What happened
During preparations for a training flight, a Robinson R44 Clipper I experienced an engine failure while the crew was engaging the rotor. The incident occurred while the helicopter was stationary on the apron in cold weather conditions, facing a wind of 23 knots gusting to 26 knots from the left 30 degrees.
Following the initial shutdown, the crew attempted a second engine start. However, the engine failed again upon engaging the rotor. While the crew waited for the clutch to disengage to facilitate the use of the rotor brake, the rotor blades unexpectedly struck the tail boom. In an attempt to manage the situation, the instructor pilot moved the collective control to the full down position and moved the cyclic control from an aft position to neutral. During this movement, the second rotor blade made light contact with the upper portion of the tail boom. No further contact occurred, and the aircraft was shut down safely. No injuries were reported, and no emergency services were required.
The investigation
SHK examined the aerodynamic forces acting on the rotor system during the period of low rotor RPM. The investigation focused on how the wind conditions interacted with the two-blade rotor system while the engine was failing to maintain steady rotation.
Findings
- The investigation identified that dynamic forces from the wind acting on the forward-rotating blade at low RPM can cause excessive blade flapping, a phenomenon known as blade sailing.
- A similar aerodynamic effect applies to the retreating blade in this specific rotor configuration.
- The primary cause of the strike was likely that the cyclic control position was not sufficiently supervised following the aborted startup procedure, especially when combined with the relatively strong wind conditions.