Accident Overview
On April 24, 2018, at approximately 1403 eastern daylight time, an experimental, amateur-built Mosquito XE helicopter (N911CY) was destroyed in an accident near Albion, New York. The private pilot, the sole occupant, sustained serious injuries. The flight was conducted under Title 14 Code of Federal Regulations Part 91 as a personal flight.
Flight History
The pilot had recently installed an autotuner unit designed to monitor and adjust the engine's air-fuel ratio (AFR). The day before the accident, he flew with the AFR set to 13.0, resulting in an exhaust gas temperature (EGT) of 1,050°F, at the low end of the normal operating range (1,050°F–1,150°F). He then adjusted the AFR to 13.5 and hovered for about 10 minutes, during which the EGT indicated 1,100°F.
On the accident day, the pilot elected to fly around the airport traffic pattern. During takeoff, at about 75 feet above ground level (agl), the engine began to backfire with resulting power surges. He decided to land straight ahead while maintaining lateral control with anti-torque pedal inputs. He reported to the helicopter designer that he experienced a second bang or pop followed by a total loss of engine power at about 50 feet agl. He attempted to autorotate, but the main rotor rpm had deteriorated, rendering controlled flight impossible. Upon impact, the helicopter turned onto its left side and a post-impact fire ensued due to impact damage to the fuel tank. The pilot egressed and was treated for his injuries.
Witness Observation
A witness driving on a road near the airport reported seeing the helicopter hovering about 6 feet agl adjacent to hangars. The witness then observed the helicopter ascend straight up to about twice the height of nearby powerlines and proceed in a westerly direction. He noted a small piece separate from the helicopter, followed by a popping sound and then a grinding sound. The helicopter entered a steep descent, during which a second piece separated. The helicopter impacted the ground and immediately burst into flames.
Post-Accident Examination
An Federal Aviation Administration (FAA) inspector examined the accident site. One air induction filter was located about 20 feet east of the wreckage. The airframe sustained extensive damage from the post-impact fire, which consumed most of the airframe and a portion of the tail boom. The tail rotor gearbox was torn from its mounts and separated from the helicopter on impact. All controls and the tail rotor drive shaft were accounted for with no evidence of preimpact failure or malfunction. The remainder of the drive train components were extensively fire-damaged. The autotuner was destroyed by fire.
Engine Examination
The engine, a two-stroke, two-cylinder, 85-horsepower Innovator Technologies Inntec 800, was examined by a representative of the helicopter designer with FAA oversight. No evidence of preimpact failure or malfunction was noted to the powertrain or lubrication system. The spark plugs exhibited a black-and-white colored crust consistent with postcrash use of a fire extinguisher; underneath, the plugs displayed a light, smoky gray color consistent with a lean fuel-to-air ratio.
Designer Statements
The designer of the helicopter stated, and the pilot agreed, that during manipulation of the autotuner to obtain proper EGT readings, the pilot did not review or accept the auto-tune-suggested new fuel map and thought he had completed the autotuner setup, but he had not. As a result, the autotuner was operating on the accident date with the initial, lean AFR setting. The designer also noted that the helicopter was equipped with a knock sensor; when detected, the engine computer (ECU) puts the engine into a first limp mode, which retards timing by 10 percent and results in partial power loss. A second knock causes the ECU to put the engine into full limp mode (idle), and the engine remains in that state until the ECU is rebooted.