Leverett, MA, USA
N9742Z
CESSNA U206G
The airplane was in cruise flight at 3,000 feet above ground level when the engine developed vibrations and, shortly after, lost total power. The pilot conducted a forced landing in an open field. The pilot cleared the trees adjacent to the field but did not see the electrical wires on the other side. The airplane’s main landing gear contacted the wires, and the airplane flipped and impacted the ground inverted. Examination of the engine revealed that several incorrect nuts for the cylinder hold-down studs were installed, and the breakaway torque values measured for the cylinder hold-down nuts were consistent with improper torque having been applied. Metallurgical examination revealed that the engine’s crankshaft fractured due to fatigue cracks that developed as a result of the No. 2 main bearing shifting in the crankcase saddle. The through-bolts that clamp the No. 2 crankcase saddle faces are secured with the hold-down nuts for the Nos. 2 and 3 cylinders. The engine had a reported total time in service of 6,629 hours with 1,475 hours since major overhaul. All six cylinders had been replaced since the overhaul, with the Nos. 2 and 3 cylinders having been replaced 692 and 549 hours before the accident, respectively.
HISTORY OF FLIGHT On December 5, 2010, about 1530 eastern standard time, a Cessna U206G, N9742Z, registered to Airborne Maintenance Incorporated and operated by an individual, crashed in an open field in Leverett, Massachusetts, during a visual flight rules flight from Dillant-Hopkins Airport (EEN), Keene, New Hampshire to Long Island Mac Arthur Airport (KISP), Islip, New York. Visual meteorological conditions prevailed at the time and flight following was activated for the 14 Code of Federal Regulations Part 91, personal flight. The airplane incurred substantial damage. One of the passengers was killed another passenger succumbed from injuries 7 days after the accident. The pilot and a third passenger received serious injuries. The flight departed EEN about 1505., Five minutes prior to the accident, the flight was handed off to the FAA Bradley Approach from FAA Boston Air Route Traffic Control Center. Shortly after this the flight was lost from radar. The last radar contact was reported at 1528, at an altitude of 1,900 feet mean sea level (msl), and at a ground speed of 87 knots. There was no distress calls received from the flight by Bradley Approach control. The pilot stated that the flight was in cruise at 3,000 feet msl when the airplane’s engine developed a vibration. Shortly after, the engine stopped producing power. To assist the pilot in trouble shooting the engine problem, the occupant seated in the right front seat switched places with the person seated in the middle right seat, who was a pilot and an airplane mechanic. The pilot saw an open field and elected to make an off field landing there. The airplane cleared the adjacent trees to the open field. He recalls clearing the tree and lowering the flaps. The next thing he remembered was unbuckling his seat belt and calling 911. The pilot indicated that he did not see the electrical wires behind the trees. PERSONNEL INFORMATION The pilot seated in the left seat, held a commercial pilot certificate with ratings for airplane single engine land, multiengine airplane land, and instrument airplane. He was issued a Federal Aviation Administration (FAA) first-class medical certificate on April 20, 2010, with no limitations. The pilot reported he has a total of 390 hours, which 90 hours are in the accident airplane. The passenger, seated in the right seat, age 62, held a commercial pilot certificate with a rating for rotorcraft –helicopter and a private pilot certificate with a rating for airplane single engine land. He held a flight instructor certificate with a rating for rotorcraft-helicopter. He was issued an FAA second-class medical certificate on August 11, 2010, with limitations of must have available glasses for near vision. He had documented 10,000 total hours at that time. He had a mechanic certificate with ratings for airframe and powerplant. AIRCRAFT INFORMATION The Cessna U206G, a six place all metal, high wing, single-engine airplane, variable-pitch propeller, with fix landing gear, serial number U20606636, was manufactured in 1982, and issued a standard airworthiness certificate, in the normal category. The airplane was powered by a Continental IO-520-DCF, 300-horsepower engine, with a Hartzell three bladed propeller. The airplane’s engine was overhauled in April of 2004. The engine’s last inspection was performed on October 25, 2010, at which the engine had a total of 1,459.2 hours since major overhauled. The airplane’s last inspection was October 25, 2010 and the airplane had a total of 6,556.8 hours at that time. The airplane was on the manufacturers 100 hours maintenance schedule. At the time of the accident, the airplane had accumulated a total of 6,572.1 hours. METEOROLOGICAL INFORMATION The closest official weather observation was at Orange Municipal Airport (ORE), Orange, Massachusetts, about 13 miles northeast of the accident site. The ORE 1552 METAR, was winds from 310 degrees at 9 knots gusting to 15 knots; visibility, 10 statute miles; overcast clouds at 4,200; temperature minus 02 degrees Celsius (C); dew point minus 10 degrees C; altimeter 29.52 inches of mercury. WRECKAGE AND IMPACT INFORMATION Examination of the accident site showed the main wreckage was located at 42 degrees, 26.391 minutes north and 072 degrees, 29.317 minutes west, about 105 feet away from electrical transmission wires. The top electrical wire was observed frayed and remained intact; drooped above the ground.The airplane was on an estimated heading of 210 degrees when the airplane’s main landing gear caught the top wire on the electrical transmission lines running east and west, at an estimated height of 72 feet above ground level. The airplane became inverted as the wire separated from its attachment points at each nearby tower. The airplane impacted the ground right wing first, in a near inverted attitude. The airplane came to rest facing approximately a 360 degrees heading. The left main gear spring detached from its securing bracket and remained attached to the fuselage. The left wheel assembly separated and was located below the wire run. The right main separated from the fuselage attachment point and came to rest about 35 feet northeast of the main wreckage. The right wing tip section was crushed, bent, and curled down (up inverted) from the strut wing attachment point. The right wing strut separated from the fuselage attachment, ripped into the cabin area, and folded over inside the cabin area. The right side of the cabin fuselage section was crushed and buckled outward. The right side of the fuselage roof area was crushed in to the top of the glare shield. The left side of the fuselage did not buckle. The left wing and wing strut remained intact and the upper side of the wing incurred impact damage. The windshield was shattered and fragmented on the ground. The vertical stabilizer was bent and curled toward the left. The lower engine cowl separated from it respective attachment points exposing the bottom the engine. The engine was separated from the engine mounts and was resting on top of the upper cowling. The lower section of the engine bay was observed covered with engine oil. The airplane’s cowl flap and belly was observed with engine oil streaking to the lower tail section of the airplane. The propeller and engine stayed attached. One of the three bladed propeller blades stuck in the soil. The propeller’s spinner and the propeller blades were observed with no damage; the blades were observed in the low pitch angle. All of the flight control surfaces were observed attached in their respective locations. Flight control cable continuity was established from all surfaces into the cockpit area. The flap actuator was observed in the flaps full extended position, the control rod from the actuator to the right flap was observed separated. Both wing fuel tanks were breached. The first responders and the responding FAA inspector observed fuel at the accident site. The fuel selector was observed in the left wing fuel tank position. Examination of the airplane’s fuel system from the wings fuel tanks to the fuel gascolator revealed no discrepancies. Fuel was observed in the gascolator. The auxiliary electrical fuel pump operated when electrical power was applied to the motor. Engine control cable continuity was established from the cockpit to their respective engine control components. All were safetied at their respective control levers. The engine incurred impact damage to the intake manifold and vacuum pump. The oil sump and crankcase incurred damage and were not breached. The oil service cap was observed installed and secured. The oil level dip stick was observed installed and secured. A borescope inspection of the cylinders did not reveal any discrepancies. No discrepancies were noted with the spark plugs. Both magnetos were observed in their respective location. Several ignition wires on the top of the engine were observed cut by impact damage. The fuel pump was observed with impact damage and safety wired. Fuel was observed when the fuel pump outlet fuel line was disconnected. The fuel pump drive shaft rotated when turned by hand. The fuel pump drive shaft coupling was intact. The bolt that secures the alternator’s belt adjustment brace to the engine was missing from its respective location. The alternator belt was observed loose. MEDICAL AND PATHOLOGICAL INFORMATION The Medical Examiner’s Office in Holyoke, Massachusetts, conducted a postmortem examination of the pilot-rated passenger. The cause of death was blunt force trauma. The FAA Civil Aeromedical Institute (CAMI) conducted toxicology testing on specimens from the pilot-rated passenger. No ethanol or drugs were detected. The passenger seated in the middle row, right seat, succumbed to his injuries 7 days after the accident. TEST AND RESEARCH An engine teardown examination was conducted at the Continental Motors Incorporated (CMI) Facility with NTSB oversight. The external examination revealed that the seven stud flange nuts between cylinder number 1 and 3, 3 and 5, and 4 and 6 were not self-aligning nuts and exhibited a flat seat. Cylinders number 1, 5, and 6 hold-down nuts near the 2, 11, and 5 o’clock positions respectively were self-aligning nuts and exhibited beveled seats. During the cylinder through-bolt and deck stud break-away torque measurements examination, several nuts where observed not at their required specified torque values. The internal examination revealed that the crankshaft fractured forward of the number 2 connecting rod and connecting rod journal. The number 2 cylinder connecting rod bolt and nut were found in the oil sump. The number 2 main bearing was observed damaged, with sections missing, and shifted on the crankcase saddle. Evidence of fretting damage was observed on the mating saddle contact surfaces of the crankcase halves for the number 2 bearing. The number 2 main bearing journal exhibited scoring consistent with bearing rotation. The crankcase, main bearings, crankshaft, and the number 2 cylinder connecting rod assembly were retained for further metallurgical examination at the NTSB Materials Laboratory. The metallurgical examination revealed the crankshaft was fractured through the cheek at the aft end of the number 2 main journal. The fracture surface was relatively smooth with curving crack arrest lines, features consistent with fatigue. Fatigue features emanated from multiple origins separated by ratchet indicated the general direction of fatigue crack propagation. The origins were at the crankshaft surface in the radius between the number 2 main journal and the cheek at the aft end of the journal. Circumferential scoring and metal transfer was observed on the crankshaft surfaces adjacent to the origin area in an arc intersecting the origin area. Fatigue features extended through nearly the entire thickness of the cheek. The number 2 main bearing was fractured into several pieces. Several of the pieces had curvature consistent with shifting and outward deformation between the crankshaft cheek and the crankcase saddle. The largest number 2 bearing piece was recovered in a shifted position with an outward curve between the crankshaft cheek and the crankcase saddle. Other pieces of the number 2 bearing were recovered from the oil sump. The bearings showed evidence of an orange color consistent with a copper-lead intermediate layer. A closer view of the fracture surface and adjacent number 2 main bearing journal revealed the mating fracture surface was obliterated by post-fracture damage. The number 2 main journal surface was tinted gold-brown consistent with exposure to elevated temperatures. Circumferential scoring was also observed on the journal surface. An overall examination of the interior and mating surfaces of the crankcase halves revealed the saddles for the number 1 and number 2 main bearings were damaged in the bearing seating area. Fretting was observed on the mating contact faces of the number 2 main bearing saddle around the through-bolt hole. Fretting was also observed on the contact faces of the number 1 main saddle and to a lesser extent on the contact face of the number 3 main bearing. The engine had a reported total time in service of 6,629 hours with 1,475 hours since major overhaul. However, all 6 cylinders had been replaced between 420.9 hours and 728.5 hours before the accident. The number 2 cylinder had been replaced on July 10, 2007, 692.0 hours before the accident, and the number 3 cylinder had been replaced on March 1, 2008, 549.3 hours before the accident.
A total loss of engine power due to the fatigue failure of the crankshaft, which resulted from improper tightening of the cylinder hold-down nuts.
Source: NTSB Aviation Accident Database
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