N756NNCessna TU206G · 1977

On September 20, 2018, about 1522 central daylight time, a Cessna TU206G, N756NN, was substantially damaged when it was involved in an accident near Brenham, Texas. The airline transport pilot and passenger were not injured. The airplane was operated by The United States Department of Agriculture as a public flight.

After completing a series of photographic assignments in Central and North Texas, the pilot determined there was adequate fuel remaining to reach the destination airport with a 45-minute reserve. After circumnavigating rain showers, he began the landing approach. About 5 miles from the airport, the engine began to lose power.

The pilot noticed that the left fuel tank gauge indicated a lower quantity than the right, so he switched the fuel selector valve from the left to right tank; however, the engine did not respond, and shortly thereafter, the engine lost total power. The pilot reported that due to the airplane's low altitude, he did not perform any items on the emergency checklist, but instead focused his attention on executing a forced landing. The airplane impacted a tree just before touchdown and impacted a fence during the landing roll.

The airplane was equipped with long-range fuel tanks as well as tip tanks, which contained a usable fuel quantity of 76 and 27 gallons, respectively. Fuel from the tip tanks was transferred to the main tanks when needed using a transfer pump.

The airplane was equipped with the standard factory main fuel tank gauges, along with a CGR-30P electronic engine monitor manufactured by Electronics International. The unit had a fuel totalizing feature and was capable of displaying and recording fuel consumption and engine parameters, including cylinder and exhaust gas temperatures, fuel flow and pressure, and fuel remaining. The pilot reported that, before departure, he filled the airplane to capacity, and the recovered data indicated that he correctly set the fuel quantity in the totalizer to 103 gallons upon system startup. The engine operated continuously for 4 hours and 54 minutes, during which the average fuel flow was about 18.5 gallons per hour (gph), fuel pressure was 10 lbs per square inch (psi), and the engine speed was about 2,450 rpm. All engine parameters remained stable throughout the flight and appeared consistent with the airplane’s performance specifications documented in the manufacturer's Pilot's Operating Handbook (POH).

The CGR-30P's calculated fuel remaining quantity decreased in a linear manner throughout, and 4 hours and 51 minutes after engine start, the quantity remaining reached 17.8 gallons. Over the final 3 minutes of the flight, the engine speed dropped to 1,510 rpm, and fuel flow and pressure decreased to 1.6 gph and 1.4 psi, respectively.

Although the left wing sustained impact damage, none of the airplane's four fuel tanks were breached. Aircraft recovery personnel drained about 18 gallons of fuel from the right main fuel tank and reported that both the tip tanks and the left main tank were empty.

The airplane was examined by a mechanic under the supervision of a Federal Aviation Administration inspector. The examination did not reveal any pre-impact airframe or engine anomalies that would have precluded normal operation.

The pilot reported that he switched fuel tanks every hour and planned an average fuel consumption of 18 gallons per hour. He reported that he had allowed a fuel tank in the accident airplane to run dry in the past, but the engine had always restarted after he switched tanks.

The emergency procedures section of the POH provided the following guidance for an engine failure in flight:

(1) Airspeed - 75 KIAS.

(2) Fuel Selector Valve and Quantity - CHECK.

(3) Mixture - RICH.

(4) Auxiliary Fuel Pump - ON for 3-5 seconds with throttle 1/2 open; then OFF.

(5) Ignition Switch - BOTH (or START if propeller is stopped).

(6) Throttle - ADVANCE slowly.

The POH permitted the exhaustion of a fuel tank in flight but required that the auxiliary fuel pump be engaged for a short period after the fullest tank had been selected. The POH also suggested that the pump be run briefly before exhausting the tank, to confirm its operation.

The first item in the before landing checklist stated, "Fuel Selector Valve – FULLER TANK".