N401WC

On July 15, 2017, about 1700 Alaska daylight time, a float-equipped Cessna A185F airplane, N401WC, was substantially damaged when it was involved in an accident near Wasilla, Alaska. The pilot was not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight.

The pilot stated that, 4 days before the accident, he fueled the airplane’s long range tanks to 62 total gallons before flying from Campbell Lake to Lake Louise, about a 1.3-hour flight. Before the return flight on the day of the accident, the fuel quantity indicators each read 3/4 full and the pilot visually verified, without a dipstick, the fuel level in each tank. He took fuel samples and noted no contamination.

About 1 hour into the flight, at an altitude of about 1,500 ft mean sea level (msl), the pilot looked at the engine analyzer and fuel flow gauge and noted a fuel flow of 13 gallons per hour (gph). Then the fuel flow decreased to 4 gph and the engine “sputtered.” The pilot pushed the throttle lever to maximum and “pumped the throttle,” trying to keep the engine running, and he maneuvered the airplane for an emergency landing. He verified that the fuel selector was in the “both” position and noted that the left fuel quantity gauge indicated empty and the right indicated about 1/2 full. The fuel flow temporarily increased to 15.6 gph and then the engine lost total power about 300 ft above ground level (agl). The pilot performed a forced landing in grassy tidal flats. During the landing, the left float contacted a log hidden in tall grass, and the airplane nosed over. The airplane came to rest inverted and the pilot safely egressed.

A Federal Aviation Administration aviation safety inspector who lived in the area of the accident site heard the airplane sputtering for about 45 seconds and watched the airplane descend toward the flats. He said that just before the airplane's floats touched down, he momentarily heard the engine power increase, which was followed by silence. He responded to the scene of the accident and noted that the airplane was inverted and no fuel was leaking from the airplane, which sustained substantial damage to the wings, left lift strut, and vertical stabilizer.

During a postaccident examination, electrical power was applied; the fuel quantity gauges indicated that the left fuel tank was empty and the right tank was 1/2 full. The digital fuel flow gauge indicated that 61.2 gallons had been used since the last time it was set and 0.8 gallon remained. Fuel samples exhibited qualities consistent with 100 low lead aviation fuel. About 8 gallons of fuel were drained from the right tank and 1.75 gallons from the left, which was less than the tank's unusable amount of 3 gallons. The fuel quantity indicators each indicated empty after draining the tanks. The four fuel tank caps were in place and secure. The fuel tank vents were in place and clear of debris. No leaks were observed. The fuel selector valve and auxiliary fuel pump were functionally tested and operated normally. An engine test run was conducted with fuel that was drained from the tanks. The engine started successfully, and a magneto test was conducted at 900 rpm. The engine was not tested at high power due to propeller damage. Examination of the airframe and engine revealed no evidence of mechanical malfunctions or failures that would have precluded normal operation.

The pilot stated that the fuel tanks were balanced before the flight and that he flew the airplane with the fuel selector in the “both” position. Based on the prevailing wind at the time of the accident, the flight would have experienced a quartering headwind, which would have decreased ground speed, increased the crosswind drag effect on the floats, and increased time and fuel required for the flight.

The Cessna A185F pilot operating handbook (POH) indicated that expected fuel flow at 2,500 ft msl at 25 inches manifold pressure and 2,500 rpm was 16 gph.

A Cessna Pilots Association Technote 003 indicated possible causes of fuel imbalance in Cessna single-engine airplanes while operated with the fuel selector in “both.” The issues that were highlighted in the article were sloshing in the tank’s interconnect vent line, fuel tank vent tube abnormalities, fuel tank cap venting, wing dihedral affects, fuel line restriction, and flight control rigging out of balance. A note on page 2-2 of the Cessna 185 POH indicated that:

Fuel may not be drained from full tanks evenly due to sloshing in the interconnect vent line, preventing absolutely equal vent pressure in each tank. However, as fuel is consumed, clearing the interconnect vent line and equalizing tank vent pressures, the fuel quantities should equalize in each tank, provided the wings are maintained exactly level.

Another note on page 2-4 indicated:

Take-off and land with the selector valve handle in the BOTH ON position to prevent inadvertent operation on an empty tank. However, when the selector is left in the BOTH ON position for extended flight, unequal fuel flow from each tank may occur if the wings are not maintained exactly level. Resulting wing heaviness can be alleviated gradually by turning the selector valve handle to the tank in the “heavy” wing.

The recommended cruise fuel management procedure for extended flight is to use the left and right tank alternately.

The POH indicated on page 2-5 that:

To ensure a prompt engine restart in flight after running a fuel tank dry, immediately switch to a tank containing fuel at the first indication of fuel pressure fluctuation and/or power loss. Then place the right half of the auxiliary fuel pump switch in the ON position momentarily with the throttle at least ½ open.

The pilot stated that, after the first sign of loss of power, he knew that the left tank was indicating empty. Once he turned and initiated an emergency landing, there was no time to perform the engine restart procedures.