Enstrom F 28A N24RB

Summary

On 29 January 2014 at about 21:15 local time, a Enstrom F 28A registered N24RB, operated by Helicopters Inc, was involved in an accident near Memphis, Tennessee, United States. 2 people were on board and nobody was injured. The aircraft was substantially damaged. The NTSB has published a probable cause for this accident; it is quoted in full below.

Date

29 January 2014 at 21:15

Classification

Accident

Location

Memphis, Tennessee, United States

Nearest airport

General Dewitt Spain (M01)

Coordinates

35.2006, -90.0539

Aircraft

Enstrom F 28A

Registration

N24RB

Category

Helicopter

Year built

1971

Engines

1

Operator

Helicopters Inc

Operating rule

Part 91: General Aviation

Purpose of flight

Instructional

Phase of flight

Not recorded

Route

Memphis → Memphis

Aircraft damage

Substantial

Weather

VMC

Light

Day

NTSB number

ERA14LA108

Nobody was injured.

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.

Quoted verbatim from the NTSB record. This site does not paraphrase or interpret it.

Read the full NTSB narrative

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.

Quoted verbatim from the NTSB record.