What happened
During a takeoff from a confined landing site, a Robinson R44 helicopter experienced a significant loss of rotor RPM, leading to a collision with the ground. The flight involved a full passenger load and occurred during the warmer part of the day, following two successful, lighter departures from the same site earlier that morning.
As the pilot attempted to climb, the aircraft's rotor RPM began to decay, triggering a low RPM warning. The pilot attempted an emergency landing but was unable to arrest the descent. The impact resulted in 4 injuries.
The investigation
The investigation focused on the aircraft's weight, balance, and performance capabilities. It was determined that the pilot had relied on estimated passenger weights based on physical descriptions rather than actual weights. This led to a calculation error where the takeoff weight was likely approximately 30 kg above the maximum gross weight and exceeded the aircraft's out of ground effect (OGE) hover performance limits.
Furthermore, the pilot did not follow the manufacturer's recommended maximum performance takeoff procedure, which involves a vertical climb to clear obstacles. Instead, a normal takeoff profile was used, which significantly reduced the time available for in ground effect (IGE) flight before encountering a tree line. The investigation also noted that the pilot did not re-evaluate performance based on the higher temperatures present during this specific flight.
Findings
- The aircraft was likely operating above its maximum gross weight due to inaccurate weight estimations.
- High density altitude and the heavy takeoff weight created a situation where the power required exceeded the power available, leading to blade overpitching.
- The pilot did not follow the Robinson Helicopter Company guidance for clearing obstacles during takeoff.
- The pilot's recovery technique for the low rotor RPM was inconsistent with the pilot operating handbook.
- The passengers were incorrectly classified as aerial work passengers rather than air transport passengers, which reduced the level of safety oversight.