What happened
On February 7, 2014, a Robinson R22 helicopter, registration F-OLJB, was performing a private flight near Marla, France. After picking up a passenger at a small helipad, the pilot attempted an oblique takeoff facing a northeast heading. This specific takeoff profile required the aircraft to climb over a 1.4-meter-high hedge.
As the aircraft passed over the hedge at a height of approximately 1.6 meters, the pilot experienced a significant drop in main rotor RPM, triggering the "LOW RPM" audible alarm. In an attempt to avoid the obstacle, the pilot executed an emergency turn to the right toward a nearby football field located at a lower elevation. During this maneuver, the helicopter struck the ground heavily, bounced, and eventually came to a halt several meters away. The aircraft sustained heavy damage.
The investigation
The investigation focused on the aerodynamic performance of the aircraft under the specific environmental conditions at the time. The temperature was 27°C (ISA + 22), and the aircraft was operating at a density altitude of approximately 8,200 feet. The investigation also reviewed a previous takeoff performed minutes earlier, where the pilot had experienced a similar loss of altitude but had successfully recovered by adjusting the collective pitch.
Investigators examined the aircraft's engine, a Lycoming O-360 J2A, and found no technical failures. The analysis centered on the transition from ground effect to out-of-ground-effect (OGE) flight. It was noted that the aircraft's weight for the second takeoff was approximately 10 to 20 kg higher than the first, further reducing the power margin.
Findings
- The primary cause of the accident was that the aircraft's performance capabilities were underestimated during an OGE takeoff in high-altitude, high-temperature conditions.
- The pilot's transition from ground effect to out-of-ground-effect flight caused an irrecoverable sink rate because the available power was insufficient to maintain altitude.
- The pilot's experience with a similar sink rate during a previous takeoff led to an erroneous interpretation of the aircraft's performance limits.
- The presence of terrain on the chosen takeoff heading prevented the pilot from gaining enough airspeed to exit the transition zone safely.