What happened
On January 11, 2022, an Eurocopter EC135 P2+ helicopter, registration N531LN, was operating as an air ambulance flight for Air Methods Corporation. The flight departed Chambersburg Hospital Heliport (PA60) and was en route to Children’s Hospital of Philadelphia Heliport (9PN2).
While cruising at approximately 1,500 feet mean sea level, the aircraft experienced an abrupt altitude excursion followed by a dive. Witnesses and video evidence described the helicopter in a near-vertical, nose-down, spiraling descent. During the flight, the crew experienced a period where the aircraft rolled inverted, causing the medical crewmembers to be pinned to the ceiling of the cabin.
The pilot managed to arrest the rotation and recover the aircraft from the dive; however, the aircraft was unable to climb or hover. The helicopter ultimately performed a hard landing on a city street, which caused the tailboom to separate and resulted in substantial damage to the aircraft. The pilot sustained one serious injury, while the two medical crewmembers and the patient were not injured.
The investigation
Investigators examined the wreckage and flight data from several onboard sources, including the engine data collection units (DCU), the cockpit warning unit, and the vehicle and engine multifunction display (VME).
Analysis of the engine data revealed that during the flight, there were overspeeds in the engine power turbines (Nf) accompanied by a reduction in engine torque to 0%. These overspeeds caused both engine control systems to function as designed by reverting to manual mode at a minimum fuel flow rate. Because the engine twist grips remained in the normal fly position, the engines continued to run at low power without automatic governing, leaving the pilot with insufficient power to maintain altitude.
Mechanical examination of the main rotor, tail rotor, drive systems, and engines showed no evidence of preimpact failures or malfunctions that would have caused the initial departure from controlled flight. While black-colored debris was found in the No. 1 hydraulic filter, testing showed no functional anomalies in the hydraulic or flight control systems that would have precluded normal operation.
Findings
- A main rotor system overspeed occurred, likely during the dive maneuver.
- The rotor overspeed caused a sudden reduction in load, leading to the overspeed of both engine power turbines.
- Both engine control systems independently reverted to manual mode at minimum fuel flow.
- The engines continued to operate at low power without automatic governing, providing insufficient power for continued flight.
- The pilot was unable to increase power because the engine twist grips remained in the normal fly position.