Enstrom F 28A N24RB accident near Memphis, Tennessee

Probable cause

A skid tube clamp weld failure due to high residual stress, which resulted in a skid collapse during landing. Contributing to the accident was the airplane manufacturer’s lack of adequate guidance for periodically inspecting the skid tube clamp.

NTSB analysis

The flight instructor reported that he and the student pilot were practicing running landings with a simulated stuck antitorque pedal. At touchdown, the left landing skid collapsed, and the helicopter subsequently came to rest in a forward-pitch attitude.

Postaccident examination of the landing skid revealed that the skid clamp that connects the forward cross tube to the landing gear skid had failed due to fatigue cracking that had initiated in the heat-affected zone adjacent to a weld. The fatigue cracks then propagated inward through the clamp. After the clamp fractured, the bolt hole ring that connected the clamp to the fuselage frame failed in overstress, which caused the landing gear skid to fail. Hardness testing revealed that the weld material, clamp, and ring, significantly differed in hardness. Materials joined with a broad hardness gradient are likely to contain high residual stresses, which in this case, would have been concentrated at the boundary between the clamp and the weld and led to a failure of the weld when the landing load was applied. Because multiple crack initiation sites were found in areas without material defects, the fatigue crack that led to the leg failure probably developed and propagated over a long time under numerous low-stress cycles. The airplane manufacturer placed no life or event limits (hard landing) for replacing landing gear skid tube clamps; it only required periodic visual inspections of the skid tube clamps, which was not adequate for detection of the fatigue cracking because the weld cracks were not visible and could only have been detected by magnetic particle or dye penetrant inspection.

Text from the NTSB. Open the NTSB docket for ERA14LA108 for the full record.