What happened
On July 25, 2014, an Airbus Helicopters EC120, registration F-GMHZ, was performing a training flight at Avignon Caumont airfield. The flight involved a student pilot and an instructor performing an autorotation exercise. During the maneuver, the instructor moved the power lever from the flight idle position to the flight position to attempt a power recovery.
As the aircraft approached the runway, the instructor noticed the student's turn was late, making the intended landing area unsafe due to nearby obstacles. The instructor decided to attempt a power recovery to avoid the obstacles. However, during this rapid increase in power, the engine suffered a flameout. The instructor took control and attempted a forced landing, but the helicopter struck the ground heavily, causing the main rotor to sever the tail boom. The aircraft slid along the runway and came to a halt.
The investigation
The investigation focused on the engine's failure during the power increase and the maintenance history of the aircraft. Investigators examined the engine on a test bench, where they successfully reproduced the engine flameout by performing a rapid movement of the power lever from flight idle to flight.
Analysis of the engine's Engine Power Check (CSM) data recorded in the VEMD (Vehicle and Engine Multifunction Display) revealed that recent tests showed a "T4 Margin" outside of acceptable tolerances. Furthermore, the investigation found that the maintenance workshop had misidentified these out-of-tolerance results as acceptable, likely due to a misunderstanding of the VEMD software standard being used.
Findings
- The primary cause of the accident was the instructor's decision to delay the power recovery despite recognizing that the aircraft's trajectory no longer allowed for a safe landing.
- The engine flameout was caused by the clogging of the priority injector with a carbon deposit (coke), which prevented the engine's anti-extinction function from maintaining sufficient fuel flow during the rapid power increase.
- The instructor did not immediately recognize the engine failure because the initial torque surge and yaw jerk provided a false indication of successful power recovery.
- Maintenance personnel failed to trigger necessary troubleshooting after reviewing engine health check results that showed abnormal temperature margins.
- The premature formation of carbon deposits in the injector was an unexplained phenomenon.