Accident Summary
On February 21, 2017, at about 1015 Atlantic standard time, an Airbus Helicopters (Eurocopter) AS 350 B3, registration N413LP, sustained substantial damage during a practice autorotation at Fernando Luis Ribas Dominicci Airport (TJIG) in San Juan, Puerto Rico. The helicopter was operated by the commercial pilot/owner under 14 Code of Federal Regulations Part 91. The flight instructor, acting as pilot-in-command, and the commercial pilot were not injured. Visual meteorological conditions prevailed and no flight plan was filed for the flight, which originated from TJIG about 0900.
Flight and Landing
According to the flight instructor, the commercial pilot was practicing autorotations. During recovery from the last 180° autorotation, the instructor observed that the engine rpm continued to increase beyond limitations, followed by a helicopter vibration. He immediately landed on a grass area near a runway. After landing, a mechanic reported the engine was on fire. The instructor completed the engine fire procedure and exited the helicopter.
The commercial pilot stated that after several uneventful maneuvers, the instructor requested an autorotation. During the flare, the commercial pilot heard an engine overrev, followed by an Nr (rotor speed) aural warning and a fire warning light. After landing, he exited with a fire extinguisher and attempted to extinguish the fire.
Review of airport security video showed the helicopter descending to a grass area adjacent to the runway. About 30 feet above ground level, smoke began emitting from the helicopter, which then climbed out of the video frame before descending again and landing on the grass while continuing to emit smoke.
Post-Accident Examination
Examination by a Federal Aviation Administration inspector revealed that the fire damaged the engine deck support structure and a portion of the tail rotor drive shaft. Components including the vehicle and engine multifunction display (VEMD), digital engine control unit (DECU), hydromechanical unit (HMU), and assembly valve were retained for analysis at manufacturer facilities under Bureau d'Enquetes et d'Analyses (BEA) supervision in France. Testing of the HMU and assembly valve found no anomalies that would preclude normal engine operation.
Data Analysis
Data from the VEMD and DECU indicated that during the accident flight, the first recorded failure was an NG/N1 failure at 1 hour, 13 minutes, and 18 seconds into the 1-hour 14-minute flight (VEMD) and at 1:13:27 (DECU). This failure signified that the gas generator rotation speed (N1) reached an out-of-limit value. At that time, fuel regulation was in mixed mode, as the collective twist grip throttle control was out of the “flight” detent and the pilot was manually controlling the throttle. A second failure recorded 2 seconds later indicated that the free turbine rotation speed (N2) reached an out-of-limit value, with a maximum recorded value of 545 rpm, corresponding to an Nr of 140%.
According to a representative from the engine manufacturer, the engine's freewheeling turbine was designed for its turbine blades to separate at 150% turbine speed to prevent the turbine disc from separating at 170% turbine speed. Examination of the engine revealed evidence consistent with turbine blade separation, resulting in an engine fire.