NTSB Factual Report

On September 21, 2007, about 1500 Alaska daylight time, a float-equipped Cessna U206 airplane, N8030Z, sustained substantial damage during a forced landing when it collided with terrain following a loss of engine power, about 77 miles north of McGrath, Alaska. The airplane was being operated as a visual flight rules (VFR) cross-country positioning flight under Title 14, CFR Part 91, when the accident occurred. The airplane, registered to Scenic Mountain Air Inc., Moose Pass, Alaska, was operated by the pilot/owner. The airline transport certificated pilot, and the sole passenger, received minor injuries. Visual meteorological conditions prevailed. The flight departed about 1430 from Galena, Alaska, for Moose Pass, and a VFR flight plan was filed.

During a telephone conversation with the National Transportation Safety Board (NTSB) investigator-in-charge (IIC), on September 24, the pilot reported that the airplane was in cruise flight with the engine set at 23 inches of manifold pressure, and the propeller was at 2,300 rpm. He began to notice a vibration, which was followed by a loud bang, and the propeller stopped. The pilot described the prop as having suddenly stopped in a feathered position, without windmilling or slowing down. The pilot noted that the top of the engine cowling was dented from the underside, near the oil dipstick inspection door. He selected an emergency landing area on a mountain ridge that had been burned during a forest fire. Just prior to touchdown, the airplane collided with several trees. The airplane's left float assembly dug into the ground when it hit the ridge, and the airplane nosed over.

PILOT INFORMATION

The pilot holds an airline transport pilot certificate with a multiengine land rating, commercial pilot privileges with airplane single-engine land and single-engine sea ratings, and a flight instructor certificate with single-engine, multiengine, and instrument airplane ratings. The pilot also holds an FAA mechanic certificate with airframe and powerplant ratings, and has FAA inspection authorization (IA).

AIRCRAFT INFORMATION

The Federal Aviation Administration (FAA) requires that aircraft, operated by air carrier certificate holders, comply with the manufacturer's recommended maintenance programs. On August 23, 2007, the pilot/owner submitted a request to his FAA principle maintenance inspector (PMI) at the Anchorage Flight Standards District Office (FSDO), Anchorage, Alaska, for a 100 hour extension beyond the engine manufacturer's time between overhaul (TBO) interval of 1,700 hours. The pilot stated that the engine had, at that time, 1,695.6 hours, all airworthiness directives (AD) had been complied with, all engine compression levels were above minimum, the oil consumption was normal, and an oil analysis was normal. He also indicated that he recently replaced an engine cylinder, and a borescope inspection had not detected any abnormalities.

On August 24, the FAA inspector granted a one-time extension of the overhaul interval to 1,800 hours. The inspector noted that all ADs must be complied with, the engine oil must be changed every 25 hours, and accompanied by an oil analysis and borescope inspection on each cylinder. The inspector also stated that any malfunction or abnormality during the extension would void the extension.

Following the accident, the pilot/owner submitted selected pages of the maintenance records to his FAA PMI. A review of the records indicated that the engine was overhauled on June 26, 1997, at tach time 4,852.36. At that time, the engine had 1,920.36 hours, and 418.56 hours since its previous overhaul due to "cylinder distress and oil contamination."

The exact tach time on the airframe at the time of the accident was not reported by the pilot/owner, however, the operator's FAA PMI reported that at the time of the accident, the engine had accrued about 1,735 hours since a major overhaul, and about 60 hours since the number 6 cylinder was changed. Using that data, the total time on the airframe would be about 6,587 hours.

The engine log had an entry on June 2, 2002, tach time 5,854.5, which listed an oil change. On the same page of the log, the next log entry was dated July 29, 2006, tach time 6,413.4, during which the number 2 and number 6 cylinders were changed, which would be about 173.6 hours before the accident.

The most recent annual inspection listed in the engine log was dated August 1, 2007, at tach time 6,527.0. The total engine time was 3,464.5, with a time since major overhaul of 1,674.6. The log also stated that the number 6 cylinder was changed, which is about 60 hours before the accident.

On August 26, 2007, the airframe logbook had an entry for an oil change, airworthiness directive (AD) compliance check of the induction airbox and seat rails, and a transponder system check. At that time, the tach time was 6,550.3, with a total time on the engine of 3,487.8, and 1,697.9 hours since a major overhaul.

The next and most recent airframe logbook entry was on September 11, 2007, at a tach time of 6,574.6. The entry noted an oil change, and that an oil sample was taken for analysis, with the total time on the engine listed as 3,512.1, and 1,722.2 since a major overhaul. Using the FAA's total time data, the airplane flew about 12.8 hours between the oil change and the accident.

ENGINE

The airplane was recovered by the pilot/owner and shipped to Anchorage. The engine was removed from the airframe at an engine maintenance facility in Anchorage, and an engine examination was done on October 25, 2007. The NTSB IIC, FAA personnel, and the parties to the investigation participated in the examination.

The examination revealed a large hole in the top of the case, adjacent to the number 4 cylinder. The cylinder was entirely broken from the engine case, and displaced outward about 6 inches. A broken portion of the upper engine case remained attached to the cylinder. The piston and connecting rod were missing.

The number 4 connecting rod cap was found in the engine compartment. It was blackened and straightened from its normal "U" shape. One rod cap bolt was bent mid shank about 90 degrees. The bolt threads were severly damaged, and was missing the nut and cotter pin. The opposite side cap bolt was bent and broken near its base. One of the number 4 connecting rod cap bearings was found on top of the engine case. The other bearing was not found.

The number 5 cylinder fins adjacent to the upper engine case through bolt were distorted by outward movement of the through bolt head. The number 3 cylinder fins were cracked through the upper spark plug hole. Fretting was noted on the joining surface of the engine case halves.

PROPELLER

The pitch of the blades is controlled by oil pressure to overcome the tension of a compressed spring in the hub. A piston within the hub changes blade pitch angle via a phenolic pitch change link between the piston and a blade actuating pin attached to the base of each propeller. In the hub, the phenolic pitch change links are in compression. Within the hub is a low pitch spacer to limit the forward travel of the piston, and a high pitch spacer to limit the aft travel of the piston.

The propeller was manufactured in 2003, and installed on the accident airplane on June 10, 2004. It was removed on September 25, 2006, due to leaking and repaired/overhauled. It was reinstalled on October 2, 2006, and operated normally until the accident. At the most recent annual inspection on August 1, 2007, (tach time 6527.0), the prop had a total time of 498.8 hours.

The propeller was examined at a propeller maintenance facility, Anchorage, on October 26, 2007, and the examination was attended by the parties to the investigation. The examination revealed that propeller blades 1 and 3 snap rings at the hub were out of their respective slots at the hub socket. The number 2 blade snap ring was beginning to move out from its slot at the snap ring gap.

The tip of the number 3 blade had leading edge gouges, some minor chordwise scratches, and large chordwise scratches on the backside of the blade. The tip of the number 2 blade had minor chordwise scratches and minor leading edge dents. The tip of the number 1 blade had minor chordwise scratches, and minor leading edge dents.

The number 2 blade phenolic link was intact, with a small gouge at one end. The number 1 and 3 phenolic links were broken.

The prop governor was bench tested on October 26, 2007, and functioned properly.

TESTS AND RESEARCH

The separated rod cap was sent to the NTSB Materials Laboratory for examination on February 6, 2008. The examination revealed that the cap mating surfaces around the bolts were damaged, but did not show evidence of fretting. The bent rod cap bolt threads had thread damage that flattened the thread profiles, and longitudinal smearing away from the bolt head, mostly in the end threads. Smeared nut remnants were found in the bolt threads. A small metallic piece was found in one of the bolt cotter pin holes, but no identifiable pieces of the pin were found.

The fractured rod cap bolt was broken near the rod cap split line. The bolt was deformed with fracture patterns consistent with a bending overstress separation. No indication of preexisting cracking was observed.

The propeller hub piston, piston cylinder, blade pitch change links, and the blade actuating pins were sent to the FAA's Aircraft Certification Office (ACO), Wichita, Kansas, and the components were examined by a representative of the propeller manufacturer, and an FAA engineer on March 12, 2008. They reported that the number 1 and 3 phenolic pitch change links were fractured in tension at the blade actuating pin end, and the number 2 link was nicked near the blade actuating pin hole. The high and low pitch spacers were installed, which limited the high pitch setting to 28.5 inches.

The manufacturer reported that the propeller pitch change mechanism in the accident airplane is incapable of traveling to, or near, a feathered position under normal operating conditions. In addition, if the pitch change mechanism failed in-flight, the blades would rotate to a flat pitch due to the blade centrifugal twisting moment of the rotating propeller. The manufacturer noted that the link fractures are typically found after ground impact or a blade strike due to the blade being forced to twist toward a higher pitch, or by blade cocking in the hub socket by ground forces.

ADDITIONAL INFORMATION

The Safety Board released the wreckage, located at Anchorage, to the owner's representatives on October 26, 2007. The propeller components and the engine rod cap were retained by the Safety Board for examination until their release on April 21, 2008.

NTSB Probable Cause

A separation of the engine connecting rod and engine cylinder due to the loss of a connecting rod nut that was stripped from the rod bolt.