N36SG

HISTORY OF FLIGHTOn April 6, 2013, about 1035 Pacific daylight time, a Beechcraft A36TC airplane, N36SG, was substantially damaged during a forced landing in Big Bear Lake, Big Bear City, California. The pilot received serious injuries, and his three passengers received minor injuries. The personal flight was conducted under the provisions of Title 14 Code of Federal Regulations Part 91. Visual meteorological conditions prevailed at the accident location. The airplane had departed on an instrument flight rules (IFR) flight plan from McClellan-Palomar Airport (CRQ), Carlsbad, California, and was destined for Big Bear City Airport (L35).

According to the pilot, his passengers include his niece, her husband, and their 5 year-old son. En route, the pilot canceled his IFR flight plan, and continued, using flight following services from SoCal Approach. The pilot planned to follow his normal route, which included a slow letdown to L35 after passing ARRAN intersection. Shortly after the airplane passed ARRAN, at an estimated altitude of between 9,500 and 9,000 feet, the airplane experienced a "very sudden" onset, high-frequency vibration. The pilot reduced the power setting, and notified the air traffic controller of the problem. Initially the manifold pressure gauge provided very erratic indications, but the pilot was unable to discern whether this was a valid indication, or the instrument responding to the vibration.

The airplane began to descend more rapidly than the pilot desired, and he adjusted the engine controls to yield normal cruise manifold pressure and rpm values. However, the airplane did not respond commensurately with the indicated power setting, and continued its descent. When the airplane was approximately abeam the "observatory" (a visual landmark) at an altitude of about 6,900 feet, the pilot switched to the airport advisory frequency, and notified them of his problem. The pilot observed a causeway that crossed the lake between his position and the airport, and began a turn to his right (south) to avoid the causeway. At some point, the pilot recognized and broadcast that he was unable to make the airport, and planned to land in a field west of the airport. The pilot banked to the right to turn for the field, but the right wingtip contacted the lake, which ended the flight. The airplane came to rest inverted in water about 2 feet deep. The impact location was about 0.3 miles southwest of the causeway, and about 1.25 miles southwest of the L35 runway 8 threshold.

An eyewitness who was located on his property at the lakeshore was attracted to the airplane by its sound. He estimated that at that time he noticed the airplane, it was about 30 feet above the lake, and headed south, towards him. He then saw it bank right, and impact the lake. He summoned 911 assistance, and then ran into the lake to render assistance. PERSONNEL INFORMATIONAccording to Federal Aviation Administration (FAA) records, the pilot held a private pilot certificate with airplane single engine land and instrument airplane ratings. He reported that he had a total flight experience of approximately 1,300 hours, including about 1,250 hours in the accident airplane make and model. His most recent flight review was completed in April 2012, and his most recent FAA third-class medical certificate was also issued in April 2012. AIRCRAFT INFORMATIONFAA records indicate that the airplane was manufactured in 1979. It was equipped with a Teledyne Continental TSIO-520 series engine, and a McCauley model 3A32C76 propeller. The airplane was originally registered as N66656. In 1986 the registration was changed to N36ML, and in 1996 to N36SG. The pilot purchased the airplane in November 1998.

Maintenance records indicated that the airplane had been upgraded with multiple avionics changeouts or additions, as well as aftermarket wing tip tanks. Both the accident engine and the accident propeller hub were different serial numbers than the ones that were originally installed during manufacture of the airplane. However, all three accident propeller blades were the same ones that were originally installed during manufacture of the airplane. METEOROLOGICAL INFORMATIONThe L35 1035 automated weather observation included winds from 260 degrees at 8 knots, variable between 224 and 284 degrees, visibility 10 miles, clear skies, temperature 15 degrees C, dew point minus 8 degrees C, and an altimeter setting of 30.14 inches of mercury. AIRPORT INFORMATIONFAA records indicate that the airplane was manufactured in 1979. It was equipped with a Teledyne Continental TSIO-520 series engine, and a McCauley model 3A32C76 propeller. The airplane was originally registered as N66656. In 1986 the registration was changed to N36ML, and in 1996 to N36SG. The pilot purchased the airplane in November 1998.

Maintenance records indicated that the airplane had been upgraded with multiple avionics changeouts or additions, as well as aftermarket wing tip tanks. Both the accident engine and the accident propeller hub were different serial numbers than the ones that were originally installed during manufacture of the airplane. However, all three accident propeller blades were the same ones that were originally installed during manufacture of the airplane. WRECKAGE AND IMPACT INFORMATIONThe airplane came to rest inverted in the lake, in about 2 feet of water, and about 100 feet from shore. The impact location was approximately 1.25 miles southwest of the L35 runway 08 threshold. The left wing had separated from the fuselage during the accident sequence, and was located about 75 feet from the airplane. Aside from loss of several transparencies, the cabin remained essentially intact, and did not lose any appreciable occupiable volume. The forward and aft cabin doors remained operable, and the pilot and passengers exited through those doors.

The airplane was recovered by dragging it inverted onto shore, where it was then righted. Some fuselage deformation resulted during the lake-recovery efforts. The airplane was transported to a secure facility for additional examination. Detailed examination of the engine and airframe did not reveal any pre-impact conditions or anomalies that would have precluded normal operation and continued flight.

The three blade, variable pitch propeller remained attached to the engine propeller flange. All three of the propeller blades were retained in the propeller hub; however, all three propeller blades were free to rotate independently of one another in the hub. All three of the propeller blades displayed aft bending, and two of the propeller blades displayed twisting deformation. The propeller flange remained attached to the rest of the crankshaft and there were no cracks noted in the crankshaft. The propeller was removed from the engine and shipped to McCauley, the propeller manufacturer, for detailed examination.

The airplane was equipped with a JPI EDM-700 engine monitor, which displayed multiple engine parameters in the cockpit, and stored time history data of those parameters in non-volatile memory. The unit was removed and sent to the NTSB Recorders laboratory in Washington DC for data download. The accident flight was successfully downloaded, but review of the recorded information did not reveal any engine-related anomalies.

The airplane was equipped with a Shadin 1530 fuel flow monitor, which displayed fuel-related information in the cockpit, and stored time history data of those parameters in non-volatile memory. The unit was removed and sent to the NTSB Recorders laboratory in Washington DC for data download. The accident flight was successfully downloaded, but review of the recorded information did not reveal any fuel-related anomalies. ADDITIONAL INFORMATIONPropeller Design

The propeller was a constant-speed design which permitted the blades to change pitch in flight to maintain a preset rpm selected by the pilot. Pitch change commands were effected by pressurized engine oil, metered by the propeller speed governor, that drove a piston fore and aft inside the hub, which, through a series of links, converted the piston translation into blade rotation.

Each propeller blade had ring-shaped steel ferrule attached to its blade butt. The ferrule was oriented so that its circular plane was perpendicular to the span of the blade. Once installed, the ferrule did not move relative to the blade. Each ferrule measured about 5 inches in outside diameter, and about 1 inch thick. The face that was oriented towards the propeller axis of rotation (center of the hub, away from the propeller blade) was referred to as the "forward" face; this is not a reference to its orientation in the airplane axis system.

Each propeller blade was also equipped with a steel actuating pin. Each pin measured about 3 1/2 inches in length. The "upper" two-thirds of the overall pin length consisted of a constant- diameter rod that terminated in a smooth cone at the top. The lower one-quarter of the overall pin was threaded, and a hexagonal set of wrenching flats was situated between the threaded section and the upper smooth rod. The wrenching flat section was approximately twice the diameter of the other two sections, and each of those sections was smoothly faired into the wrenching flat.

An actuating pin was threaded into the forward face of each ferrule. The pin extended parallel to, but in the opposite direction of, the blade. A special washer served as part of the interface between the pin and the ferrule. One side of the washer was planar with concentric grooves. The center hole was chamfered on the opposite side to accommodate the stress relief fillet of the pin. The planar side of the washer abutted the ferrule face, and the chamfered side of the washer accommodated the stress relief fillet of the pin that was situated between the pin threads and the wrenching flats. When properly installed, there was no relative motion between the pin, washer, and ferrule. The wrenc...