Accident Overview
On September 20, 2020, about 2047 eastern daylight time, a Piper PA-28R-180, registration N3790T, was substantially damaged when it was involved in an accident near Moosic, Pennsylvania. The pilot-in-command sustained serious injury, and the pilot-rated passenger, who was also the airplane owner, sustained minor injuries. The airplane was being operated as a personal flight under Title 14 Code of Federal Regulations Part 91.
Pilot and Passenger Accounts
According to the pilot-in-command, before departure the owner checked the fuel and oil and reported the airplane was “looking good.” She did not recall if the owner mentioned the oil quantity. No discrepancies were noted during engine run-up or after takeoff. The flight climbed to 8,500 ft mean sea level under visual flight rules with flight-following. During cruise, about 30 miles from Wilkes-Barre Scranton International Airport (AVP), the pilot noted the owner could not obtain the desired rich-of-peak setting. After the owner enrichened the mixture, the engine monitor showed a decrease of about half in the No. 2 cylinder exhaust gas temperature (EGT).
Both occupants felt a vibration, and control was transferred to the owner as previously agreed. An emergency was declared with FAA air traffic control. They reviewed nearby airports but deemed none suitable. The pilot elected to proceed to AVP at best glide speed. During descent, the pilot advised the owner that the No. 1 cylinder was also failing. They performed the partial loss of engine power checklist. The pilot reported oil pressure dropped to zero, though oil temperature remained normal, and she noticed smoke, which she attributed to fire. Both noted the engine rpm exceeded maximum redline and was uncontrollable by the propeller control.
Although close to AVP, they realized they would not reach the airport. The owner turned toward a parking lot but saw a tall building in the way and maneuvered for a forced landing to a residential street. He attempted to extend the landing gear but did not see three green lights indicating gear down and locked. Before touchdown, the owner saw a flash consistent with hitting a powerline. Neither recalled the impact sequence. Both were taken to a hospital. The accident site was about 1.8 nm and 019° from the approach end of runway 22 at AVP.
Engine Examination
National Transportation Safety Board examination, with assistance from the engine manufacturer, revealed that the No. 2 cylinder remained attached only by the exhaust and had a section of crankcase attached. The cylinder and attached crankcase were retained for materials laboratory examination. A fractured anchored-thru stud at the 10 o’clock position was found finger-tight. The No. 3 and 4 cylinders could not be removed due to damaged skirts. A borescope inspection via a drilled hole in the upper crankcase showed the No. 4 cylinder connecting rod was separated. The engine was not disassembled further. Excised or fractured studs and three cylinder hold-down nuts were also retained.
Materials laboratory examination found that the fracture surface on the crankcase remnant was consistent with overstress fracture. The threaded hole for the free-thru stud at the four o’clock position showed batter and attrition, with threads almost entirely worn away. All stud remnants exhibited diffuse-origin fatigue cracks initiating at thread roots, propagating through about 70% to 80% of the stud diameter before final overstress fracture. Some anchored-thru studs had thread locking compound on threads engaged in the crankcase.
Maintenance History
According to engine logbooks, the engine was overhauled in July 2005. Routine inspections occurred until March 2013, when the No. 3 cylinder was removed and sent for repair. On September 11, 2017, the No. 2 cylinder was removed and a broken anchored-thru stud at the eleven o’clock position was extracted and replaced with a new stud using “lock tight.” At that time, airframe total time was 7,405.79 hours and tachometer time 5,100.42 hours.
An entry dated June 20, 2018, indicated torquing of engine top backbone bolts per the Lycoming IO360 maintenance manual, along with tightening oil cooler and return lines. Two days later, a 100-hour inspection also mentioned torquing the backbone bolts. The last inspection entry on December 5, 2019, showed normal compression readings in all cylinders. The tachometer time at the accident was not determined. According to the hour meter, the engine had accrued about 1,260 hours since overhaul and 270 hours since the No. 2 cylinder was removed and reinstalled.
Engine Data Analysis
Read-out of the engine data monitor by the NTSB Vehicle Recorder Division showed that No. 2 cylinder head temperatures (CHT) were higher than the other cylinders throughout recorded data, including initially during the accident flight. However, those values later decreased to match other cylinders, a pattern not seen on the previous two flights. No. 2 EGT values decreased sharply about 2000:30, with a corresponding CHT drop. Oil pressure, steady until then, began decreasing about 2004:27 and reached 0 psi at 2011:06. EGT readings for the remaining three cylinders were normal until 2011:34 when No. 1 and No. 4 began decreasing, and at 2012:52 for No. 3.
ADS-B data correlated with the engine data showed that at the time of No. 2 cylinder loss, the flight was about 30 nautical miles west-northwest of Albany International Airport, Albany, New York. When oil pressure reached zero, the airplane was about 57 nautical miles east-northeast of Greater Binghamton Airport (BGM) and about 77 nautical miles from AVP.
Additional Information
According to Lycoming Service Instruction 1029D, Avco Lycoming engines incorporate bolts and long thru studs extending through crankcase halves primarily to hold them together and secure cylinders. Two types are used: anchored-thru studs, threaded into one crankcase half, and free-thru studs, which extend through both halves and serve as dowels for alignment.