The calculator
Runs in your browser. Inches or millimetres at the top. Measure the depth at the deepest point, after all corrosion has been removed.
How the rule works
A pit is a stress riser. Removing the corrosion leaves a hole with steep sides, and that is where a crack starts. In critical and highly stressed areas, AC 43.13-1B par. 6-115b wants every pit left after corrosion removal blended out, to prevent stress corrosion cracking.
The dish-out: 20:1, length to depth. Par. 6-115c: “Rework depressions by forming smoothly blended dish-outs, using a ratio of 20:1, length to depth.” Figure 6-14 dimensions it on the finished part: the cleaned-up area is at least 20 times the depth long and 10 times the depth wide. The figure for multiple pits measures from the bottom of the depression: 10 times the depth to each end, 5 times to each side.
It is an ellipse, and the direction matters. Par. 6-115a: the major axis runs spanwise on wings and horizontal stabilizers, longitudinally on fuselages, and vertically on vertical stabilizers. That lines the long, shallow slope up with the main direction of stress.
Closely spaced pits become one dish-out. Par. 6-115c also says to remove the material between closely spaced pits, rather than leave a surface of separate craters. Measure the depth of the combined area at its deepest point.
The limit is the SRM's. The AC's aluminum procedure ends with “determine depth of faired depressions to ensure that rework limits have not been exceeded” (par. 6-136i), and does not say what those limits are. When removal exceeds the SRM's limits, par. 6-134b sends you to a DER or the manufacturer. That is why this calculator takes the limit as an input.
Worked example
Pitting on a 0.040" bare 2024-T3 wing skin. After cleanup, the deepest point measures 0.004". The SRM for this area allows 10% of the skin thickness.
- Minimum length: 20 × 0.004 = 0.080", spanwise.
- Minimum width: 10 × 0.004 = 0.040", chordwise.
- Thickness removed: 0.004 ÷ 0.040 = 10%.
- SRM limit: 10% × 0.040 = 0.004". The depth is at the limit, not over it.
Within the limit, with nothing to spare. Any further sanding to finish the surface takes it over. Stop, measure, and if the finish needs more material, that is a call for the SRM's repair section or a DER, not for the next grade of abrasive paper.
Limits — when NOT to use this
Geometry from the AC, and a comparison against a figure you supply. Nothing more.
- No generic depth limit exists. Neither AC 43.13-1B nor this calculator says how deep you may go. Without the SRM figure, the answer is not “probably fine.”
- The limit is specific. SRM limits differ by part, by zone, by distance to fasteners and edges, and by how much other rework is nearby. A figure for one area does not carry to the next.
- Measure after the corrosion is gone. The depth that counts is the deepest point once every corrosion product is removed, not the depth of the visible pit.
- Clad sheet is different. AC 43.13-1B Table 6-1 limits sanding on clad aluminum to the removal of minor scratches.
- Not for exfoliation or intergranular corrosion. Their extent is not visible at the surface. Par. 6-136 notes that exfoliation is not to be removed by abrasive blasting; these are not cosmetic blend-outs.
- Remove steel fasteners and nut-plates first. Par. 6-115c: steel or copper particles embedded in aluminum become a point of future corrosion.
Questions people actually ask
How deep can I blend out corrosion?
As deep as the aircraft's SRM allows for that part, and no deeper. AC 43.13-1B gives the shape of the blend, not the depth limit. Past the SRM's limits, par. 6-134b says to contact a DER or the manufacturer.
What does a 20:1 blend ratio mean?
The dish-out is at least 20 times as long as it is deep. AC 43.13-1B Figure 6-14 sets the width at 10 times the depth. A 0.005" deep blend is at least 0.100" long and 0.050" wide.
Which way should the blend run?
Spanwise on wings and horizontal stabilizers, fore and aft on fuselages, vertically on vertical stabilizers (par. 6-115a).
What do I do after blending?
The AC's aluminum procedure: finish with progressively finer abrasive to 400 grit, clean with dry cleaning solvent, check the depth against the rework limits, then apply chemical conversion coating (MIL-C-81706) immediately (par. 6-136g to j).
Sources
- FAA AC 43.13-1B CHG 1, Chapter 6, par. 6-115 and Figures 6-14 to 6-16 — the 20:1 dish-out, its elliptical shape and orientation, and the dimensions on the figures: length 20 times and width 10 times the depth (Figure 6-14); 10D and 5D from the bottom of the depression for multiple pits. Table 6-1 for clad aluminum.
- FAA AC 43.13-1B CHG 1, Chapter 6, par. 6-134 and 6-136 — “When corrosion removal exceeds the limits of the structural repair manual, contact a DER or the aircraft manufacturer” (6-134b), and the aluminum rework procedure (6-136).