CEN14LA261

The pilot reported that the accident occurred during his fifth glider aero-tow flight since the airplane had last been refueled. He stated that the aero-tow and glider release were uneventful. However, as he was returning to the departure airport, the engine began to run intermittently before it eventually experienced a total loss of engine power. During the subsequent forced landing, the airplane became entangled with a chain-link fence and impacted a road before it slid into a drainage ditch. A postaccident examination of the airplane's single fuel tank established that it was undamaged and void of any usable fuel. Before the first flight of the day, the airplane was refueled, and the total usable fuel was about 32.5 gallons. The airplane recording tachometer indicated that 2.2 tachometer hours had been accumulated since that time. Although the airplane operator reported that, according to historical fueling and flight data, the airplane's average fuel consumption rate was about 10.2 gallons per tachometer hour, the calculated average fuel consumption rate was 14.8 gallons per tachometer hour since the last refueling. Although the total loss of engine power was caused by fuel exhaustion, the investigation could not determine the reason for the above-normal fuel consumption rate. However, if the pilot had determined the actual fuel consumption rate between flights, he should have identified that insufficient fuel was available to complete the accident flight.

On May 28, 2014, about 1500 mountain daylight time, a Piper model PA-25-235 airplane, N8808L, was substantially damaged during a forced landing near Boulder, Colorado. The commercial pilot sustained minor injuries. The airplane was registered to and operated by the Soaring Society of Boulder under the provisions of 14 Code of Federal Regulations Part 91 without a flight plan. Day visual meteorological conditions prevailed for the local glider aero-tow flight that departed Boulder Municipal Airport (BDU), Boulder, Colorado, about 1430.The pilot reported that the purpose of the accident flight was to aero-tow a glider to 10,200 feet mean sea level (msl) before returning to the departure airport. He stated that the aero-tow and glider release were uneventful. However, as the airplane was returning to BDU, the engine began to run intermittently as the airplane crossed over the foothills southwest of the airport. The engine eventually lost total power around 10,000 feet msl. The pilot reported that he did not attempt to restart the engine following the loss of engine power. He initially thought that the airplane had sufficient altitude to safely glide to the airport, but it ultimately descended to altitude that required an off-airport landing. He decided to land on a nearby soccer field; however, as he approached the field he realized that there were power lines situated alongside the road that bordered the soccer field. The airplane landing gear collided with a chain-link fence as he maneuvered the airplane below the power lines. After the landing gear became entangled with the fence, the airplane collided with the road before it came to rest in a drainage ditch. The right wing sustained substantial damage during the collision with the fence and terrain. The airplane's single fuel tank held 38 gallons, of which 2 gallons were considered unusable. The pilot reported that, before his first flight of the day, the fuel level was about 1-inch from the top of the filler neck and that the fuel quantity sight gauge indicated that the tank was near capacity. The airplane operator reported that a 1-inch void at the top of the tank equated to about 3.5 gallons of unused tank capacity. Therefore, before the pilot's first flight of the day, the total useable fuel was about 32.5 gallons. The recording tachometer indicated 3,607.7 hours before the pilot's first flight of the day and 3,609.9 hours following the accident, equating to 2.2 tachometer hours having been accumulated since the last refueling. The calculated average fuel consumption rate was about 14.8 gallons per tachometer hour since the last refueling. The airplane operator reported that, according to historical fueling and flight data, obtained from January-March 2014, the airplane's average fuel consumption rate was about 10.2 gallons per tachometer hour. Additionally, the previous pilot had flown 13 aero-tows with an average fuel consumption rate of 10.3 gallons per tachometer hour. The previouspilot also confirmed that he had refueled the airplane following his final flight, and left a 1-inch void at the top of the fuel tank. The airplane was equipped with a fuel quantity sight gauge that was calibrated to correctly indicate when there was 10 gallons of usable fuel remaining. The operator's standard operating procedure was to refuel the airplane whenever the sight gauge indicated 10 gallons remaining. The accident pilot reported that the airplane had accumulated 1.9 tachometer hours during his first 4 aero-tow flights and that there was slightly more than 10 gallons when he referenced the fuel quantity sight gauge before the accident flight. The investigation determined that the average fuel consumption rate for the previous 4 flights was at least 11.3 gallons per hour. According to the pilot, the accident flight departed at 3,609.6 tachometer hours, and it indicated 3,609.9 at the accident site. As such, the duration of the accident flight was at least 0.3 tachometer hours before the engine lost power. An on-site examination was completed by a Federal Aviation Administration Inspector with the Denver Flight Standards District Office. The FAA inspector reported that his visual examination of the airplane's single fuel tank established that it was undamaged and void of any useable fuel. At 1456, the BDU weather observing system reported: calm wind, 10 miles visibility, clear sky conditions, temperature 30 degrees Celsius, dew point 5 degrees Celsius, and an altimeter setting of 30.10 inches-of-mercury.