Edge Distance

[old man emu]

While reading the ATSB report on the Morgan -v- Ferris Wheel collision, we noted that there was a design error that was not commented upon by the ATSB in relation to the control column bracket.

 

Have a look at the bolt hole through the control column support bracket. See how close to the upper edge of the bracket the hole is?

 

When a load is transmitted from one part to another through a rivet, bolt, or pin, the fastener exerts compressive forces against the edges of the hole, tending to crush the metal ahead of the fastener or tear the hole out if it is near the edge of the piece.

 

Standard engineering practice is to have the centre of any hole in metal a minimum of [(2.5 x bolt diameter) + 0.05"] from the closest edge of the metal. The 0.05 allows for maintaining the 2D distance after repair using oversize fasteners, and allowance for manufacturing and repair tolerances.

 

For composite structures 2.5D + 0.05 inch is used by several manufacturers. Composites are more sensitive to edge distances and hole spacing than metal joints. The brittle nature of composites and the absence of local yielding creates higher peak stresses.

 

Another thing that could have been done to make the bracket a bit lighter, without affecting its strength, would have been to take the square edges off and make the bracket look like a triangle with the top cut off. ( I think the shape is called a trapezoid)

 

Old Man Emu

 

Control Column.pdf

 

Control Column.pdf

 

Control Column.pdf