N911CY

On April 24, 2018, about 1403 eastern daylight time, an experimental, amateur-built Mosquito XE helicopter, N911CY, was destroyed when it was involved in an accident near Albion, New York. The private pilot was seriously injured. The helicopter was operated as a Title 14 Code of Federal Regulations Part 91 personal flight.

The pilot stated that he had installed an autotuner unit, which monitors and adjusts the engine’s air fuel ratio (AFR) and he had been performing flight testing to determine the proper AFR setting. The day before the accident, he flew with the AFR set to 13.0, with a resulting exhaust gas temperature (EGT) reading of 1,050° F, which was at the low end of the normal operating range (1,050° F to 1,150° F). He adjusted the AFR to 13.5 and hovered for about 10 minutes, during which the EGT indicated 1,100° F.

On the day of the accident, he elected to fly around the airport traffic pattern. During takeoff, about 75 ft above ground level (agl), the engine started to backfire with resultant power surges. He decided to land straight ahead while keeping lateral control with anti-torque pedal input, and 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 ft 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 postimpact fire ensued due to impact damage to the fuel tank. The pilot egressed and was treated for his injuries.

A witness, who was driving on a road near the airport, reported seeing the helicopter hovering about 6 ft agl adjacent to hangars. He then watched the helicopter ascend "straight up" to about twice the height of nearby powerlines, and proceed in a westerly direction. He observed a "small piece" separate from the helicopter and then heard a popping sound followed by a grinding sound. The helicopter then entered a steep descent, during which a "second piece" separated from the helicopter. The helicopter impacted the ground and immediately burst into flames.

Examination of the accident site by a Federal Aviation Administration (FAA) inspector revealed that one air induction filter was located about 20 ft east of the wreckage. The airframe sustained extensive damage due to the post-impact fire, which consumed most of the airframe and a portion of the tail boom. The tail rotor gear box 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 accounted for but were extensively fire-damaged. The autotuner was destroyed by fire.

Examination of the two-stroke, two-cylinder, 85 horsepower Innovator Technologies Inntec 800 engine was performed 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 the postcrash use of a fire extinguisher, underneath which the plugs displayed a light, smoky gray color consistent with a lean fuel-to-air ratio.

The designer of the helicopter stated, and the pilot agreed that, during manipulation of the autotuner to obtain the proper EGT readings, the pilot did not review or accept the auto-tune-suggested new fuel map and thought he had completed the autotuner set-up, but he had not. As a result, the autotuner was operating on the accident date with the initial, lean AFR setting. The designer also stated that the helicopter was equipped with a knock sensor, which when detected, causes the engine computer (ECU) to put the engine into a first "limp mode," which retards the timing 10 percent and results in a partial loss of power. The designer of the helicopter also reported that a second knock would cause the ECU to put the engine into full "limp mode," or idle, and the engine will remain in that state until the ECU is rebooted.