CEN15LA146 · Piper PA 28R-180

The private pilot had purchased the airplane and had a mechanic perform a prepurchase inspection. During the inspection, the mechanic determined that a cylinder had recently been changed. The pilot subsequently departed at night for a cross-country flight to his base airport. The pilot reported that, during cruise flight at 8,500 ft mean sea level and about 2.2 hours into the flight, he heard an "explosion"; the airplane then decelerated and started rapidly descending. The pilot was unable to restart the engine and chose to perform forced landing on a road. The pilot placed the landing gear selector in the "down" position, but only the main landing gear extended, so he conducted the emergency landing gear extension procedures; however, the nose landing gear still did not extend. During the forced landing, the airplane impacted a highway sign pole, departed the road, impacted a culvert, traveled over a ditch, and came to rest about 50 ft from the road.

According to the mechanic who had recently replaced the No. 4 cylinder, he removed the cylinder and installed the replacement cylinder in accordance with the engine manufacturer overhaul manual directions. Specifically, he replaced the rod bolts and nuts and reinstalled the connecting rod. The mechanic initially reported that he performed the cylinder break-in procedures and that the cylinder functioned normally. However, he subsequently reported that the break-in flight time was not flown continuously as recommended in the service instruction.

Initial examination of the engine revealed that the No. 4 cylinder had separated from the crankcase. During subsequent disassembly and examination, a large hole in the crankcase was observed between the Nos. 3 and 4 cylinders. The No. 4 connecting rod was severely bent with the piston pin still in the connecting rod small end; the piston was fractured around the skirt and through the pin bosses. The remaining skirt portions exhibited a burnished appearance, consistent with contacting the bore. The upper right anchor through bolt exhibited stretching, and part of the fracture surface was separated in a flat pattern perpendicular to the length of the through bolt. Some of the cylinder through bolts were missing, but one 1/2-inch through bolt from the No. 4 cylinder was found, and it exhibited a granular appearance and was broken perpendicular to the length of the bolt. The upper sections of the No. 4 cylinder deck and a section of the upper right anchor bolt exhibited fretting, indicative of a loss of torque.

Based on the observed witness marks and damage, the No. 4 piston failed, which liberated the connecting rod. The connecting rod with the piston pin still in the small end bore contacted the camshaft and fractured it and the adjacent crankcase structure. The separated rear section of the camshaft and crankcase structure was displaced toward the No. 4 cylinder and led to its separation, which caused the engine to stop running; this scenario is consistent with the pilot's report of an explosion and engine stoppage. Further, the fretting damage and bolt failures indicated that the No. 4 cylinder likely separated from the crankcase due to a loss of torque on the cylinder bolts. The break-in procedures used by the mechanic were likely not a factor in the accident.

Verbatim NTSB analysis and probable cause from the NTSB dataset