Accident Overview
On September 25, 2017, at approximately 1740 eastern daylight time, an experimental amateur-built Mosquito XE 285 helicopter, registration N926KB, was substantially damaged during an accident in Odessa, Florida. The helicopter was operated as a personal flight under Title 14 Code of Federal Regulations Part 91. The pilot, the only occupant, sustained minor injuries.
Flight and Loss of Power
According to the pilot, the flight was uneventful after departure with no indications of mechanical problems. He flew locally at altitudes between 800 and 1,000 feet above mean sea level for about 45 minutes. While returning to the departure site and beginning a slow descent, the pilot noticed a substantial loss of engine and rotor rpm. Attempting to increase power yielded no response. He immediately initiated an autorotation and searched for a landing spot. Although he aimed for an open area, the helicopter could not reach it and impacted a house roof.
Aircraft and Engine Details
The helicopter's special airworthiness certificate was issued on April 20, 2017. At the time of the accident, the helicopter and engine had accumulated approximately 18 total hours of operation. The engine was a liquid-cooled, fuel- and oil-injected, two-cylinder, two-cycle type with aluminum pistons and steel-sleeved cylinders. The aircraft was equipped with an MGL Avionics Stratomaster Velocity engine monitor.
Wreckage Examination
Post-accident examination revealed the helicopter came to rest on its left side on the roof. The cockpit was forward of the roof's peak, and the tailboom, still attached but bent, lay on the back side of the peak. The left landing gear skid had penetrated the roof. Two holes in the roof corresponded to main rotor blade tip locations. One main rotor blade had a slight kink on its trailing edge. Both main rotor blades were attached to the rotor head, and the tail rotor was attached to the tailboom. Control continuity was established to both main and tail rotors.
Engine Examination
The engine was clean and free of debris, with oil present in the oil tank. The coolant return line, rated for a maximum temperature of 200°F, was found melted off its brass connector. Metallic spatter was observed on the No. 1 cylinder's spark plug electrode. Scoring was present on the sides of both Nos. 1 and 2 pistons. Additionally, the sides of the No. 1 piston were fractured with missing pieces. The tops of both pistons were intact, and oil was present.
Engine Monitor Data
When the engine monitor was powered up, it displayed the last recorded maximum temperatures: No. 1 cylinder exhaust gas temperature 1,122°F, No. 2 cylinder exhaust gas temperature 1,144°F, cylinder head temperature 600°F, and radiator temperature 600°F.