Mariposa, CA, USA
N4791C
Cessna T210N
The aircraft collided with multiple obstacles during a forced landing in rough terrain following a catastrophic engine failure. While en route about 6,000 feet mean sea level, one cylinder in the engine indicated a loss of exhaust gas and cylinder head temperature. Thereafter, an increasingly strong vibration was felt, oil dispersed onto the front windscreen, and all engine power was lost. During the landing flare, the airplane impacted an oak tree, a fence, and a rock outcrop. The engine was found separated from the airplane's firewall. Also, the number 6 cylinder was observed broken from the engine case. The engine was torn down and the case was opened. The evidence of the torque values found on all the engine through-bolts, combined with the fretting signature at the base of the number 6 cylinder, and the oil residue at the union of the case halves, indicates that when the cylinders were installed on the engine their respective bolts were inadequately torqued. During an interview with the mechanic who had replaced the 6 cylinders on the engine, he reported that he followed the instructions in TCM's maintenance manual when he had replaced the cylinders in May 2005. He said he torqued the nuts on the cylinders to about 500 inch pounds. According to TCM's maintenance manual, 1989 edition, the required torque for the through-bolts was 690 to 710 inch pounds. By 1996, TCM had revised its manual by issuance of service bulletin 96-7. In that bulletin the required torque was increased to between 790 to 810 inch pounds.
HISTORY OF FLIGHT On January 24, 2006, about 0951 Pacific standard time, a Cessna T210N, N4791C, experienced a total loss of engine power during cruise flight near Mariposa, California. During the subsequent forced landing on rough terrain in the open field, the airplane collided with a tree and a fence. The airplane was substantially damaged. Representations, LLC., Santa Ynez, California, operated the airplane. The private pilot sustained minor injures, and the passenger was not injured. The flight was performed under the provisions of 14 CFR Part 91, and no flight plan had been filed. Instrument meteorological conditions prevailed at the accident site. The business flight originated from Visalia, California, about 0912. The pilot reported to the National Transportation Safety Board investigator that he intended to fly to Sacramento, California. However, because of inclement weather at his destination, he planned to divert to an alternate location. While en route and in visual meteorological conditions, one cylinder in the engine indicated a loss of exhaust gas and cylinder head temperature. Thereafter, an increasingly strong vibration was felt and oil dispersed onto the front windscreen. The pilot descended and made a forced landing in instrument meteorological conditions. During the landing flare the airplane impacted an oak tree, a fence, and a rock outcrop. WRECKAGE AND IMPACT INFORMATION The engine was found separated from the airplane's firewall. Also, the number 6 cylinder was found in the open field, separated from the engine's case. TESTS AND RESEARCH Logbook Data and Mechanic Statements. The airplane engine's logbook was examined, and the following was noted: The airplane received an annual inspection in February 2005. On May 27, 2005, all 6 cylinders were replaced, and the indicated tachometer time was 3,908.15 hours. The Hobbs time was 787.1 hours. (The Hobbs time at the accident was noted at 924.5 hours.) No maintenance manuals were referenced for the removal and installation of the cylinders in the logbook sign off. The National Transportation Safety Board investigator interviewed the mechanic who had replaced the 6 cylinders on the engine. In pertinent part, the mechanic made the following statements: 1. I maintained the accident airplane and do not employ or use anyone else in the maintenance of the engine. 2. In May 2005, I personally installed all 6 cylinders in the engine following the instructions in TCM's maintenance manual. I do not have a copy of the manual's associated Service Bulletin 96-7C. 3. During the cylinder installation, I torqued the nuts on the cylinders to about 500 inch pounds. 4. I do not put oil on the bolt threads; I put them on dry. 5. After the accident I had my torque wrenches calibrated. They were not within calibration. Teledyne Continental Motors (TCM), Service Bulletin (SB) 96-7C, issued February 8, 2005. According to TCM, compliance with SB96-7C will enhance safety. The SB provides torque values for fasteners on all TCM engines. The following "WARNING" note is printed on the SB's cover page: "Proper torqueing [sic] practices cannot be over emphasized. Torque values are provided as a convenient method of achieving correct pre-loading of highly stressed fasteners. If the fasteners are not...properly lubricated, the correct fastener pre-load will not be achieved even though the given torque value is reached. For this reason, it is critical that all fasteners be...correctly lubricated prior to torquing. Failure to verify a fastener's serviceability or to correctly lubricate the fastener prior to assembly and torquing will result in the fastener not being properly pre-loaded and subsequent failure of the fastener may occur." SB96-7C further indicates the following: "Proper cylinder installation requires a multiple step torquing process. Cylinder base stud threads, through bolt threads and nuts must be lubricated with clear 50 weight aviation oil....Torque the cylinder through bolt nuts and cylinder base stud nuts to the specified torque value for the cylinder base stud nuts." The prescribed torque value for the accident engine's 12-point-through bolt at the cylinder flange is between 790 and 810 inch pounds. According to TCM, in 1996 it revised the engine's maintenance manual by issuance of SB96-7. In that bulletin the required torque for specific through bolts (pertinent to the accident engine) was indicated at 790 to 810 inch pounds. The most recent version of this SB was issued in 2005, and the subject torque value was not changed. Engine Findings. Under the direction and observation of the Safety Board investigator, the TCM participant tore down the engine. The number 6 cylinder was observed separated from the engine crankcase mounting through bolt and studs. Additionally, the respective piston was broken at the piston pin; the connecting rod was broken from the crankshaft and piston. The internal cylinder area of the number 6 cylinder was not impact damaged. The lower skirt area of the cylinder exhibited witness marks (signatures) consistent with having been impacted by its respective connecting rod, when the cylinder was held at a location commensurate with it being partially dislodged/separated from its attachment mount. Several of the number 6 cylinder hold down studs were broken or damaged, and several engine crankcase through bolts were found without damage (see photographs). The TCM participant noted the following: After the case was split open, the forward set of crankcase through bolts exhibited fretting at the center of the bolts. The entire length of the crankcase backbone exhibited a rough surface from fretting, and the top of the engine visually appeared to have been seeping oil. The TCM participant indicated that during the teardown all of the observed torque values on the through bolts were less than 500 inch pounds. This observation, combined with the fretting signature at the base of the number 6 cylinder, and the oil residue at the union of the case halves, indicates that when the cylinders were installed on the engine their respective bolts were inadequately torqued. (See the TCM participant's "Engine Field Inspection Report" for additional details.) ADDITIONAL INFORMATION The wreckage was released to the owner's assigned insurance adjuster. No parts or original records were retained.
The failure of the maintenance personnel to follow the procedures in the TCM maintenance manual and adequately torque the engine through-bolts, which led to the separation of the No. 6 cylinder and the catastrophic failure of the engine.
Source: NTSB Aviation Accident Database
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