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Dark stains in wood structure

Aviation Mechanic Airframe (AMA) practice question 1 of 30 on Non-Metallic Structures. ACS element AM.II.B.K2 · difficulty: standard. Want to answer it in a timed quiz? Use the Aviation Mechanic Airframe (AMA) practice mode.

Question bank updated 2026-09-22

QUESTIONWhile inspecting a wooden aircraft structure, a mechanic finds dark discolorations and gray stains running along the grain. What do these markings indicate?

  1. AWater penetration of the wood
  2. BLocal staining from a synthetic adhesive hardener
  3. CNormal color variation between the annual rings
Show the answer and explanation

Correct answer: A. Water penetration of the wood

  • A — correct The handbook states that dark discolorations of the wood or gray stains running along the grain are indicative of water penetration, and the part must be replaced if light scraping does not remove the discoloration.
  • B — incorrect Local staining by dye from a synthetic adhesive hardener is a separate condition that the handbook says to disregard; it does not run along the grain as a gray stain.
  • C — incorrect Color variation between growth rings is not what the handbook associates with these markings; gray staining along the grain is treated as evidence of moisture, not normal figure.

BACKGROUND

Moisture is the principal enemy of wood aircraft structure. All fungi that cause decay require considerable moisture for growth, so trapped water leads directly to decay, dry rot, and glue joint failure. Because of this, the handbook treats staining as a moisture indicator rather than a cosmetic issue: dark discolorations or gray stains running along the grain point to water penetration. If light scraping does not remove the discoloration, the part is replaced. Corrosion found on screws or bolts removed from a suspect joint is another useful indicator of water penetration in the surrounding wood.

STUDY TIP

Learn the specific visual cues the handbook ties to moisture in wood, and remember the decision rule that a stain which will not scrape away means replacement rather than repair.

KEY POINTS

  • Dark discoloration or gray stains along the grain indicate water penetration.
  • If light scraping does not remove the stain, replace the part.
  • Staining from synthetic adhesive hardener dye is disregarded.

SOURCES

  • FAA-H-8083-31B ch06 "Wood Condition"
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MORE ON NON-METALLIC STRUCTURES

SkyDrill is not affiliated with the FAA. This is an original practice item based on FAA-H-8083-31B Aviation Maintenance Technician Handbook – Airframe, and 14 CFR part 43. Regulations and handbooks are revised; confirm details against the current edition.