Moosic, PA, USA
N3790T
Piper PA28R
During a personal cross-country flight, the owner seated in the right front seat noted the No. 2 cylinder exhaust gas temperature had decreased about 1/2 of its value. The pilot-in-command (PIC) and owner both then felt a vibration and positive control of the airplane was transferred to the owner as was briefed before departure. An emergency was declared with air traffic control while (best glide speed) airspeed was maintained. The PIC reported there were no suitable airports nearby and proceeded to a diversion airport 77 miles away. While continuing toward the diversion airport, the oil pressure decreased to zero. Realizing the flight would not reach the airport, the pilot maneuvered for a forced landing to a residential street and collided with a powerline. The accident site was located less than 2 nautical miles north-northeast from the approach end of the intended runway at the diversion airport. Postaccident examination of the engine revealed that all anchored-thru or free-thru studs that secured the No. 2 cylinder to the crankcase exhibited fatigue. The fatigue originated at the roots of the threads and propagated though about 70% to 80% of the diameter of the studs before finally fracturing due to overstress. The fatigue-fractured studs led to the cylinder separating from the engine, the loss of oil supply, and subsequent total loss of engine power. Although the pilot was not aware of the extent of the problem with the No. 2 cylinder, about the time the problem arose, the flight was about 30 nautical miles west-northwest of an international airport with available runways of sufficient length. As the flight continued in a southerly direction, about the time and location the oil pressure decreased to zero, another airport with a runway of more than adequate length was located 20 nm closer than the intended diversion airport. Thus, had the pilot diverted to the nearest airport immediately after the loss of the No. 2 cylinder, or diverted to an airport about 20 nautical miles closer than his intended airport after the oil pressure decreased to zero, it is likely an uneventful on-airport forced landing could have been performed. Twelve years after the engine was overhauled, or 3 years and 270 hours prior to the accident, the No. 2 cylinder was removed to replace a fractured anchored-thru stud. After the fractured anchored-thru stud was replaced, the cylinder was installed. Several entries indicated that the hardware that clamped the crankcase halves together on the top portion of the engine was re-torqued. It is likely that during installation of the No. 2 cylinder following replacement of the fractured anchored-thru stud, the anchored-thru and free-thru studs were inadequately torqued, which resulted in fatigue fracturing of all studs followed by cylinder separation, oil exhaustion, and resulting total loss of engine power.
On September 20, 2020, about 2047 eastern daylight time, a Piper PA-28R-180, N3790T, was substantially damaged when it was involved in an accident near Moosic, Pennsylvania. The pilot sustained serious injury, and the pilot-rated passenger/airplane owner sustained minor injuries. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. The pilot-in-command (PIC) stated that before departure, the airplane owner checked the fuel and oil, and said the airplane was “looking good.” She did not recall if the owner said anything about the oil quantity. No discrepancies were noted during the engine run-up and after takeoff, with visual flight rules flight-following, the flight climbed to 8,500 ft mean sea level. During cruise flight about 30 miles from Wilkes-Barre Scranton International Airport (AVP), Wilkes-Barre/Scranton, Pennsylvania, the PIC noted that the owner could not obtain the desired rich-of-peak setting and asked him if she should adjust the fuel to air ratio. The owner enrichened the mixture and looked at the engine monitor and noted the No. 2 cylinder exhaust gas temperature (EGT) had decreased about 1/2 of its value. They both felt a vibration and positive control of the airplane was transferred to the pilot-rated passenger/owner as had been agreed before departure. An emergency was declared with Federal Aviation Administration (FAA) air traffic control, and they reviewed nearby airports, but determined none were suitable. The PIC elected to proceed to AVP and maintained best glide speed while flying toward AVP. During the descent, the PIC advised the owner that they were losing the second cylinder (No. 1 position). The partial loss of engine power checklist was performed, and the flight continued toward AVP. The PIC reported that the oil pressure went to zero, though the oil temperature was OK, and she reported smoke which she attributed to be from fire and called that out to the owner. Both reported that the engine rpm went past maximum redline and that it was not controlled by the propeller control. Although the flight was close to AVP, both realized the flight would not reach the airport and the owner turned to the right toward a parking lot but realized there was a tall building between their position and the parking lot. He then maneuvered for a forced landing to a residential street and attempted to extend the landing gear but neither occupant saw three green landing gear down and locked lights. Before touchdown, the owner saw a flash of light consistent with collision with a powerline. Neither occupant recalled the impact sequence. Both were taken to a hospital. The accident site was located about 1.8 nm and 019° from the approach end of runway 22 at AVP. Examination of the engine by the National Transportation Safety Board with assistance from the engine manufacturer revealed the No. 2 cylinder remained attached by the exhaust and had a section of crankcase attached. The cylinder with attached section of crankcase was retained for examination by the NTSB Materials Laboratory. During further examination of the engine, a fractured anchored-thru stud at the 10 o’clock position was found finger-tight and could be backed out by finger pressure. The Nos. 3 and 4 cylinders could not be removed because the skirts of each cylinder were damaged. A hole was drilled into the upper crankcase half for borescope inspection, which revealed the No. 4 cylinder connecting rod was separated. The engine was not disassembled further. Excised or fractured studs (either anchored-thru or free-thru), and three cylinder hold-down nuts were also retained. According to the NTSB Materials Laboratory examination, the fracture surface on the crankcase remnant attached to the cylinder contained fracture features consistent with overstress fracture in an aluminum casting. The threaded hole for the free-thru stud at the four o’clock position exhibited batter and mechanical attrition such that the threads were almost entirely worn away and the hole was enlarged. The fracture surfaces on the stud remnants all exhibited diffuse-origin fatigue cracks that initiated at the roots of the threads and propagated though about 70% to 80% of the diameter of the stud before they succumbed to final overstress fracture. Studs in the eight, ten, and eleven o’clock positions that were anchored-thru studs exhibited a thread locking compound on some of the threads engaged in the crankcase. Review of the engine logbook revealed the engine was overhauled by an airframe and powerplant mechanic in July 2005. Routine scheduled inspections occurred after overhaul until March 2013, when the No. 3 cylinder was removed and sent for repair and September 11, 2017, when the No. 2 cylinder was removed and a broken anchored-thru stud at the eleven o’clock position was extracted. A new stud, part number 50-15 was installed at the eleven o’clock position using “lock tight.” The airframe total time at that time was 7,405.79 and the tachometer time was 5,100.42. An entry dated June 20, 2018, indicated, “torqued engine top back-bone bolts per Lycoming IO360 Maintenance Manual. Tightened oil cooler lines. Inspected cooler for cracks. Tightened oil return lines.” The next entry 2 days later associated with a 100-hour inspection also mentioned torquing the engine top back-bone bolts with a slight increase in the tachometer time reading. The last entry associated with an inspection of the engine on December 5, 2019, revealed normal compression readings in each cylinder. The tachometer time at the time of the accident was not determined. At the time of the accident 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. Read-out of the engine data monitor by the NTSB Vehicle Recorder Division revealed the cylinder head temperature (CHT) readings for the No. 2 cylinder were higher than the other cylinders when all recorded data was reviewed. That was also the case initially during the accident flight, but the values then decreased in line with the values for the other cylinders, which had not been seen for the previous 2 flights. The data indicated that the No. 2 EGT values decreased sharply about 2000:30, with a corresponding drop in CHT temperatures for the same cylinder. The oil pressure, which had been steady, began to decrease about 2004:27, and was at 0 psi at 2011:06. The EGT readings for the remaining three cylinders were at normal values until 2011:34, when the Nos. 1 and 4 cylinders readings began to decrease and 2012:52, when the EGT values for the No. 3 cylinder began to decrease. According to the ADS-B data correlated with the engine data monitor report, at the time and location when the loss of the No. 2 cylinder occurred, the flight was located about 30 nautical miles west-northwest of Albany International Airport, Albany, New York. The ADS-B data indicated that the flight continued towards AVP, and at the time and location where the oil pressure went to 0 psi, the airplane was located about 57 nautical miles east-northeast of the Greater Binghamton Airport/Edwin A Link Field (BGM), Binghamton, New York, or about 77 nautical miles from AVP. According to Lycoming Service Instruction 1029D, all Avco Lycoming aircraft engines incorporate bolts and long thru studs that extend through the crankcase halves primarily for holding them together. The studs also secure the cylinders to their mounting decks on the crankcase. Two basic types of thru studs are used: anchored and free-thru. The anchored-thru studs are threaded into one of the crankcase halves; the free-thru studs are not. Instead, the free-thru studs extend through both crankcase halves. Because of their interference fit in the main bearing webs, the free-thru studs also serve as dowels to align the crankcase halves.
The inadequate torquing of anchored-thru and free-thru studs during installation of the No. 2 cylinder, resulting in their fatigue fracture and subsequent total loss of engine power. Contributing to the undershoot landing was the pilot’s failure to divert to closer airports than the original diversion airport after experiencing engine performance issues.
Source: NTSB Aviation Accident Database
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