WPR09LA390

The airplane's engine lost power during cruise flight and the pilot executed a forced landing to a field. During the landing, the main landing gear encountered tall grass and collapsed. Examination of the airplane revealed that the fuselage sustained structural damage. Examination of the engine revealed no evidence of any preimpact mechanical discrepancies that would have prevented normal operation. The airplane was equipped with two 12.5-gallon wing fuel tanks, each of which had its own two-position (on-off) fuel selector valve. The left wing fuel tank was found empty and the right wing fuel tank contained about 4.5 gallons of fuel. During the accident flight, both fuel selector valves were in the 'ON' position, which was the pilot’s normal operating procedure. The airplane does not use an interconnected fuel vent system, but instead uses individual vents in the caps of each tank. With both fuel tank selector valves in the 'ON' position, if one vent is plugged, or partially misaligned, the vent pressure difference will result in unequal fuel flow to the engine and will move fuel from the higher pressure vent/tank to the lower pressure vent/tank. Also, if both fuel tank selector valves are in the 'ON' position, prolonged uncoordinated flight with one wing lower than the other will result in unequal fuel flow to the engine and in fuel flowing from the high to the low wing tank. The loss of engine power was likely due to unequal fuel feed to the engine from the two wing fuel tanks, which resulted in the left tank running dry and interrupting fuel flow to the engine. The reason for the unequal fuel feed was not determined.

On August 5, 2009, about 2020 Pacific daylight time, a Luscombe 8A, N28425, was substantially damaged during a forced landing following a loss of engine power near Eatonville, Washington. The private pilot and his passenger were not injured. The pilot/owner was operating the airplane under the provisions of Title 14 Code of Federal Regulations Part 91. Visual meteorological conditions prevailed for the local personal flight, and no flight plan was filed. The flight had originated from Pierce County Airport, Puyallup, Washington, approximately 1900. The pilot reported that while in cruise flight, the airplane’s engine "died." He noted that he had fuel in the right tank, banked slightly, and checked that the fuel mixture was full rich. The pilot initiated a forced landing to a field. During the landing, the main landing gear encountered tall grass and collapsed. Postaccident examination of the airplane revealed that the fuselage sustained structural damage. On August 19, 2009, an NTSB investigator examined the airplane’s Continental A65 engine, serial number 3744968R, which had been removed from the airframe by salvage personnel. The engine crankshaft was rotated by hand and compression, suction, and valve train continuity were verified to all cylinders and the accessory gears. The left and right magnetos produced spark on all ignition harness leads. The carburetor was separated from the crankcase just below its mount due to impact forces. The carburetor was disassembled and no anomalies were noted. When compared to the Champion Check-A-Plug comparison card, the top spark plugs exhibited normal operating signatures. No engine anomalies were noted during the examination. The airplane was equipped with two 12.5-gallon wing fuel tanks, each of which had its own two-position (on-off) fuel selector valve. When the airplane was retrieved by salvage personnel, they found that the left wing fuel tank was empty and the right wing fuel tank contained about 4.5 gallons of fuel. The tanks were intact and undamaged by the impact. The pilot reported that during his pre-flight inspection, he checked the fuel tanks and determined that there were 6.5 gallons of fuel in the left tank and 5 gallons in the right tank. He stated that the airplane burned about 5 gallons of fuel per hour. During the accident flight, both fuel selector valves were in the “ON” position, which was the pilot’s normal operating procedure. According to its type certificate data sheet the Luscombe 8A was certified under Part 4a of the Civil Air Regulations (CARs). CAR 04a.621 prescribed the certification standard for fuel tank installations and stated, in part, "if two or more tanks have their outlets interconnected, they shall be considered as one tank, and the airspace in the tanks shall also be interconnected to prevent differences in pressure at the air vents of each tank of sufficient magnitude to cause fuel flow between the tanks." An article dated February 20, 2008, and published inedition #193 of the Luscombe Association Newsletter describes the Luscombe 8 series fuel systems. The article states that the Luscombe does not use an interconnected fuel vent system, but instead utilizes individual vents in the caps of each tank. The article further states that “there is no pressure equalizing provision in the system, therefore, only ONE tank may be selected 'ON' at a time.” With both fuel tank selector valves in the 'ON' position, if one vent is plugged, or partially misaligned, the vent pressure difference will result in unequal fuel flow to the engine and will move fuel from the higher pressure vent/tank to the lower pressure vent/tank. Also, if both fuel tank selector valves are in the 'ON' position, prolonged uncoordinated flight with one wing lower than the other will result in unequal fuel flow to the engine and in fuel flowing from the high to the low wing tank.