Rectifying cracks in flap rib lightening hole flanges

While deburring/smoothing etc, I have found cracks in the edges of the lightening holes of FL-RIB-003-L-A-0 from the wing kit, as shown circled red. The cracks are full thickness. Clearly they were caused when the lightening hole was flanged - this metal is 0.41mm. Its right hand counterpart was not affected, and an inspection of all 0.41mm parts in the flap and aileron kits showed no further cracks. All other ribs of other thicknesses were also inspected and no cracks were found.

Consulted with TAF - they have confirmed that this rib can be used if the cracks are filed out with a semicircular notch and doubler plates are constructed. This will avoid significant delays in obtaining a replacement part, as well a being an instructive exercise.

A "stray" 0.41mm sling 4 rib was adapted to make suitable doubler plates.

The design and rivet pattern is shown and was checked with and approved by TAF. The rivet pattern was designed with 2D (rivet-to-edge)/3D (rivet-to-rivet) rule of AC 43.13-1B pp4-20 in mind.

The rivet-rivet separation is 15mm/17mm >> 3D (9.6mm)

The parallel row separation is at least 10mm ~ 3D

The rivet-corner fold separation is 7mm ~ 2D(6.4mm)

However as I have repurposed a 0.4mm rib, note that the width of the doubler flange is less than that of the flap rib itself. When match drilling to the flap rib holes, the distance between the doubler hole centres (here clecoed) to the edge of the doubler flange is 5.5mm, slightly less than the 2D requirement of 6.4mm. TAF has confirmed this is not an issue, as the rib flange has sufficient width.

The repurposed 0.4mm rib also has existing holes which are now unused. These are well over 3D (9.6mm) from the active flange rivets.  TAF have also confirmed this is not an issue.

Finally TAF have confirmed that the use of 3.2mm x 8mm (Gesipa) blind rivets (as supplied) is appropriate for stitching the doubler to the rib. The grip range goes down to 0.5mm which is less than the 0.8mm combined thickness of the doubler and rib.

After further consideration, I shall not be using this rib as repaired above. I have requested a replacement from TAF