What happened
On November 14, 2006, a Robinson R22 helicopter, registration F-GLPA, was performing a local flight at Bourg aerodrome. The flight involved an instructor and a student pilot. Following a series of maneuverability exercises and two autorotations on an unpaved runway, the instructor landed the aircraft and conducted a debriefing.
After the debriefing, the instructor decided to return the aircraft to the parking area. To transition into a hover, the instructor rapidly increased the collective pitch. This action caused the engine RPM to drop below 80%, triggering the low rotor RPM warning alarm, which is designed to activate at 97%. While at an altitude of approximately 1.5 meters, the instructor attempted to increase the collective pitch further to gain altitude without simultaneously increasing engine power. Additionally, the pilot pushed the cyclic control forward to gain forward airspeed. Because the rotor RPM remained below the 90% limit required for sustained flight, the helicopter lost lift and began to sink. The left skid struck the ground, causing the aircraft to tilt to the left and resulting in the destruction of the aircraft.
The investigation
The investigation focused on the flight controls and the pilot's response to the low rotor RPM alarm. Investigators reviewed the emergency procedures outlined in the Robinson R22 flight manual, specifically regarding power loss between 8 and 500 feet above the ground. The manual mandates that in the event of a low RPM alarm, the pilot must immediately increase engine power via the twist grip while lowering the collective pitch. The investigation also examined safety notices which emphasize that restoring rotor RPM through simultaneous power increase and collective reduction should be a conditioned reflex.
Findings
- The primary cause of the accident was inappropriate flight control inputs during the transition to flight.
- The instructor failed to increase engine power while increasing collective pitch, leading to a critical drop in rotor RPM.
- The decision to continue the takeoff attempt with insufficient power contributed to the loss of lift.
- The rotor speed remained below the necessary 90% threshold for stable flight.