Accident Overview
On July 15, 2017, about 1700 Alaska daylight time, a float-equipped Cessna A185F airplane, N401WC, was substantially damaged during a forced landing in grassy tidal flats near Wasilla, Alaska. The pilot, who was the sole occupant, was not injured. The flight was conducted as a personal flight under Title 14 Code of Federal Regulations Part 91.
Preflight Fueling and Inspection
The pilot reported that four days before the accident, he had filled the long-range fuel tanks with 62 gallons of fuel before flying from Campbell Lake to Lake Louise, a flight lasting about 1.3 hours. On the day of the accident, before departing Lake Louise for the return flight, the fuel quantity indicators showed each tank at 3/4 full. The pilot visually checked the fuel level in each tank without using a dipstick and took fuel samples, finding no contamination.
Inflight Events
Approximately one hour into the flight, at an altitude of about 1,500 feet mean sea level (msl), the pilot noted a fuel flow of 13 gallons per hour (gph) on the engine analyzer and fuel flow gauge. The fuel flow then dropped to 4 gph, and the engine began to sputter. The pilot advanced the throttle to maximum and pumped the throttle in an attempt to keep the engine running, while maneuvering for an emergency landing. He confirmed the fuel selector was in the "both" position and observed that the left fuel quantity gauge indicated empty, while the right showed about half full. The fuel flow temporarily increased to 15.6 gph, but the engine lost all power when the airplane was about 300 feet above ground level (agl).
Forced Landing
The pilot executed a forced landing in grassy tidal flats. During touchdown, the left float struck a log hidden in the tall grass, causing the airplane to nose over and come to rest inverted. The pilot safely egressed from the overturned aircraft.
Witness and Postaccident Examination
A Federal Aviation Administration (FAA) aviation safety inspector who lived nearby heard the engine sputtering for about 45 seconds and watched the aircraft descend. He reported hearing a momentary increase in engine power just before the floats contacted the ground, followed by silence. Responding to the scene, he found the airplane inverted with no fuel leaking. Substantial damage was noted to the wings, left lift strut, and vertical stabilizer.
During a postaccident examination, electrical power was applied to the aircraft. The fuel quantity gauges indicated the left tank empty and the right tank half full. The digital fuel flow gauge showed that 61.2 gallons had been used since its last reset, with 0.8 gallon remaining. Fuel samples were consistent with 100 low lead aviation fuel. Draining produced about 8 gallons from the right tank and 1.75 gallons from the left—less than the left tank's unusable fuel amount of 3 gallons. After draining, both fuel quantity indicators showed empty. All four fuel tank caps were secure, vents were clear, and no leaks were found. The fuel selector valve and auxiliary fuel pump passed functional tests. An engine test run using drained fuel succeeded, with a magneto check at 900 rpm; high-power testing was not performed due to propeller damage. No mechanical malfunctions or failures that would preclude normal operation were found.
Fuel Management Considerations
The pilot stated that the fuel tanks were balanced before departure and that the fuel selector remained in the "both" position throughout the flight. Based on prevailing wind, the flight faced a quartering headwind, which would have reduced ground speed and increased drag on the floats, thereby increasing fuel consumption and flight time.
The Cessna A185F Pilot Operating Handbook (POH) specifies an expected fuel flow of 16 gph at 2,500 ft msl with 25 inches manifold pressure and 2,500 rpm. A Cessna Pilots Association Technote discussed possible causes of fuel imbalance in Cessna single-engine airplanes when operating with the fuel selector in "both," including sloshing in the interconnect vent line, vent tube abnormalities, tank cap venting, wing dihedral effects, fuel line restriction, and flight control rigging imbalance.
The POH notes that fuel may not drain evenly from full tanks due to sloshing in the interconnect vent line, but quantities should equalize as fuel is consumed if wings are kept level. It advises taking off and landing with the selector in "BOTH ON" to prevent inadvertent operation on an empty tank, cautions that extended flight with the selector in "BOTH ON" can lead to unequal fuel flow if wings are not exactly level, and recommends switching to the tank on the "heavy" wing to alleviate wing heaviness. For extended cruise, the recommended procedure is to use left and right tanks alternately.
The POH also provides instructions for restarting the engine after running a tank dry: immediately switch to a tank containing fuel and momentarily turn on the auxiliary fuel pump. The pilot reported that after the initial power loss, he recognized the left tank was empty but had no time to attempt restart procedures once turning and initiating the emergency landing.