What happened
On the morning of October 7, 2021, a privately owned Robinson R22 Beta, registration JA7975, departed from the Akada Heliport Operation Site in Kanagawa Prefecture. The pilot was conducting a familiarization flight toward Kisarazu City in Chiba Prefecture. During the flight, the helicopter entered cloud cover, transitioning from visual meteorological conditions to instrument meteorological conditions.
As the aircraft climbed, the pilot requested radar vectors from Atsugi Tower to return to the Akada Site. However, during this period, the rotor speed decreased significantly, triggering the "LOW RPM HORN." In response to the warning, the pilot likely made sudden, abrupt control inputs. This resulted in an unstable, low-G flight state that led to catastrophic mast bumping. The aircraft's structural integrity failed mid-air, with the main rotor blades breaking and the tail cone separating, before the helicopter crashed into a field in Hadano City. The pilot sustained one fatal injury, and while the aircraft was destroyed, no fire occurred.
The investigation
The Japan Transport Safety Board (JTSB) conducted an investigation involving wreckage analysis and a detailed review of digital records. Investigators utilized metadata and 360-degree video from a GoPro camera mounted in the cockpit to reconstruct the flight dynamics. This allowed the team to analyze the pilot's line of sight, the timing of the low RPM warning, and the specific mechanical movements of the rotor system. The investigation also examined the engine's performance and the use of carburetor heat during the climb.
Findings
- The primary cause of the accident was the occurrence of catastrophic mast bumping, triggered by abrupt control movements during a low-G flight condition.
- The drop in rotor speed was likely caused by a loss of engine power, potentially due to the pilot failing to use carburetor heat while climbing through clouds.
- The pilot entered instrument meteorological conditions (IMC) because the flight was initiated without a sufficient plan for maintaining visual meteorological conditions (VMC) at the intended cruise altitude.
- The pilot's ability to manage the aircraft was compromised by a lack of proper altitude corrections and an inability to maintain stable flight while attempting to communicate with air traffic control.