Amateur-built MOSQUITO XE 285 N926KB accident near Odessa, Florida

Probable cause

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.

NTSB analysis

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.

Text from the NTSB. Open the NTSB docket for ERA17LA335 for the full record.