Aeronca 15AC N1301H accident near Talkeetna, Alaska

Probable cause

Maintenance personnel’s failure to perform required fuel system tests to ensure that the airplane met its original type certification basis after modifying the fuel system and the Federal Aviation Administration’s improper approval of the fuel tank modification via the field approval process during which it did not ensure that the required fuel system tests were performed, which led to the pilot’s inability to determine the airplane’s actual amount of usable and unusable fuel and the subsequent loss of engine power due to fuel starvation.

NTSB analysis

The pilot was flying on a cross-country flight to check on his son who was participating in a mountaineering class. He had exhausted the fuel in the left fuel tank and was operating the airplane on the right fuel tank; he reported that he thought the airplane had about 12 gallons of fuel remaining. After circling the camp twice about 8,000 ft mean sea level, the engine lost all power. In an effort to restore engine power, he switched fuel tanks, applied carburetor heat, and pumped the throttle. However, the engine sputtered and then lost total power again. He turned the airplane toward the nearest airstrip and was able to restore intermittent engine power by continuing to pump the throttle and rocking the wings. While the pilot was performing an emergency landing, the engine sputtered and had a short burst of power, which resulted in the airplane overshooting the intended landing area and overrunning the departure end of the runway. The airplane nosed over and sustained substantial damage to the rudder and left lift strut. No fuel was found on the ground or vegetation at the accident site. About 8 gallons of fuel was removed from the right fuel tank, and the left fuel tank was empty. No preaccident engine anomalies were noted.

A review of the airplane's maintenance records revealed that its original bladder fuel tanks had been replaced with two 24-gallon aluminum-alloy fuel tanks and that this modification was approved by the Federal Aviation Administration (FAA) under its field approval process; this process required that the maintenance information meet the original type certification basis for major alterations to aircraft, engines, and propellers certificated under the Civil Air Regulations. However, no fuel flow tests, usable/unusable fuel quantities, placarding, or flight manual supplement was referenced in the description of work when the tanks were installed or when the alteration was approved by the FAA as was required to meet the original type certification basis for the aircraft.

Examination of the wing fuel tanks revealed that the left tank's internal baffle had half-moon lightening holes at the bottom of the fuel baffle, that the right tank's internal baffle had lightening holes that started about 1 inch from the bottom of the tank, and that the baffle fit tight against the bottom of the right fuel tank. Given the lack of lightening holes at the bottom of the right fuel tank's internal baffle it is likely that the unusable fuel in the right tank would have been significantly greater than the unusable fuel in the left tank.

Each tank had a placard near the filler cap on the exterior of the wing indicating that the tank had 24 gallons of usable fuel. The fuel selector inside the cockpit had a placard indicating 36 gallons. However, postaccident calculations estimated that the total usable fuel was actually 22.5 gallons and that the unusable fuel was about 4 to 5 gallons per fuel tank.

Due to maintenance personnel's failure to conduct the required fuel system tests after the system was modified, the FAA's improper approval of the fuel tank modification, and the inconsistencies in the construction of the fuel tank baffles and in the fuel-related placarding, the pilot would have had no clear idea of how much usable or unusable fuel was available in each wing tank. Given the lack of mechanical deficiencies with the engine, it is likely that the engine lost power due to fuel starvation.

Text from the NTSB. Open the NTSB docket for ANC14LA038 for the full record.