What happened
On May 14, 2017, an EC 135 P2+ helicopter, operating a Helicopter Emergency Medical Service (HEMS) mission between Suwałki and Olecko, experienced technical anomalies during its return flight. While flying from the hospital back to the Suwałki base, the crew observed a 20% discrepancy between the torque readings of the two engines, with the left engine at 5/5% and the right engine at 35%.
Following this observation, the pilot decided to continue the flight to the base but opted to land on a grassy area of the Suwałki airport instead of the designated hangar platform for safety reasons. During the subsequent takeoff from the grass area to return to the platform, the aircraft experienced torque (TQ) limit exceedances. Analysis of the flight data revealed that the left engine reached a maximum TQ of 82.5% for 0.375 seconds, while the right engine reached 87.7% for 1.125 seconds.
The investigation
The investigation examined the flight data and the maintenance requirements triggered by the exceedances. While the left engine excursion did not necessitate immediate maintenance, the excursion on the right engine required inspections according to the Aircraft Maintenance Manual (AMM). Consequently, maintenance tasks ranging from category A to D were performed, including inspections of the main gearbox oil filter, chip detectors for the main gearbox and fenestron, and the main rotor hub. All inspections were completed within the required timeframe, and the aircraft was returned to service.
Findings
- The primary cause of the event was the unintentional movement of the ENG TRIM switch located on the collective lever, which altered the N1 compressor speed.
- The initial torque imbalance was identified during the flight from the hospital to the base.
- The decision to land on a grass area was made by the pilot to ensure safety following the initial engine discrepancy.