What happened
On June 20, 2017, an EC 135 P2+ helicopter was performing a medical transport mission (inter-hospital flight) along a route connecting Zielona Góra, Zgorzelec, and Wrocław. During the return leg from Wrocław to the base in Zielona Góra, near the town of Głogów, the crew observed a brief "Rotor RPM" warning message on the warning panel accompanied by an audible alert indicating low main rotor speed.
Upon receiving the alert, the pilot silenced the warning and conducted a thorough check of the engine parameters. The crew found that the power plant parameters were normal, with no further warnings appearing on the Crew Alerting Device (CAD), the main rotor tachometer, or the engine tachometers. Based on these findings, the pilot decided to continue the flight to the base.
As the aircraft approached for landing, the red "Rotor RPM" warning light flickered on and off several times. The landing was completed without further incident, and the pilot subsequently reported the event to the Technical Coordination Center.
The investigation
The investigation focused on identifying the source of the intermittent warning signals. Following the event, the aircraft was removed from service for technical inspection. Maintenance personnel from the Szczecin Technical Service Station diagnosed a malfunction in the main rotor speed sensor. The faulty sensor was replaced with a new unit in accordance with the Aircraft Maintenance Manual (AMM). Following the replacement, successful functional tests were performed, and a ground test and subsequent test flight confirmed the aircraft was fully operational. The helicopter was then returned to service.
Findings
- The primary cause of the incident was the operational wear of the main rotor speed sensor.