CHI95LA097

According to the pilot, approximately ten minutes into the flight, the engine began to run rough. The pilot stated he began to troubleshoot and everything appeared normal except the fuel flow was low. At this time he advanced the throttle, switched fuel tanks, and turned the fuel boost pump on. The pilot stated he then heard a 'bang' from the engine and all engine power was lost. The pilot made a forced landing on an access road adjacent to a highway. Postaccident examination revealed the number four cylinder had separated from the crankcase. Fractures of the cylinder hold down studs showed no clear evidence of fatigue cracking. Fractures are representative of overstress separations. Fretting damage was observed on the mating cylinder flange, crankcase boss, and metal folds on the cylinder flange/hole wall corners.

On March 1, 1995, at 1730 central standard time, a Piper PA-R, N1978J, registered to Lowe Automotive, St. Louis, Missouri, was substantially damaged following a loss of engine power and subsequent off-airport forced landing near Joliet, Illinois. The private pilot and one passenger reported minor injuries. The 14 CFR Part 91 flight originated in West Chicago, Illinois, at 1710 with an intended destination of Bloomington, Illinois. An IFR flight plan was filed and visual meteorological conditions prevailed at the time.

According to the pilot, approximately ten minutes after departing DuPage County Airport, West Chicago, Illinois, the engine began to run rough. The pilot stated he began to troubleshoot and "everything appeared good except [the] fuel flow was low." At this time, he advanced the throttle, switched fuel tanks, and turned the fuel boost pump on. The pilot stated he then heard a "bang" from the engine and all engine power was lost. The pilot made a forced landing on an access road adjacent to a highway. While landing, the airplane hit an embankment before coming to rest.

Postaccident examination revealed the number four cylinder was separated from the crankcase but still attached to the exhaust stack and ignition wires. The base of the cylinder was substantially damaged but there was no obvious damage to the inside walls of the cylinder. The bottom skirt of the number four piston was also substantially damaged.

The lower left cylinder hold down stud was found completely out of the crankcase, but still in the hole of the cylinder base flange with the nut attached. Another stud, which was found at the accident site, was fractured nearly flush with the underside of the assembled nut. Both studs were sent to the NTSB Materials Laboratory, Washington, D.C., for further examination.

Examination revealed the threads of the intact stud were damaged between the underside of the nut to a position corresponding to the plane of the crankcase cylinder boss when the stud is assembled to the crankcase. The Metallurgist Factual Report stated "the thread damage was as if the thread crowns had been flattened by the cylinder flange moving against this area." Examination of the other stud disclosed features indicative of an overstress separation with no evidence of preexisting cracking.

A second postaccident examination of the cylinder and crankcase was done by an NTSB Metallurgist at DuPage Airport where the airplane was being stored. Examination showed no clear evidence of fatigue cracking. All fractures appeared to be representative of overstress separations. No welding repair to the crankcase was observed. Fretting damage was observed on the mating cylinder flange, crankcase boss, and metal folds on the cylinder flange/hole wall corners. (See attached Metallurgist Factual Report)