Overview
On 28 November 2016, at approximately 08:50 local time, the crew of a Polish Medical Air Rescue (LPR) helicopter, an EC 135 P2+, received a mission to fly to a road accident in Załachowo near Żnin. During the first engine start of the day using a ground power unit, the pilot initiated a normal start of the left engine. While the left engine started successfully, the pilot heard an unidentified sound lasting about one second. When attempting to start the right engine, the pilot observed no compressor rotation, no temperature rise in the combustion chamber, and no rotation of the power turbine. Another unidentified sound was heard during that attempt. The pilot aborted the right engine start, then shut down the left engine after cooling. A subsequent re-attempt to start the right engine produced no characteristic sounds of engine spooling up.
Details
Following the unsuccessful start attempts, the pilot informed the LPR Operational Center, the emergency dispatcher in Żnin, and the on-duty engineer at the Technical Coordination Center (CKT). The CKT engineer instructed the pilot to perform a cold start of both engines to verify the symptoms. During the cold start of each engine, the pilot noted:
- No indication of compressor turbine rotation (0% on the indicator)
- No audible spooling sounds from the engines
- Audible sound of the starter-generators operating
After these tests, the helicopter was taken out of service.
Investigation Findings
Subsequent analysis revealed that the drive shafts on both starter-generators had sheared. According to the Aircraft Maintenance Manual (AMM 24-30-00, 4-2), maintenance personnel replaced both starter-generators with new units (serial numbers P2668 and P3122). After adjusting the generator voltage and verifying their operation during a test run, the helicopter was returned to service.
The investigation also considered that the helicopter had been operated the previous day in rain and near-freezing temperatures, and was stored overnight in an unheated hangar where the temperature dropped below freezing. The weather on the day of the incident at EPBY airport was CAVOK with a temperature of -0.3°C. The report noted that weather may have influenced the event.
Probable Cause
The probable cause of the shearing of the starter-generator drive shafts was the freezing of the compressor rotors to their housings. This condition may have led to a high torque moment, resulting in the destruction of the drive shafts.