LYNCHBURGH, VA, USA
N1559X
Beech A36
Shortly after takeoff, the pilot heard a loud bang, felt a thump, and saw the engine cowling deflect upward. He reduced engine power, and turned towards the departure airport. After a second loud bang, followed by severe vibration the pilot shut down the engine, and performed a forced landing to a field. Examination of the wreckage revealed two holes in the top of the engine crank case which were over 8 inches in diameter. One of the engine's number six cylinder connecting rod bolts showed signs of a catastrophic failure, the other number six connecting rod bolt was intact and did not contain a nut. Examination of the number six connecting rod, and connecting rod bolts revealed severe mechanical impact damage to the threads of the intact bolt. The broken bolt had three different fracture features. One was consistent with failure in shear, a second was consistent with a tensile overstress separation, and the majority of the third had a rubbed appearance. The connecting rod bolts and nuts had been replaced 375 hours before the accident flight. The last annual inspection of the airplane was performed about 16 months prior to the accident.
On March 5, 1998, about 1600 eastern standard time, a Beech A36, N1559X, was substantially damaged during a forced landing after takeoff from the Lynchburg Regional/Preston Glenn Field Airport (LYH), Lynchburg, Virginia. The certificated airline transport pilot and two passengers were not injured. Visual meteorological conditions prevailed, and no flight plan had been filed for the flight destined for Winston Salem, North Carolina. The personal flight was conducted under 14 CFR Part 91. In a telephone interview, the pilot said the airplane was climbing through 2,500 feet, when he heard a loud bang, felt a "thump", and saw the engine cowling deflect upward. He reduced engine power and initiated a turn back toward LYH. The pilot determined the airplane would not reach the airport at his current rate of decent and he applied some power. He then heard a second loud bang, followed by severe vibration. He feathered the propeller, shut down the engine and performed a forced landing to a field, about 1 mile from LYH. Examination of the wreckage performed by a Federal Aviation Administration Inspector revealed two holes in the top of the engine crank case which were over 8 inches in diameter. Three of the six connecting rods separated from the crankshaft throws. Two connecting rod throws exhibited signs of lack of lubrication and high temperature. The number six cylinder connecting rod throw was clean, polished, and exhibited signs of adequate oil. One of the number six cylinder connecting rod bolts showed signs of a catastrophic failure. The other number six connecting rod bolt was intact and did not contain a nut. The number six cylinder connecting rod, number six cylinder connecting rod cap with bolts, and material found in the crank case bay of cylinders three and four, were sent to the NTSB Materials Laboratory for examination. The NTSB Materials Laboratory Factual Report stated: "...One of the bolts from the # 6 connecting rod cap was fractured. The head of this bolt was trapped in the cap when received. The other bolt was intact but was also trapped within the cap. This intact bolt showed severe mechanical impact damage (mushroom deformation) to the threads. The connecting rod cap had multiple impact damage areas and deformation in the vicinity of the intact bolt...The material found in the crank case bay of cylinder #3 and # 4 showed signs of exposure to heat....All crack surfaces on these parts were damaged either by exposure to heat or by rubbing...." Scanning electron microscope (SEM) examination of the broken bolt revealed three different fracture features. One feature was consistent with a failure in shear, one feature was consistent with a tensile overstress separation, and the majority of the other feature had a rubbed appearance. A review of the airplane's engine log book revealed that on March 10, 1998, about 375 hours prior to the accident, work was performed on the engine which included the replacement of the connecting rod bolts, and nuts. The last annual inspection of the airplane was performed on November 12, 1996, at 263.1 total engine hours. At the time of the accident, the airplane's engine had accumulated a total of 465 hours.
Failure of the number six connecting rod bolt due to improper maintenance.
Source: NTSB Aviation Accident Database
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