continuity was established throughout the airframe to all primary flight controls. Separations in the aileron control system was due to removal of the wing and impact damage. Examination of the cockpit revealed that the throttle, mixture, propeller, and control lever continuity was established from the cockpit controls to the engine. The landing gear lever was "up," magnetos were "off", cowl flaps open, oil cooler half door open, lock lever mid span, carburetor heat off, fire valve mid-way between open and closed, throttle full forward, propeller pitch about 1 inch forward of the aft stop, lock lever full aft, primer in and locked, ventilation control lever full aft. Throttle, mixture, and propeller control lever continuity was established from the cockpit controls to the engine. The engine remained attached to the airframe via its mounts. All fuel and oil lines appeared to remain secure and attached to their respective fittings. The intake and exhaust systems were impact damaged. No evidence of any exhaust leaks/preexisting damage was observed. The number 8 front and number 3 front spark plugs were impact damaged. The propeller remained attached. One propeller blade separated at the hub, and the opposing propeller blade was fractured/splintered about mid span. The front spark plugs were removed and examined. The nos. 1, 8, and 9 spark plugs exhibited black deposits within the electrode area. Nos. 7 and 3 were impact damaged. Nos. 6, 5, 4, and 2 were oil soaked. Rotational continuity of the engine crankshaft and internal valve train was established throughout all cylinders. All intake and exhaust pushrods moved equally when the crankshaft was rotated by hand. The accessory gears rotated accordingly when the crankshaft was rotated by hand. The fuel vents appeared to be free of debris. The airframe fuel filter was removed and contained a liquid, consistent with 100 low lead fuel within the screen housing. The screen was free of debris. Both wing fuel tanks had been breached. The carburetor, engine driven fuel pump, and both magnetos were removed for further examination. Examination of the carburetor, engine driven fuel pump, and magnetos was conducted at the facilities of M14 Inc., Kingman, Arizona, under the supervision of the NTSB IIC. The left and right magnetos were found to be within E-gap and point gap specifications. Both magnetos produced spark and all ignition leads when the magneto drive shafts were rotated. Both magnetos were placed in an oven and heated to 225° F, and subsequently reinstalled on the test stand; both magnetos operated normally for 15 minutes with no anomalies noted. The engine driven fuel pump was operated and found to function normally with no anomalies noted. The carburetor was disassembled and inspected visually. No evidence of any preexisting anomalies were noted with the carburetor.