N6364VAlon A2 · 1965

On April 27, 2014, about 1700 eastern daylight time, an Alon A2, N6364V, experienced a total loss of engine power shortly after takeoff from Deck Airpark (NC11), Apex, North Carolina. The pilot subsequently made an off-airport forced landing into a forest. The private pilot sustained minor injuries, and the passenger was not injured. Visual meteorological conditions prevailed and no flight plan was filed for the local flight. The airplane was registered to and operated by a private individual under the provisions of Title 14 Code of Federal Regulations Part 91 as a personal flight.

The pilot reported that the taxi and initial takeoff were "normal;" however, when the airplane was about 100 feet above the trees that bordered the airport, the engine experienced a total loss of power. The pilot retarded the throttle and immediately reapplied full throttle. The engine subsequently restarted, but immediately lost power again. He then performed a forced landing into the trees, about 500 feet past the departure end of the runway. The pilot reported that he had about 15 gallons of autogas in the fuel tanks prior to departure.

According to pilot provided information and Federal Aviation Administration (FAA) records, the pilot held a private pilot certificate with a rating for airplane single-engine land. He held a third-class medical certificate, which was issued on August 20, 2013. The pilot reported 1471.6 total flight hours, with 38.3 of those hours in the accident airplane make and model.

The accident airplane, an Alon A2 (serial number A-40), was manufactured in 1965. It was registered with the FAA on a standard airworthiness certificate for normal operations. The airplane had a total time of 1,872.36 hours as of the last annual inspection, which was completed on January 4, 2014. The airplane was powered by a Continental C90 series engine. As of the last annual inspection, the engine had accumulated a total of 1,661 hours, with 287 hours since last major overhaul.

The 1651 recorded weather observation at Raleigh-Durham International Airport (RDU), Raleigh, North Carolina, located approximately 10 miles to the northeast of the accident location, included wind from 210 degrees at 6 knots, 10 miles visibility, scattered clouds 25,000 feet above ground level, temperature 28.3 degrees C, dew point 8.9 degrees C; altimeter setting 29.92 inches of mercury

The accident airplane, an Alon A2 (serial number A-40), was manufactured in 1965. It was registered with the FAA on a standard airworthiness certificate for normal operations. The airplane had a total time of 1,872.36 hours as of the last annual inspection, which was completed on January 4, 2014. The airplane was powered by a Continental C90 series engine. As of the last annual inspection, the engine had accumulated a total of 1,661 hours, with 287 hours since last major overhaul.

Initial examination of the aircraft by an FAA inspector revealed that the airplane impacted several trees and came to rest inverted between two trees, resulting in substantial damage to the wings and fuselage. Local authorities reported to the inspector that there was a strong fuel smell at the accident scene; however, the fuel tank selected at takeoff was not noted or provided to the NTSB.

A subsequent examination, of the engine at a storage facility, under NTSB oversight, revealed minimal external damage. The throttle, mixture, and carburetor heat controls remained attached to the engine and operated without anomalies. The engine remained attached to the airframe and was fitted with a 5 gallon fuel container, between the wing root and the carburetor, to facilitate an engine run. The wings were removed at the wing root to facilitate transport, and continuity was not confirmed from fuel tank to the separation point. The engine started and operated through various power settings with no abnormalities noted. No anomalies were noted from the 5 gallon fuel container through the fuel system.

FAA Guidance

According to an NTSB accident report (ERA12LA131), testing conducted at the FAA Technical Center revealed that vapor pressure of autogas can vary widely as formulations are changed seasonally, and according to local requirements. High vapor pressure can promote vapor lock in aircraft fuel systems causing engine power to be reduced or the engine to completely fail and testing by the FAA William J. Hughes Technical Center concluded that autogas with high vapor pressure can accelerate the formation of carburetor ice.

The Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25A) defines vapor lock as "A problem that mostly affects gasoline-fuelled internal combustion engines. It occurs when liquid fuel changes state from liquid to gas while still in the fuel delivery system. This disrupts the operation of the fuel pump, causing loss of feed pressure to the carburetor or fuel injection system, resulting in transient loss of power or complete stalling. Restarting the engine from this state may be difficult. The fuel can vaporize due to being heated by the engine, by the local climate, or due to a lower boiling point at high altitude."

DOT/FAA/CT-87/05 "Autogas in General Aviation Aircraft," states in part "The conditions which define the worst case for vapor lock testing (i.e., most likely to result in vapor lock) are as follows.

1. Takeoff fuel flow

2. Initial fuel temperature between 38 and 43 degrees Celsius (100 to 110 degrees Fahrenheit)

3. Ambient air temperature of 29 degrees Celsius or higher (85 degrees Fahrenheit).

4. Engine at operating temperatures typically found after a prolonged idling or a hot soak.

5. ASTM class E (winter grade) fuel if the testing is for autogas"

An FAA chart, titled "Conditions Favoring Carb Ice Formation," indicated that with the ambient temperature and dew point, "Icing at glide and cruise power" was probable.