What happened
On December 20, 1999, at 16:20 local time, a Bell 47 G helicopter, registration F-GGSU, was conducting an instructional flight near Colmar, France. While flying over a flooded area, the instructor observed sheep in distress and decided to land to provide assistance. Following an initial approach and landing, the instructor attempted a translation approximately 1.2 meters above the ground toward a shepherd. During this maneuver, the aircraft lost directional control, experiencing two rapid rotations around its vertical axis. The pilot subsequently reduced power, resulting in a forced landing.
The investigation
The investigation focused on the structural failure of the tail rotor assembly. Investigators examined the broken tail rotor blade and the tail rotor drive shaft. Analysis of the broken blade revealed a fracture approximately 10 centimeters from the hub at the blade attachment zone. Microscopic examination of the fracture surface showed paint scaling on the leading edge, indicating tensile stress, and openings on the trailing edge, indicating compressive stress. The fracture surface was clean and matte, characteristic of a ductile failure of thin sheet metal, with no evidence of pre-existing damage. The blade failed abruptly due to static bending loads within the drag plane.
Additionally, the drive shaft exhibited a static-type fracture. Because there were no signs of a high-impact collision, investigators determined that the tail rotor and crossbar likely struck the water surface or the saturated ground during the low-altitude translation.
Findings
- The primary cause of the structural failure was the tail rotor striking the water or saturated ground during a low-altitude maneuver.
- This impact generated extreme loads on the tail rotor assembly, leading to the breakage of the drive shaft and a tail rotor blade.
- The pilot's perception of the environment may have been compromised due to the unusual appearance of the flooded terrain.