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Blend-out length for a corrosion pit

Aviation Mechanic General (AMG) practice question 17 of 31 on Cleaning and Corrosion Control. ACS element AM.I.G.K7 · difficulty: applied. Want to answer it in a timed quiz? Use the Aviation Mechanic General (AMG) practice mode.

Question bank updated 2026-09-22

QUESTIONA corrosion pit 0.025 inch deep is being blended out of a critical structure using the handbook's length-to-depth ratio for dish-outs. What length of blended depression does that ratio give?

  1. A0.25 inch
  2. B0.50 inch
  3. C1.00 inch, twice the ratio for safety
Show the answer and explanation

Correct answer: B. 0.50 inch

  • A — incorrect A 0.25 inch length would be a 10:1 ratio; the stated length-to-depth ratio for dish-outs is 20:1.
  • B — correct The handbook calls for reworking depressions as smoothly blended dish-outs using a ratio of 20:1, length to depth; 20 × 0.025 inch = 0.50 inch.
  • C — incorrect No doubling of the ratio is specified; the ratio is 20:1, giving 0.50 inch for a 0.025 inch pit.

BACKGROUND

All depressions resulting from corrosion rework must be faired or blended with the surrounding surface. Rough edges and all corrosion are removed first, and dish-outs are made elliptical with the major axis running spanwise on wings and horizontal stabilizers, longitudinally on fuselages, and vertically on vertical stabilizers. In critical and highly stressed areas all remaining pits must be blended out to prevent stress risers that could cause stress corrosion cracking. Depressions are reworked as smoothly blended dish-outs using a 20:1 length-to-depth ratio, and steel nut-plates and fasteners are removed before blending corrosion out of aluminum.

STUDY TIP

Remember the 20:1 length-to-depth rule and the orientation of the ellipse for each structure; both are easy calculation targets.

KEY POINTS

  • Dish-outs are blended at a 20:1 length-to-depth ratio
  • Ellipse major axis runs spanwise on wings, longitudinally on fuselages
  • Pits in highly stressed areas must be blended out to avoid stress risers

SOURCES

  • FAA-H-8083-30B ch08 "Fairing or Blending Reworked Areas"
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MORE ON CLEANING AND CORROSION CONTROL

SkyDrill is not affiliated with the FAA. This is an original practice item based on FAA-H-8083-30B Aviation Maintenance Technician Handbook – General, 14 CFR part 43 and part 65. Regulations and handbooks are revised; confirm details against the current edition.