If the original rib stitching spacing is unknown, how should you determine the correct spacing?

• A. Consult the chart in AC 43.13.
• B. Use the spacing from adjacent ribs on the same aircraft.
• C. Apply standard 2 inch spacing uniformly.
• D. Take measurements from a different model.

Answer: (A)

When you don’t know the original rib stitching spacing, use FAA-approved data to determine it. The chart in AC 43.13 provides the recommended rib-stitch spacing for different fabric types and rib geometries, so you apply the spacing that matches your specific construction. This ensures the fabric remains properly tensioned and the stitches carry the load as intended.

Relying on spacing from adjacent ribs can be unreliable because each rib and fabric setup may differ. Applying a uniform standard distance like two inches everywhere ignores the exact material and rib size, which can lead to improper tension or failure. Taking measurements from a different model isn’t appropriate because it may have different construction. So the correct approach is to consult the chart in AC 43.13 to determine the proper spacing.