What happened
On April 6, 2016, a Robinson R44 Raven I, registration OK-SCI, was undergoing engine checks at Roudnice Airport (LKRO) in the Czech Republic. The pilot was preparing for a series of solo training flights. During the functional test of the low rotor RPM warning system, the helicopter began an uncontrolled rotation to the left.
As the pilot attempted to manage the sudden movement, the aircraft drifted approximately 21 meters from its original position. During this rotation, the tail boom struck a wooden fence, and the tail rotor and main rotor blades subsequently struck branches of a tree located just beyond the perimeter. The impact caused extensive damage to the aircraft, including the detachment of the tail rotor gearbox and deformation of the tail boom. The pilot was uninjured.
The investigation
The ÚZPLN investigation focused on the sequence of events during the engine run-up and the pilot's control inputs. Investigators examined the aircraft's mechanical condition, the pilot's flight experience, and the meteorological conditions at the time. Technical inspections confirmed that the engine, transmission, and flight controls were fully functional and that no mechanical failure had occurred. The investigation also reviewed the pilot's adherence to the Robinson R44 flight manual procedures regarding pedal positioning and engine start-up sequences.
Findings
- The pilot failed to perform essential pre-start procedures in full accordance with the flight manual, specifically failing to ensure the pedals were in a neutral position.
- The sudden leftward rotation was likely caused by a change in torque during the RPM reduction, exacerbated by the pilot's use of the collective lever and the fact that the left pedal was held depressed.
- The pilot's reaction to the sudden rotation was inadequate; the pilot did not immediately apply the necessary right pedal correction or lower the collective to the bottom stop to neutralize the reactive torque.
- The pilot's previous habits from operating the Robinson R22, where a depressed left pedal was common during start-up, contributed to the error.
- The aircraft was within its allowable weight and CG limits, and the weather conditions were not a contributing factor to the accident.