EUROCOPTER AS 350 near San Juan, PR — 2017-02-21

Probable cause

The flight instructor's incorrect manipulations of the twist grip throttle control during a practice autorotation, which resulted in an engine overspeed and subsequent fire.

NTSB narrative

The pilot/owner of the turbine helicopter was practicing autorotations with a flight instructor. After completing several autorotations uneventfully, the instructor asked if he could perform one, and the pilot agreed. Near the flare at the end of the maneuver, the pilot heard the engine overrev, followed by an Nr (rotor speed) aural warning, followed by a fire warning light illumination on the instrument panel. After landing, the pilot exited the helicopter with a fire extinguisher and attempted to extinguish an engine fire. Review of data downloaded from a vehicle-engine multifunction display and digital engine control unit revealed that the first failure recorded during the flight indicated that the gas generator rotation speed (N1) reached an out-of-limit value. At that time, the 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 was recorded 2 seconds later, which indicated that the free turbine rotation speed (N2) reached an out-of-limit value. The failure was triggered by the maximum recorded value of 545 rpm, which equated to a turbine speed (Nr) of 140%. The engine's freewheeling turbine was designed to separate turbine blades at 150% Nr in order to prevent the turbine disc separating at 170% Nr. It is likely that the flight instructor excessively opened the fuel metering unit via the twist grip throttle manual control, which resulted in an engine overspeed, turbine blade separation, and subsequent engine fire.

Analysis

Primary failure mode

Mechanical failure

First missed decision gate

Throttle control should have remained in the 'flight' detent to prevent overrev.

NTSB coding

• Fire/smoke (non-impact) · Autorotation

Evidence available

• Video
• Photos
• 10 docket documents

Docket documents10

• Evidence Control Formform
• Pilot/Operator Aircraft Accident Report, NTSB Form 6120.1form
• Interview Summaryinterview
• Photo Arrayphotos
• BEA Report of HUM and Assembly Valve Testsreport
• BEA Report of VEMD and DECU Data Downloadreport
• Engine Manufacturer Report of DECU Data Downloadreport
• Engine Manufacturer Report of HMU and Assembly Valve Testsreport
• Helicopter Manufacturer Report of VEMD Data Downloadreport
• Airport Security Video Excerpt (45 Seconds in MP4 Format, 14MB)video