N3804UCessna 336 · 1964

On July 3, 2014, about 1846 central daylight time, a Cessna 336, N3804U, force landed following a partial loss of engine power at Mobile, Alabama. The airline transport pilot was seriously injured, and the airplane was substantially damaged. The airplane was operated by the Mobile County Health Department. Day, visual meteorological conditions prevailed for the local, public use, aerial application flight, and no flight plan was filed. The local flight originated at St. Elmo, Alabama (2R5) about 1829.The pilot reported in a written statement to the NTSB that he was assigned to mosquito control spraying operations in the Mobile area. He departed 2R5 with about 48 gallons of fuel, including 36 gallons in the auxiliary tanks. While established on the southbound leg of a spray pattern, he transferred the engines, one at a time, to the full, 18 gallon auxiliary tanks. Shortly after, the engines began to "sputter" and would not maintain rpm. He aborted the spray run, "zoom climbed" to about 600 feet above the ground (agl), and turned to a heading of about 210, in the direction of Mobile Downtown Airport (BFM), about 4 to 5 nautical miles away. The available power was intermittent with rpm surges on both engines, and power available appeared insufficient to reach BFM. He set up for a forced landing on an island between a container terminal and a coal terminal. The airplane touched down on soft soil and came to a stop abruptly, resulting in extensive damage to the airplane.

An inspector with the Federal Aviation Administration responded to the accident site and examined the wreckage. Structural damage to the fuselage was confirmed. An examination of the fuel system revealed that the fuel selector handle for the front engine was in the right main tank position. The fuel selector handle for the rear engine was broken off from impact forces. It was later determined that the rear engine fuel selector was also in the right main tank position. The left and right main fuel tanks were empty of fuel and only residual fuel (about 2 ounces) was drained from the left and right main sump tanks. The tanks were not breached. The design of main fuel tanks allows them to completely drain into their respective sump tanks. The left and right auxiliary tanks contained sufficient fuel; however, it was not quantified.

The FAA inspector interviewed the pilot on July 9, 2014. He stated in the interview that he had been flying at 100 feet agl when the rear engine began "sputtering and coughing." He switched to the auxiliary fuel tank; however, this did not remedy the situation. He climbed the airplane to 500 feet agl and continued to troubleshoot by switching tanks and turning the boost pumps on. The front engine then began to lose power. He continued to troubleshoot the loss of power and realized that he could not maintain altitude and needed to perform an emergency landing short of the airport.

The engines were removed from the airframe and sent to the manufacturer's facility for examination. The engines were test run during the week of July 21 through 25, 2014. After repair of impact-related damage, the engines ran normally in a test cell. The test runs did not reveal any abnormalities that would have prevented normal operation and production of rated horsepower.

On January 16, 2015, the aircraft fuel system was re-examined under the supervision of the NTSB investigator-in-charge. The both engine fuel selectors were confirmed to be in the right main tank positions. Flaky, rust-colored debris was observed inside the right sump tank. The left sump tank was relatively clean. The left and right wing auxiliary fuel pumps, which operate the left and right main tank fuel sources only, were found to be functional when energized with a dc electrical source. The fuel lines from the fuel selector valves to the main and auxiliary tanks were unobstructed.

The rear engine fuel strainer was opened for examination. It contained about 1/8 teaspoon of light gray, powder-like debris in the bowl. The screen was unobstructed. The supply line from the strainer to the engine-driven fuel pump was obstructed; no air could be blown through it by mouth. The obstruction was removed with a clean piece of wire; about one teaspoon of light gray, powder-like debris was removed.

The front engine fuel strainer was opened for examination. The bottom of the bowl was corroded with a rust-like substance. The screen was unobstructed. The supply line from the strainer to the engine-driven fuel pump was obstructed; no air could be blown through it by mouth. The obstruction was removed with a clean piece of wire; about one half teaspoon of rust-colored, granular debris was removed.

According to the aircraft manufacturer, the auxiliary fuel tanks are intended for level, cruise flight. The auxiliary fuel tanks gravity feed directly to the fuel strainer; no boost pumps are available. The Cessna 336 pilot's operating handbook (POH) states that, in the pre-flight exterior inspection and the before landing checklist, the main tanks are to be selected. Also, the POH states that, in the event of an engine out situation during flight, the main tanks are to be selected if the auxiliary tanks are in use.