What type of glue may have been used in older wooden aircraft construction that requires careful inspection to detect deterioration?

• A. Epoxy resin glue
• B. Polyvinyl glue
• C. Casein glue
• D. Resorcinol glue

Answer: (C)

The type of glue used in older wooden aircraft that requires careful inspection for deterioration is casein glue. Casein is a milk-protein adhesive that was commonly used in early wooden aircraft because it bonded well when applied and dried, but it is highly susceptible to moisture. Over time, exposure to humidity or water can cause casein to soften, crack, or crumble, and the deterioration can occur inside joints where it isn’t readily visible. Because the damage can be hidden within the wood-laminated joints, meticulous inspection is necessary to detect weakening bonds and potential rot.

Epoxy resin and resorcinol glues are much more resistant to moisture and age pretty well under typical service conditions, so they don’t exhibit the same deterioration patterns as casein. Polyvinyl glue is a more modern wood glue and would not be the characteristic choice for older aircraft construction. Thus, casein glue best fits the scenario of older wooden construction that warrants careful inspection for deterioration.