What happened
On November 28, 2016, at approximately 08:50, a crew prepared an EC135 P2+ helicopter for a medical flight mission to Załachowo, near Żnin. During the initial start sequence using a ground power unit, the pilot successfully started the left engine, noting a brief, unidentified noise during the process. However, when attempting to start the right engine, the pilot observed no increase in compressor speed, combustion chamber temperature, or turbine RPM, accompanied by another brief unidentified noise. The pilot subsequently shut down the left engine and attempted a second start of the right engine, which also failed to show characteristic engine spool-up sounds.
Following the failed attempts, the pilot notified the LPR Operations Center, the Żnin Emergency Medical Dispatcher, and the Technical Coordination Center engineer. To verify the symptoms, the engineer ordered a cold start attempt of both engines. During this second attempt, the pilot reported that both engines showed 0% compressor turbine speed and no audible engine spooling, though the starter motors themselves were heard running.
The investigation
The investigation, conducted by the operator's internal commission, focused on the mechanical failure of the engine starting system. Technical analysis revealed that the drive shafts of both starter motors had sheared. Following the Aircraft Maintenance Manual (AMM) protocols, maintenance personnel replaced both starter motors with new units. The investigation also examined the ground power unit used for the start, noting it had recently undergone overhaul; testing under load showed no irregularities with the power source.
Findings
- The investigation identified that the compressor rotors had frozen to their housings prior to the start attempt.
- This freezing was likely caused by the aircraft's operation in rain and near-freezing temperatures the previous day, followed by storage in an unheated hangar where temperatures dropped below freezing overnight.
- The frozen components created excessive torque during the start attempt, which directly led to the shearing of the starter motor drive shafts.
- Ambient weather conditions at the time of the incident were CAVOK with a temperature of -0.3°C.