Odessa, FL, USA
N926KB
BRAD J BATES MOSQUITO XE 285
According to the pilot of the experimental amateur-built helicopter, after departure, the flight was uneventful with no indications of any mechanical problems. After flying around the local area between 800 and 1,000 ft above mean sea level for about 45 minutes, the pilot started to return to the departure site and began a slow descent, after which, he noticed a substantial loss of engine and rotor rpm. He then tried to increase power with no response, so he entered an emergency autorotation and started looking for a landing spot. He aimed to land in an open area, but the helicopter was unable to reach it and impacted a house’s roof. Download and examination of data from the helicopter’s electronic engine monitor indicated that the engine had experienced an overheat event. The overheating was confirmed during a postaccident examination of the helicopter’s engine when it was discovered that the coolant return line, which carried heat away from the engine’s cylinders and was rated for a maximum temperature of 200° F, had melted off its brass connector. Additionally, both the Nos.1 and 2 pistons exhibited scoring on their sides. The No.1 piston’s sides were also fractured, and pieces were missing. Additionally, the No.1 spark plug electrode exhibited evidence of metallic splatter. This evidence indicates that the engine likely experienced cold scoring, which can occur if power is applied before the engine has been brought up to normal operating temperature. When this occurs, it results in the piston heating up and expanding faster than the cylinder, losing clearance between the piston and cylinder, and the piston contacting the sides of the cylinder bore, and seizing. It is likely the No. 1 piston experienced a cold seizure, which resulted in its failure and a subsequent total loss of engine power.
On September 25, 2017, about 1740 eastern daylight time, an experimental amateur-built, Mosquito XE 285 helicopter, N926KB, was substantially damaged when it was involved in an accident in Odessa, Florida. The pilot sustained minor injuries. The helicopter was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. According to the pilot, after departure, the flight was uneventful with no indications of any mechanical problems. After flying around the local area between 800 and 1,000 ft above mean sea level for about 45 minutes, the pilot started to return to the departure site and began a slow descent, after which the pilot noticed a substantial loss of engine and rotor rpm. He then tried to increase power with no response, so he immediately entered an autorotation and started looking for a landing spot. He aimed for an open area, but the helicopter was unable to reach it, and impacted a house’s roof. The helicopter’s special airworthiness certificate was issued on April 20, 2017, and at the time of the accident, the helicopter and engine had accrued about 18 total hours of operation. The helicopter was equipped with a liquid-cooled, fuel- and oil-injected, two-cylinder, two-cycle engine, which had aluminum pistons and steel-sleeved cylinders. It was also equipped with an MGL Avionics Stratomaster Velocity engine monitor. Examination of the accident site and the wreckage revealed that the helicopter had come to rest on its left side on the house’s roof. The cockpit was on the forward side of the roof’s peak, and the tailboom, which was still attached but bent, was on the back side of the roof’s peak. The left landing gear skid had penetrated the roof, and two holes were found in the roof that corresponded to the location of the main rotor blade tips. The trailing edge of one of the main rotor blades had a slight kink. Both main rotor blades were attached to the rotor head, and the tail rotor was attached to the tailboom. Control continuity was established to the main and tail rotors. Examination of the engine revealed that it was clean and free of debris and that oil was present in the oil tank. The coolant return line. which carried heat away from the engine’s cylinders and was 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 observed on sides of both the Nos. 1 and 2 pistons. Additionally, the sides of the No.1 piston were fractured, and pieces were missing. The tops of both pistons were intact, and oil was present. When the engine monitor was powered up, it showed the last recorded maximum temperatures as follows: the No.1 cylinder exhaust gas temperature was 1,122°F, the No. 2 cylinder exhaust gas temperature was 1,144°F, the cylinder head temperature was 600°F, and the radiator temperature was and 600°F.
A total loss of engine power due to cold seizure of the No.1 piston, which resulted in its failure and a subsequent emergency autorotation and impact with a house’s roof.
Source: NTSB Aviation Accident Database
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