HOMEWOOD, MS, USA
N1477D
Cessna 421A
During cruise flight, a slight vibration was felt and the right propeller completely separated contacting the right horizontal stabilizer. The flight diverted and landed uneventfully. The right engine No. 3 cylinder connecting rod failed; evidence of exposure to elevated temperatures and higher stress fatigue crack propagation was noted. Extensive fretting noted on the faying surfaces of the right propeller gear and hub, and on both sides of the propeller bulkhead and fillet assembly. The holes in the propeller bulkhead assembly and propeller gear were elongated/eggshaped. Fatigue noted on all eight studs; wear noted on the shanks of seven studs; the eighth stud was not examined. Four of the recovered seven spacers were damaged shortening their length. Only two of the eight fractured studs were extended the specified minimum distance beyond the nut. No discrepancies with the threads in the hub holes of the left propeller, or to the threads in the hub holes of the right propeller that were checked. Overstress failure noted on the fractured propeller blade from the right propeller. The mechanic who installed the right propeller used un-calibrated torque wrenches and an outdated Service Manual. The airplane had accumulated approximately 50 hours since installation of the right propeller.
On July 25, 2000, about 1450 central daylight time, a Cessna 421A, N1477D, registered to Smart Lease, Llc, experienced separation of the right propeller during cruise flight near Homewood, Mississippi. Visual meteorological conditions prevailed at the time and an instrument flight rules (IFR) flight plan was filed for the 14 CFR Part 91 personal flight. The airplane was substantially damaged and the commercial-rated pilot, airline transport-rated copilot, and four passengers were not injured. The flight originated about 1330 from the Destin-Fort Walton Beach Airport, Destin, Florida. According to the pilot-in-command, during cruise flight at 12,000 feet mean sea level, a slight vibration was felt; the right propeller suddenly separated from the airplane. An emergency was declared, and the flight diverted to the Jackson International Airport, where an uneventful landing occurred. Examination of the airplane by a Federal Aviation Administration (FAA) airworthiness inspector revealed that the right propeller was completely separated from the engine (see photograph 1). The upper surface of the leading edge of the right horizontal stabilizer was crushed nearly full span and the outboard section of the horizontal stabilizer was displaced down approximately 35 degrees (see photograph 3). Compression wrinkles were noted on the bottom skin of the right horizontal stabilizer. Examination of the right engine revealed a hole on the top of the crankcase (see photograph 8). Five fractured studs with attached nuts; three of which also had a spacer attached, and 3 separated spacers were initially located in the engine compartment area; one fractured stud with attached nut was later recovered (see photograph 15). The studs with attached nuts, and attached or loose spacers were retained for further examination by the NTSB. The right engine was removed for further examination. The right propeller was found on November 10, 2000, at approximately 32 degrees 18.268 minutes North latitude and 089 degrees 34.506 minutes West longitude; a reward had been offered by the insurance representative (see photographs 12, 13, and 14 for views of the recovered right propeller). The breakaway torque of the left propeller mounting nuts was checked on August 17, 2000, in the presence of the FAA inspector and an investigator from the airplane manufacturer. The condition of each stud and nut whether they were dry or wet with lubricant material was also documented. A copy of the FAA inspector statement documenting the findings is an attachment to this report. The left propeller was then removed for further examination. Examination of the right engine at the manufacturer's facility with FAA oversight revealed that the No. 3 cylinder connecting rod was failed (see photograph 11). Metallic debris consisting of connecting rod pieces were found in the oil sump (see photograph 9). No failure or malfunction was noted with the oil pump. The No. 3 cylinder connecting rod bearing was burnt and extruded, pieces of the bearing were found in the oil sump. The No. 3 cylinder connecting rod journal was black and partially melted (see photograph 10); heat distress was noted to the No. 2 cylinder connecting rod journal. Examination of the propeller flange which had entrapped one fractured propeller mounting stud with attached spacer and nut revealed several holes were elongated (see photographs 6 and 16). A fractured propeller mounting stud with attached nut was found on top of the No. 6 cylinder (see photograph 7). The crankshaft counterweights movements were determined to be free and unrestricted. The propeller shaft with entrapped fractured stud with spacer and nut attached, the located fractured propeller mounting stud with attached nut, and the fractured No. 3 cylinder connecting rod were retained for further examination by the NTSB. A copy of the report from the engine manufacturer is an attachment to this report. Examination of the left propeller at the manufacturer's facility with FAA oversight revealed that all but one of the eight studs were protruding the correct length from the hub. All studs were removed from the hub for examination of the threads with a thread go-no go gage, all were found to be within limits. The right propeller bulkhead and fillet assembly and the recovered right propeller were also examined at the manufacturer's facility with FAA oversight. All eight studs were fractured near the hub mounting surface; sections of the fractured studs remained inside the hub. The No. 1 propeller blade was fractured at approximately the 12-inch station; the remaining segment of blade was bent rearward. Examination of the propeller bulkhead and fillet assembly revealed the propeller stud holes were elongated/eggshaped. One propeller blade was removed from the propeller hub and one propeller blade was saw cut near the propeller hub. The propeller hub with the fractured and saw cut blades attached, and the propeller bulkhead and fillet assembly were retained for further examination by the NTSB. A copy of the report from the propeller manufacturer and a copy of a statement from the FAA inspector who observed the propeller examinations are attachments to this report. The right propeller hub with fractured and saw cut propeller blades, propeller shaft with entrapped fractured stud with attached spacer and nut, right bulkhead and fillet assembly, fractured No. 3 cylinder connecting rod, fractured studs, and the attached or loose spacers were examined by the NTSB Materials Laboratory located in Washington, D.C. Examination of the No. 3 cylinder connecting rod revealed indication of exposure to elevated temperatures and evidence of higher stress fatigue crack propagation on both fracture surfaces of the strap of the connecting rod. Examination of the fracture surface of the No. 1 propeller blade revealed features typical of overstress separation (see photograph 12 for view of the fracture surface). Extensive fretting damage was noted on the faying surfaces of the propeller hub, propeller shaft flange, and on both surfaces of the bulkhead and fillet assembly. The shanks on all of the six smaller diameter studs exhibited wear on the clockwise and counterclockwise sides; the wear was greater on the counterclockwise side. The shank of the large stud entrapped in the propeller gear was not examined; the shank of the other large stud was worn completely around its circumference (see photograph 15 for view of six of the fractured studs and photograph 17 for view of one of the fractured studs). Crack arrest lines indicative of fatigue cracking was noted on all fracture surfaces of all eight studs. Contact damage and deformation was noted to the end of all the spacers adjacent to the mounting flange of the propeller gear. Four of the recovered seven spacers were later determined to be damaged which shortened their length to less than the specified length. Only two of the eight fractured studs were found extended the minimum distance beyond the nut specified by McCauley Propeller Systems. No discrepancies were noted to the threads in two of the stud holes in the propeller hub or to the threads of the studs in those holes. A copy of the NTSB Materials Laboratory Report is an attachment to this report. The airplane was inspected in accordance with an annual inspection that signed off on January 5, 2000; both propellers were removed during the inspection for Airworthiness Directive (AD) compliance and were subsequently replaced due to economic reasons. At the time the mechanic signed off the annual inspection, neither propeller was installed. An entry in the right engine logbook with the same date as the annual inspection indicates that a "0 time" propeller was installed. The replacement propellers were overhauled on February 3, 2000, and installed by a mechanic other than the mechanic who performed the annual inspection on an unknown date after overhaul. The maintenance records do not reflect the date when the replacement propellers were actually installed. Copies of excerpts from the maintenance records are an attachment to this report. The mechanic who installed in part the right propeller used uncalibrated torque wrenches and an outdated Service Manual. The airplane was not operated between January 20 and May 11, 2000, and at the time of the accident had accumulated approximately 50 hours since installation of the right propeller. A copy of an FAA inspector statement regarding an interview with the mechanic who installed both propellers is an attachment to this report. An additional participant in the investigation was William B. Welch, of Cessna Aircraft Company, Wichita, Kansas. The airplane minus the retained right engine and left propeller, was released to Mr. Deanes L. Rowedder of Kern & Wooley, on September 22, 2000. The right propeller was retained on November 10, 2000, after being found that date. Additional components that were retained include the right engine propeller gear, seven loose fractured studs from the right propeller, seven spacers from the right propeller, eight mounting stud nuts from the right propeller, the No. 3 cylinder connecting rod from the right engine, and the right bulkhead and fillet assembly. All retained components were released to Mr, Deanes L. Rowedder of Kern & Wooley, on May 16, 2001.
The inadequate installation of the right propeller by the mechanic for his failure to properly torque the eight nuts resulting in fatigue failure of the studs and separation of the right propeller during cruise flight. Findings in the investigation were 1) the mechanic who installed the right propeller used un-calibrated torque wrenches, and 2) the mechanic who installed the right propeller used an outdated Service Manual.
Source: NTSB Aviation Accident Database
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