WPR10LA130

The pilot reported that about 30 minutes into the 45 minute-long flight, the engine began vibrating and he maneuvered the airplane back towards the airport. As the airplane joined the downwind leg of the traffic pattern, the pilot heard a "bang," followed by the engine experiencing complete loss of engine power. Unable to land at the airport due to altitude loss, the pilot performed a forced landing in an adjacent park. During the landing sequence the airplane struck a building while the pilot attempted to avoid a pedestrian. The airplane sustained substantial damage to the right wing and forward fuselage. Examination of the radial engine revealed that one of the cylinder heads and its associated piston had become separated. The remaining internal sections of the engine sustained catastrophic damage, consistent with continued rotation after the failure of the cylinder. Half of the studs attaching the failed cylinder to the head exhibited fatigue signatures consistent with insufficient clamping pressure applied to their nuts. A full service history for the engine was not located; however, maintenance records indicated that the cylinder head had not been removed in the 11 years and 317 flight hours prior to the accident.

HISTORY OF FLIGHT On February 7, 2010, about 0840 mountain standard time, a Nanchang China CJ-6A, N6263D, experienced a loss of engine power and the pilot made a forced landing in a park near Falcon Field Airport, Mesa, Arizona. The pilot was operating the airplane under the provisions of Title 14 Code of Federal Regulations Part 91. The certificated commercial pilot was not injured, and the passenger sustained minor injuries. The airplane sustained substantial damage. The local flight departed Falcon Field, about 0810. Visual meteorological conditions prevailed, and no flight plan had been filed. According to the pilot, about 30 minutes into what was planned to be a 45-minute flight, he began to feel the engine vibrating. He then turned the airplane back to Falcon Field, and as the airplane joined the downwind leg, he heard a loud, "bang." The engine then lost power, such that the airplane was no longer able to maintain altitude. The pilot then made a 180-degree turn with the intention of landing in an adjacent park. During the final approach, he observed someone walking on his intended landing area. He banked the airplane to the left and landed; during the ground roll, the airplane collided with a restroom facility on the park property. During the accident sequence, the right wing, firewall, and forward fuselage structure sustained substantial damage. AIRCRAFT INFORMATION The airplane (serial number 4732023) was manufactured in 1984, and subsequently imported from China to the United States. It was equipped with a Housai H6, air-cooled, nine-cylinder radial engine, serial number 201-H6A822361, and a metal two-bladed constant speed propeller. On June 1, 1998, the airplane was issued the Federal Aviation Administration (FAA) registration number of N6263D, and an experimental-exhibition airworthiness certificate. No maintenance records were located for the airplane dating prior to 1998. Airframe logbook records indicated that the last recorded maintenance action was a conditional inspection, which was conducted at 2,338.8 total flight hours, on April 9, 2009. The accompanying engine logbook reported an 'unknown' total engine time at that inspection. The first entry in the logbook, dated November 22, 1999, indicated a total airframe time of 2,022 hours. No entries were noted indicating any major maintenance or overhaul to the engine. TESTS AND RESEARCH The engine was examined by the NTSB investigator-in-charge (IIC) and an Airframe and Powerplant mechanic, who specializes in the Housai series of radial engines. The engine remained attached to the radial portion of its mount, which had become separated from the airframe. External examination of the engine revealed that cylinder number four had become detached from the crankcase at its flange, with the eight cylinder head studs separated at the crankcase. The stud shanks remained affixed within the crankcase, fracturing at varying lengths. The cylinder head remained partially attached by the sparksplug leads and fuel lines. The piston, upper section of its connecting rod, both pushrods, and all cylinder head nuts had been ejected from the engine, and were not located. Rotation of the propeller by hand resulted in free, smooth, and unrestricted movement. Green-colored, translucent oil was observed within the crankcase and rocker covers. The oil shutoff valve, located on the firewall, was noted in the open position. Internal examination of the engine revealed catastrophic damage. All remaining piston rods were still attached to their respective pistons, but had become separated from the master rod bearing. The rod ends for pistons 1, 2, 3, 5, and 6 remained attached to the master rod flange, and had separated about 2 inches from their knuckle pins. Movement of the rods by hand resulted in smooth and free motion. The rod ends for pistons 7, 8, and 9 had become detached from their knuckle pins; their associated master rod pin mating surfaces were clean and shiny. With the exception of cylinder and piston number 5, all separated rod surfaces exhibited, 'peening' damage with obliterated surface features consistent with continued movement after failure. All cylinder sleeves and the areas of the case adjacent to the carry-through bolts had sustained gouging damage, consistent with connecting rod contact. The crankshaft balance weights sustained radial scoring along their outer edges. The pistons had sustained scoring damage to their lower skirts consistent with contact with the rotating crankshaft weights and piston rods. External examination of the cylinder flanges revealed no obvious indications of oil leak. Examination of the remaining ancillary engine components revealed no defects that would have affected engine operation. Cylinder number four, a section of its associated engine case flange, and piston number five with its connecting rod were sent to the NTSB materials laboratory for further examination. Laboratory examination of one cylinder head stud within the case flange revealed a smooth arced fracture surface in the root of the stud thread, oriented perpendicular to the axis of the stud. A closer view of the fracture surface revealed ratchet marks initiating in the root of the thread. Adjacent to the ratchet marks, a section of thread appeared to have been torn from the stud, and bent in the direction away from the arced surface. Further examination of the flange area revealed that three more studs also displayed similar fracture features. Examination of the number five piston connector rod fracture face revealed a deformed section with bent flanges that displayed slant planes and mechanical damage. The damage was consistent with continued engine rotation subsequent to failure.