The pilot reported that the flight departed Gatlinburg/Pigeon Forge Airport (KGKT) in Sevierville, Tennessee. He proceeded northbound toward his destination, climbing to 2,500 feet mean sea level.
About 15 minutes into the flight, over a forested area near Rutledge, Tennessee, the Beech V35’s engine made a “loud bang” and started running rough.
As the pilot turned to a southerly heading and an open field, the engine seized. Unable to land in the field, he performed a forced landing into trees.
The wreckage came to rest inverted, and the pilot and passenger were met by first responders.
FAA inspectors responded to the accident site and examined the wreckage. The fuselage, wings, and empennage were substantially damaged. The external surfaces of the fuselage were covered with engine oil.
After recovery of the wreckage, the engine was removed from the airframe for examination.
Several small (about 0.75 inch) holes were found in the upper engine case, and one large hole was found under the area of the left magneto. The oil pan was removed and it contained engine oil and numerous metallic fragments.
Further disassembly of the engine revealed that one of the No. 2 crankshaft cheek counterweights was missing. The largest hole in the engine case was directly adjacent to the area where the counterweight liberated.
The retaining circlips and plates from the counterweight were found inside the engine. Two of the circlips were fractured, and two were bent but intact.
The plates and circlips were forwarded to the NTSB Materials Laboratory for analysis.
The circlips were arbitrarily numbered 1 though 4.
Circlips 1 and 2 were intact and bent.
Circlip 3 was fractured at both leg ends. An examination of both fracture surfaces revealed localized necking and cup-shaped fracture surfaces consistent with overstress.
Circlip 4 also had both leg ends fractured. One of the fractures was similar to the circlip 3 fractures, however the other fracture exhibited two thumbnail-shaped cracks along the inner surface. There were also fatigue striations consistent with fatigue crack propagation.
A review of the aircraft maintenance records revealed that the engine was disassembled and inspected for a propeller strike on Nov. 2, 2020, at 5,061 total airframe hours. The airplane had accrued about 130 hours since this inspection.
Probable Cause: A total loss of engine power due to the fatigue failure of a retaining circlip in one of the No. 2 crankshaft cheek counterweights, which resulted in liberation of the counterweight.
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This September 2023 accident report is provided by the National Transportation Safety Board. Published as an educational tool, it is intended to help pilots learn from the misfortunes of others.
Ok
Is it counterweight release or counterweight liberation ?
Inquiring minds need to know !!
AD 2023-04-08 was issued re the counterweight clips on big Continentals a few years ago, due to the factory installing some of them incorrectly.
https://www.flyingmag.com/potential-crankshaft-flaw-prompts-faa-to-issue-continental-engine-ad/
I wonder if that same sort of incorrect installation is what caused this failure?
JimH in CA, hard to say but counterweight pins, plates, snaprings, and bushings are 100% replacement parts in such cases. When assembled properly and operated normally, these parts show little if any wear for the life of the engine. But to your point, stress and shock between a counterweight and its crankshaft blade during a prop strike event is enormous. Thus, the teardown requirements.
Counterweight release at 130 hours TIS is rare when properly assembled using new, certified OEM parts. Regardless, critical adherence to AD 2023-04-08 and Mandatory Service Bulletin 23-01A during counterweight installation cannot be overstated.
Additionally, operation in accordance with CSB09-11 (minimum RPM limit of 2300) and correct propeller indexing will lessen counterweight wear and tear.
I wonder what affect the prop strike some time ago, that resulted in a teardown and reassembly had on this failure.?