Cessna 150K N6059G accident near Huntsville, Texas

Probable cause

A partial loss of engine power due to fuel starvation caused by a fuel system blockage, and the flight instructor’s subsequent failure to maintain adequate airspeed after the loss of engine power, which resulted in the airplane exceeding its critical angle of attack and entering an aerodynamic stall at a low altitude.

NTSB analysis

The student pilot and his flight instructor were conducting touch-and-go landings during an instructional flight when a partial loss of engine power occurred during the initial climb from the runway. Two witnesses reported that they were on the ramp preparing for a flight when they heard a sudden decrease in engine rpm. They turned and saw the airplane flying about 500 ft above the runway in a level attitude with the wings rocking. The airplane then entered a left turn in a nose-down attitude. One of the witnesses believed that the pilot was attempting a descending left 180° turn to land on the opposite runway direction used for the takeoff. The airplane completed about 90° of turn when it entered an aerodynamic spin and descended to the ground in a nose-down attitude.

Postaccident examination revealed debris that obstructed the inlet elbow fuel fitting to the fuel shutoff valve. Similar debris was also recovered from the fuel shutoff valve and the left and right fuel lines. Borescope examination of the fuel tanks revealed additional loose debris inside each fuel tank. Laboratory analysis determined the elemental composition of the fuel system debris consisted mostly of silicon, iron, and lead. Silicon and iron are consistent with soil. The presence of lead was consistent with transfer contact with fuel system components where leaded fuel, such as aviation 100 low-lead fuel, had been present. Based on the available evidence, the partial loss of engine power was likely due to fuel starvation from a blockage of the inlet elbow fuel fitting to the fuel shutoff valve.

According to the airplane’s maintenance logbooks, the last maintenance of the fuel shutoff valve was completed more than 8 years before the accident. However, about 9 months before the accident, the fight instructor, who was also the airplane owner, made an entry in the airplane’s discrepancy record that the airplane was using fuel from the left tank “slower” than the right tank. The discrepancy was deferred until the next annual inspection. The discrepancy log entry further noted that the issue could not be duplicated during the annual inspection that was completed about 14 flight hours before the accident. Comparing the signatures on the discrepancy entry to signatures found in the flight instructor’s pilot logbook established that it was the flight instructor who entered and closed out the discrepancy entry. As described, the discrepancy was likely indicative of a fuel system blockage forming that resulted in uneven fuel usage between the left and right fuel tanks.

The wreckage examination did not reveal any preimpact flight control or mechanical malfunctions or failures of the engine that would have prevented normal operation. The airplane’s nose-down attitude and aerodynamic spin are consistent with the flight instructor failing to maintain adequate airspeed during the return to the runway.

Text from the NTSB. Open the NTSB docket for CEN23FA401 for the full record.