What happened
On Tuesday, December 30, 2003, a Robinson R44 helicopter, registration F-OIAI, was performing a private flight from Koumac to Nouméa in New Caledonia. Approximately 15 minutes after takeoff, while cruising at an altitude of roughly 700 feet and a speed of 110 knots, the pilot heard an intensifying whistling sound accompanied by significant vibrations through the cyclic control and the airframe.
In response to the deteriorating flight conditions, the pilot initiated an emergency descent and landed in a field at the "Ouaco" site near Kaala-ground. During the landing maneuver, the tail rotor blades struck a shrub. Upon inspection on the ground, the pilot identified damage to both the tail rotor blades and a deformation on the upper surface of one main rotor blade.
The investigation
The investigation focused on the condition of the rotor blades following the impact and the onset of vibrations. Investigators found significant erosion on the leading edges of both the main and tail rotor blades, which had worn the metal down to the bare surface.
On one main rotor blade, approximately one meter from the tip, the metallic cladding on the upper surface was torn perpendicular to the leading edge. The cladding had also delaminated for about 20 centimeters on either side of this tear. The examination determined that the blade had suffered excessive wear, likely caused by erosion from an abrasive atmosphere, such as sand.
As the cladding thinned due to erosion, a crack developed parallel to the chord. This crack progressed along the rear of the leading edge spar. The resulting aerodynamic depression caused the front of the cladding to peel away under the mechanical force of the relative wind (traveling at approximately 550 km/h). This peeling created a small cavity and caused the leading edge to lift, generating the noise and vibrations reported by the pilot.
Findings
- The primary cause of the vibrations was the failure to detect abnormal wear on the rotor blades.
- The erosion of the blade leading edges was underestimated.
- The main rotor blade involved had 1,492 hours of total use, well below the manufacturer's 2,200-hour limit.
- No anomalies were noted during the previous 50-hour inspection, and the blade had only flown 96 hours since its last 100-hour inspection.