What happened
On March 15, 2008, at 15:25, a Robinson R4-4 Raven, registration G-WMWM, was attempting to depart from the Courchevel Altiport for a flight to Grenoble. The aircraft was carrying a pilot and two passengers.
Prior to departure, the pilot estimated the aircraft's weight at 1,028 kg, which was 61 kg below the maximum takeoff weight of 1,089 kg. Based on the day's conditions, the pilot determined that available power was only sufficient for hovering in ground effect. Due to the surrounding mountainous terrain, a headwind takeoff was not possible. Consequently, the pilot decided to take off using runway 04, which would involve a takeoff with a tailwind to utilize the 123-meter horizontal section of the runway to gain speed.
During the acceleration phase, sudden turbulence caused the helicopter to descend, triggering the low rotor RPM warning. One skid momentarily brushed the runway. As the aircraft reached a 19% slope change, the combination of decreased rotor RPM and insufficient translational speed caused the helicopter to descend further. To avoid the tail rotor striking the ground, the pilot attempted a left turn to utilize a steeper slope to regain acceleration. During this maneuver, the left skid struck the snow-covered ground adjacent to the runway, causing the helicopter to slide several dozen meters before coming to rest on its left side. The aircraft was destroyed.
The investigation
The investigation focused on the performance calculations and the impact of wind conditions on the takeoff. Investigators analyzed the power requirements for the Robinson R44 at the pressure altitude of 6,527 feet. Under the day's temperature conditions, the maximum takeoff weight for hovering in ground effect was approximately 1,070 kg, while the limit for hovering out of ground effect was significantly lower, at approximately 940 kg.
Technical analysis of the power curves revealed that the available power was momentarily insufficient to maintain the necessary acceleration. The investigation established that the pilot's decision to take off with a tailwind increased the distance required to transition through the transition zone, a calculation that is not covered by standard performance charts.