NTSB Factual Report

On August 6, 2001, at 1030 central daylight time, a Piper PA-32-300, N5481T, sustained substantial damage during a forced landing to a soybean field after a loss of engine power. The private pilot was not injured. The 14 CFR Part 91 pleasure flight had departed Meigs Field (CGX), Chicago, Illinois, at 1000, en route to Batten Airport (RAC), Racine, Wisconsin. Approximately 8 miles south of RAC, the airplane lost power and a forced landing was accomplished. Visual meteorological conditions prevailed and no flight plan was filed.

The pilot reported he was flying in cruise flight at 2,800 feet mean sea level. The airplane was operating normally with no abnormal engine indications. He reported that he heard a "loud bang" and experienced a complete loss of engine power. He attempted to restart the engine but with no success. He declared an emergency and executed a forced landing to a soybean field.

The on-site inspection of the airplane revealed that the number 3 cylinder and connecting rod had separated from the engine. All of the number 3 cylinder hold down studs were found broken.

The IO-540-K1A5, serial number L-9929-48, Lycoming engine was removed from the airframe and shipped to Textron Lycoming for an engine teardown. An engine teardown and metallurgical examination of engine parts were conducted on October 11, 2001, with a National Transportation Safety Board (NTSB) investigator present at the inspections.

The engine teardown revealed the following concerning the cylinders:

"The cylinders were all TEXTRON Lycoming Nitride. The rocker arms, valves, valve seats and valve springs were all in normal condition. The cylinder combustion chambers were consistent with normal operation except #3 cylinder which was separated from the engine and #5 cylinder which had heavy secondary debris damage. The pistons, piston rings, and cylinder walls displayed normal operational conditions with no pre-mishap mechanical discrepancies except for #3 cylinder assembly which had separated from the engine."

The engine teardown revealed the following concerning the crankcase and main bearings:

"Crankcase parting surface at the #2 main position had heavy fretting and the #3 and #4 top face had light fretting. The crankcase had field rework numbers stamped on it. All four crankcase ribs adjacent to the #3 cylinder deck were possibly welded. All eight hold down studs on the #3 cylinder deck were broken. There were no abnormalities observed with the main bearings, which were 'Superior' bearings. The anchored thru-studs were submitted to the Material Lab at TEXTRON Lycoming by the request of the NTSB Investigator in Charge (IIC). (See TEXTRON Lycoming "Air Safety Investigation Single Engine Teardown Report")

The TEXTRON Lycoming metallurgical report stated that no part markings or identifying markings were observed on any of the thru-studs. The report stated, "In order to evaluate the thru-studs and the short studs to engineering drawing requirements the following Textron Lycoming part numbers were used: 69679, 667634, 50-15, and 38-13."

The report indicated the following information concerning the upper front thru-stud:

"The upper front thru-stud fractured through the root of the sixth complete thread from the neck between the two thread sets. The stud fractured through approximately 95% of the cross-section by fatigue as characterized by the smooth transgranular topography and beach marks…...The hardness of the stud was 49HR30N, which is below the engineering drawing requirement of 56-60 HR30N. Chemical analysis of the stud conformed to AMS 6322 steel. Examination of the microstucture revealed tempered martensite and ferrite…...However, the ferrite was present in greater amounts and larger size than typically observed in properly heated AMS 6322. This indicates this stud was most likely not properly heat treated."

The report indicated the following information concerning the lower front thru-stud:

"The upper front thru-stud fractured through the root of the fifth complete thread from the neck between the two thread sets. The stud fractured through approximately 90% of the cross-section by fatigue as characterized by the smooth transgranular topography and beach marks…...The hardness of the stud was 52-54 HR30N, which is below the engineering drawing requirement of 56-60 HR30N. Chemical analysis of the stud conformed to AMS 6322 steel. Examination of the microstucture revealed tempered martensite and ferrite…...There was slightly more ferrite than typical of properly heated AMS 6322. This indicates this stud was most likely not properly heat treated."

The report indicated that the lower rear 1/2 inch stud fractured through approximately 80% of the cross-section by fatigue. The hardness of the stud was 33 HRC, which conformed to the engineering drawing requirement of 26-38 HRC.

The report indicated that the upper rear 1/2 inch stud fractured through approximately 70% of the cross-section by fatigue. The hardness of the stud was 30-31 HRC, which conformed to the engineering drawing requirement of 26-38 HRC.

The report indicated the following information concerning the 3/8 inch cylinder studs:

"The four studs all fractured in fatigue through approximately 65-75% of the cross-section and initiated from multiple origins from the root of a thread. These stud fractures are secondary to the anchored thru-stud, P/N 69679, fractures."

The TEXTRON Lycoming metallurgical report summarized the findings as follows:

"Based upon the fractures of the #3 cylinder studs, the four 1/2 inch studs fractured in the order: upper front, lower front, lower rear, and upper rear. The four 3/8 inch studs fractured as a secondary result of the ½ inch stud fractures.

It was also determined that the upper and lower front thru-studs, P/N 69679, had low hardnesses and microstuctures most likely indicating improper heat treatment. The other thru-studs in the crankcase, P/N 69679 and P/N 66734, also had low hardnesses and microstuctures most likely indicating improper heat treatment.

It is considered that the improper heat treatment of the upper and lower front through studs, P/N 69679, is most likely the primary contributing cause to the separation of the #3 cylinder." (See TEXTRON Lycoming Materials Laboratory Report No. 13143)

The TEXTRON Lycoming Materials Laboratory Report No. 13143 and the associated thru-bolts were sent to the NTSB Materials Laboratory for examination. The NTSB lab verified that the #3 cylinder 1/2 inch studs had failed as a result of fatigue. The NTSB lab verified that the hardness of the thru-studs was below the requirements outlined in engineering drawings.

The airplane maintenance records revealed that the last 100 hour maintenance inspection was conducted on December 12, 2000. The airplane had flown about 104 hours since the last inspection. The maintenance records revealed that the engine had been overhauled at an engine overhaul facility on February 11, 1999. It had accumulated about 532 hours of flight time since the last major overhaul. The engine overhaul facility did not, and was not required by the Federal Aviation Administration regulations or by the engine manufacturer's overhaul manual, to test the engine thru-studs for hardness during the engine overhaul.

NTSB Probable Cause

The total loss of engine power during cruise flight due to the number 3 cylinder thru-studs fatigue fractures, and the thru-stud's low hardness as a result improper heat treatment of the thru-studs, and the unsuitable terrain encountered during the forced landing.