Eurocopter Deutschland Gmbh EC135 P2+ N531LN accident near Drexel Hill, Pennsylvania

Probable cause

An inflight attitude upset for undetermined reasons that resulted in a rotor system overspeed, a reduction of power from both engines, and a subsequent hard landing.

NTSB analysis

Flight track data from the helicopter air ambulance flight indicated that, while in cruise flight at an altitude of about 1,500 ft mean sea level (msl), the helicopter departed normal cruise flight with an abrupt increase in altitude, followed by a dive. The recovered data from various sources onboard the helicopter did not contain information as to whether the helicopter rolled inverted during this altitude excursion, as recalled by the crewmembers. Surveillance video showed the helicopter in a near-vertical, nose-down, spiraling descent. The pilot arrested the rotation and recovered the helicopter from the dive but was unable to climb or hover due to insufficient engine power, thereby resulting in a hard landing to a city street and substantial damage to the helicopter. Examination of the helicopter revealed no evidence of malfunction that would result in an abrupt departure from cruise flight.

Because of the limited control authority of the Stability Augmentation System (SAS) actuators, it is unlikely that a malfunction of a SAS actuator would have resulted in an inflight upset before the pilot could react to the malfunction. Additionally, a malfunction of a trim actuator would not result in an inflight upset as the pilot would notice an attitude deviation before the trim actuator, whose rate of movement is limited by design, would be able to move the helicopter into an unusual attitude.

Data indicated that a main rotor system overspeed, which likely occurred during the dive maneuver, resulted in the overspeed of both engine power turbines due to the sudden reduction in load from the main rotor. As a result of the power turbine overspeed, both engine control systems, independent of each other, functioned as designed and reverted to manual mode while at a minimum fuel flow rate. Both engines continued to run at low power without automatic governing, resulting in insufficient power to continue normal flight as the engine twist grips remained in the normal fly position for the duration of the flight.

Text from the NTSB. Open the NTSB docket for ERA22FA105 for the full record.