What happened
On July 2, 1996, a private Robinson R44 helicopter, registration F-GPHS, was conducting a local VFR flight near Annemasse, France. The aircraft, operated by a French citizen with appropriate licensing and recent experience on the type, took off from Bourg-Ceyzeriat for a short trip to Annemasse. After a lunch break, the pilot conducted a thorough preflight inspection, noting no abnormalities.
Approximately five minutes into the return leg at an altitude of 2,300 feet and airspeed exceeding 80 knots, severe vibrations began affecting the cyclic control stick and fuselage. The pilot reduced speed to 65 knots to initiate an autorotation. Although the engine remained running and rotor RPM stayed within limits, the vibrations intensified before subsiding during the descent. The pilot flared near the ground and applied collective pitch, but the helicopter rolled onto its right side upon impact with a sloped field in Chancy, Geneva. All three occupants escaped without injury from the substantially damaged aircraft.
The investigation
Investigators from the Aircraft Accident Investigation Bureau, alongside representatives from the FAA and Robinson Helicopter Company (RHC), disassembled the main gearbox. They discovered that the 18 bolts clamping the ring gear to the gear carrier had loosened significantly during service, with torque dropping to levels comparable to normal self-locking nut drag rather than proper clamping force.
Surface finish measurements of the ring gear clamping area revealed a roughness far exceeding the required 63 micro-inch specification, showing a "phonograph groove" appearance. This insufficient surface finish prevented adequate clamping, leading to severe fretting around the bolt holes. A fatigue crack propagated from a fretted hole across the flange, eventually separating the gear mounting flange from the center section of the carrier.
Findings
The primary cause was a failure in the main gearbox drive system due to insufficient surface finish on the ring gear clamping surface. This manufacturing defect allowed the clamping bolts to loosen under normal operational loads. The resulting fretting initiated a fatigue crack that ultimately fractured the gear carrier, causing a loss of positive drive to the main rotor.
While the pilot executed the emergency procedure correctly, the intermittent drive caused by the partially meshed ring gear likely prevented power transmission to the main rotor blades during the flare, contributing to the rollover. The weather conditions were determined to have no bearing on the accident. The aircraft's maintenance records were up to date, and the last 100-hour inspection had been performed just weeks prior.
Safety action
In response to these findings, Robinson Helicopter Company issued Service Bulletin No. 15 on August 2, 1996, mandating inspections and modifications for R44 main rotor gearboxes. The FAA released Airworthiness Directive 96-18-22 on August 29, 1996, requiring immediate action. The Swiss Federal Office for Civil Aviation (FOCA) adopted this directive as mandatory by September 18, 1996. Subsequent inspections revealed that gear sets on several other R44 helicopters required removal or rework due to similar fretting issues.