WPR14LA147

The pilot was performing an introductory flight lesson for a prospective student pilot and a passenger. The pilot reported that, about 20 minutes after takeoff, while in cruise flight, the engine began producing only partial power. The pilot performed troubleshooting steps, including applying carburetor heat, but the engine did not respond and subsequently lost total power. He performed a forced landing into a field of tall grass. The airplane came to rest nose down after the nose gear dug into the ground, which resulted in substantial damage. A postaccident examination of the engine and airframe revealed no evidence of mechanical malfunctions or failures that would have precluded normal operation. The operator's mechanic reported that the airplane's fuel tank vent line was partially blocked; however, the airplane was equipped with a secondary vented fuel cap, so it is unlikely that fuel starvation occurred.

According to the Cessna 172 service manual, fuel tank venting is accomplished by an overboard vent line incorporated in the left fuel tank. The vent line protrudes through the bottom of the left wing into the airstream. In addition, a vent crossover line connects the airspace in the left tank to the airspace in the right tank where a vented fuel tank cap is installed. The Cessna 172P Pilots Operating Handbook, "EMERGENCY PROCEDURES, Rough Engine Operation or loss of power, Carburetor Icing" section states the following: "A gradual loss of engine RPM and eventual engine roughness may result from the formation of carburetor ice. To clear the ice, apply full throttle and pull the carburetor heat knob full out until the engine runs smoothly; then remove carburetor heat and readjust the throttle. If conditions require the continued use of carburetor heat in cruise flight, use the minimum amount of heat necessary to prevent ice from forming and lean the mixture for smoothest engine operation." The carburetor icing probability chart from FAA Special Airworthiness Information Bulletin: CE-09-35 Carburetor Icing Prevention, June 30, 2009, shows a probability of serious icing at glide power at the temperature and dew point reported at the time of the accident. Maintenance records revealed that the airplane had undergone an annual inspection that was completed on January 30, 2014. At that time the airframe had accrued a total of 18,766.1 flight hours since manufacture in 1986, with the engine accruing 2,360 hours since overhaul. The airplane flew for 90 hours between the annual inspection and the accident. On March 26, 2014, at 1743 local time, a Cessna 172P, N9551L, landed in a field following a loss of engine power near Inarajan, Guam. The airplane was registered to, and operated by, Trend Vector Aviation International, under the provisions of 14 Code of Federal Regulations Part 91 as an introductory flight lesson. The flight instructor (CFI) and two passengers were not injured. The airplane sustained substantial damage to the forward fuselage and left wing during the accident sequence. The local flight departed Guam International Airport, Barrigada, Guam, about 1725. Visual meteorological conditions prevailed, and no flight plan had been filed. The CFI reported that after takeoff he initiated a left turn, and having reached an altitude of 1,500 feet mean sea level (msl), he began a turn to the southwest towards Apra Harbor. He continued the flight while maintaining an altitude of about 1,500 feet msl, an airspeed of between 95 and 100 knots indicated, and an engine speed of 2,300 rpm. About 20 minutes after takeoff the engine speed began to decrease. He confirmed the mixture control was set to full rich, and then applied full forward throttle control. The engine speed increased momentarily, but then decreased such that only partial power was being produced. He then applied full carburetor heat, but the engine did not respond. The airplane would notmaintain altitude, so the pilot initiated an approach to a landfill for a forced landing. As he approached the landing area he could see heavy equipment on the intended landing zone, so he turned the airplane towards a tall-grass field for a tailwind landing. He stated that during the final approach the engine was no longer producing any power, and the propeller appeared to be wind-milling. The airplane came to rest nose-down after the nose gear dug into the ground. The closest aviation weather observation station was located at Guam International Airport, which was 14 miles north-northwest of the accident site. An aviation routine weather report was recorded at 1754. It reported: wind from 070 degrees at 13 knots gusting to 20 knots; visibility 10 miles with clear skies; temperature 28 degrees C; dew point 23 degrees C; altimeter 29.91 inches of mercury. The airplane was recovered from the accident site and examined by a local mechanic. The mechanic reported that each fuel tank contained about 12 gallons of fuel, and that no fuel or oil leaks were apparent. Oil was present in the sump, and the oil was free of contamination or metal particles. The carburetor bowl was full of fuel, the spark plugs all exhibited signatures consistent with normal operation, and both magnetos were intact and set to the correct engine timing. The cylinder compressions were all about 64/80, and the engine controls were intact at their respective linkages, with the carburetor heat control observed in the full forward (carburetor heat off) position. The examination did not reveal any anomalies with the airframe or engine that would have precluded normal operation. A mechanic from Trend Vector Aviation subsequently examined the airplane and reported that the left wing fuel tank vent line allowed passage of air, but appeared to offer greater resistance when compared to other Cessna 172 airplanes in their fleet. According to representatives from Trend Vector Aviation, the airplane was refueled on the morning of the accident, and subsequently performed an uneventful flight. Another Cessna 172 was refueled from the same pump, and flew all day without experiencing any problems. After the accident the fuel storage facility was checked for possible contamination, and none was found.