WPR15FA045

A loss of engine power for reasons that could not be determined due to extensive damage sustained during the collision and postaccident water immersion.

The private pilot was conducting a routine cross-country flight to the destination airport. The pilot had conducted this same flight once a month for the previous 5 years to serve as a community rural doctor. Numerous witnesses reported observing the airplane maneuvering toward the airport at a very low altitude just below the cloud layer. Several witnesses near the accident site stated that they did not hear any engine noise. The airplane impacted a tree about 2,260 ft short of the approach end of the runway and came to rest in a river, partially resting on a gravel/sand embankment. Following the accident, the water level quickly rose, nearly submerging the entire airframe. Although a postaccident examination revealed no evidence of preimpact mechanical malfunctions or failures, the extent of the damage sustained during the collision and the water immersion precluded a detailed determination of the engine and fuel system preimpact condition, settings, and functionality. One of the three propeller blades was bent aft about 80 degrees near the blade root, but the other two blades were undamaged. Based on this minimal damage and the witnesses' accounts of not hearing engine noise, it is likely that the engine experienced a loss of power while the pilot was on approach to the airport. Fuel was found in the fuel lines forward of the firewall; therefore, it is unlikely that fuel exhaustion or starvation occurred. The fuel mixture cable was found disconnected from the mixture arm on the carburetor. Given the minimal damage to the mixture arm and that it was not spring-loaded to any position, it is possible that the cable became disconnected in flight and adversely affected the engine operation. However, there was insufficient evidence to determine when the cable became disconnected or whether or when it affected the engine operation.

No records were located of the pilot, or a pilot using the airplane's registration number, contacting any Air Traffic Control tower, or Common Traffic Advisory Frequency, during the duration of the flight. The Airport/ Facility Directory (AFD), indicated that the Garberville Airport (O16) runway 36 was about 2,780 feet long and 75 feet wide. The runway surface was composed of asphalt. The airport elevation was 550 feet msl. The uncontrolled airport did not have any published instrument approaches. In the remarks section of the AFD was a note "25 ft. trees, 200 ft. from runway, 70 ft. right of centerline." There was 100LL avgas available at all times. The airplane was examined both after being removed from the river and staged at the airport and then at a later date at a recovery facility. A detailed analysis of the engine and fuel system control continuity was not possible due to the extent of the damage sustained during the collision and the water submersion. Airframe The right wing was separated for recovery purposes and contained crush damage to the leading edge. The fuel tank on the outboard right wing was found full of fluid consistent with the odor and appearance of avgas. The right flap remained attached and was physically in the retracted position. About six feet outboard from the wing root, the right wing exhibited aft crush damage, and debris was embedded in the wrinkled skin. The wing tip was absent. The left wing was carried downstream and the fuel tanks were breached. The rudder and vertical stabilizer remained intact. The right horizontal stabilizer sustained crush damage to the tip. The left horizontal stabilizer and attached elevator were creased in a slight downward V-shape. The left horizontal stabilizer tip was absent. The nose gear appeared retracted and the right main landing gear remained retracted in the wheel well. The landing gear position knob in the cockpit was in the down and locked selection. Engine The engine remained attached to the airframe and mounts, with buckling observed on the firewall. An external visual examination of the engine revealed no evidence of pre impact catastrophic mechanical malfunction or fire. Damage was noted on the No. 1 rocker box cover, where crush deformation was observed. Investigators removed the upper spark plugs of all cylinders and three lower (the lower Nos. 4 and 6 were not able to be removed due to oil filling the cylinders and potential leakage). According to the Champion Aviation Check-A-Plug AV-27 Chart, the spark plug signatures corresponded to normal engine operation although numerous plugs were coated with oil. Mechanical continuity was established throughout the rotating group, valve train, and accessory section during hand rotation of the crankshaft. "Thumb" compression was observed in proper order on four of the six cylinders. A subsequent borescope examination of cylinders Nos. 3 and 5, the cylinders that did not yield thumb compressions, revealed post impact foreign debris had been trapped under the valve seats. Additionally, the combustion chamber of each cylinder was examined through the spark plug holes utilizing a lighted borescope. The combustion chambers remained mechanically undamaged, and there was no evidence of foreign object ingestion (pre-impact) or detonation. The valves were intact and undamaged. There was no evidence of valve to piston face contact observed. The three-bladed constant speed propeller remained attached at the crankshaft flange. The spinner was attached to the propeller and crushed aft. The propeller blades remained attached to the propeller hub. One of the three propeller blades was bent aft about 80 degrees near the blade root. The remaining blades were undamaged, consistent with the engine producing little if any power at the time of impact. The propeller governor was securely attached at the mounting pad with the pitch control rod securely attached at the control arm. The governor was removed for examination. The drive was intact and free to rotate by hand. Fuel System The cockpit fuel selector handle was positioned on the left main (30 gal) tank. The fuel selector valve could not be accessed due to the damage sustained to the cockpit. The right outboard tank (15 gal right aux) was full with a fluid consistent with smell and appearance of avgas; the right inboard fuel tank was filled with silt and mud. The gascolator had been submerged in river water. Disassembly of the bowl revealed that there were several ounces of fluid consistent with smell an appearance of avgas. Some water and mud residue was also found in the bowl. The filter screen was found free of contamination. Investigators removed the fuel line from the electric fuel pump to the gascolator, and a fluid consistent with smell and appearance of avgas dribbled out of the line. The line from the mechanical pump to the carburetor contained a similar liquid. The carburetor sustained crush damage and was broken open, having come loose from the engine. The carburetor bowl was breeched and empty. The carburetor heat and throttle cables remained attached to their respective arms on the carburetor and continuity to the cockpit could not be established due to the compromised airframe. The mixture cable was detached from the intact mixture-arm on the carburetor; the hardware could not be located. According to Lycoming, the carburetor is not equipped with a spring-loaded mixture arm. Therefore the arm would stay positioned wherever the linkage became detached, but would be able to vibrate into any position. There was no evidence of any pre-impact mechanical malfunctions or failures found during the examination. The County of Humboldt Coroner completed an autopsy of the pilot. The FAA Civil Aeromedical Institute (CAMI) performed toxicological screenings on the pilot. According to CAMI's report (#201400266001) the toxicological findings were negative for carbon monoxide and tested drugs. The accident site was located in the South Fork Eel River about 2,260 feet south of the approach end of runway 36 at Garberville. The airplane came to rest in the water, partially resting on a gravel/sand embankment. The left wing was separated and entangled in vegetation/debris on the south side of the river. Following the accident, the water level quickly rose, nearly submerging the entire airframe. The first identified point of contact consisted of a tree that contained a large gouge on its south-facing trunk. That gouge was located about 60 to 70 feet above ground level (agl), and about 20 feet below the top of the tree. Numerous branches, and several pieces of airplane debris, were located immediately beneath the tree, consistent with the left wing colliding with the trees prior to descending into the river. A complete pictorial of the wreckage location and surrounding terrain is contained in the public docket for this accident. A routine aviation weather report (METAR) was generated by an Automated Surface Observation System (ASOS) in Fortuna, California, located about 35 miles from Garberville on a heading of about 330 degrees. The 0955 METAR from that station included wind from 120 degrees at 15 knots; a scattered layer of clouds at 3,600 and 4,200; a broken layer of clouds at 5,000; with 9 miles visibility. It recorded the temperature at 52 degrees Fahrenheit; dew point 50 degrees Fahrenheit and an altimeter setting of 30.03 inHg. Witnesses in Garberville reported that there was a low cloud layer at the time of the accident and there was a light rain drizzle. The Piper Aircraft PA-24-250 Comanche, serial number (s/n) 24-2616, was manufactured in 1961 and purchased by the pilot in 2012. The low-wing airplane was equipped with four seats, retractable tricycle landing gear, and traditional flight control surfaces. The airplane was equipped with a Lycoming O-540-A1D5, s/n L-5661-40, and, according to the manufacturer, is rated at 250 shaft horse power (SHP). Th