What type of glue, commonly used in older aircraft, requires careful inspection for deterioration?

Casein glue is a protein-based adhesive derived from milk, historically used in older aircraft construction, especially in joining wood components. Its natural composition and water solubility make it effective for wood-to-wood bonding but also vulnerable to environmental conditions. Over time, casein glue can deteriorate due to moisture, temperature changes, and exposure to UV light, leading to weakened bonds.

Regular inspection for deterioration is crucial to ensure the structural integrity of aircraft components where casein glue has been used. Signs of failure may include peeling, softening, or discoloration of joints bonded with this adhesive. Unlike synthetic adhesives such as epoxy or polyurethane, which have more stable properties and typically exhibit high resistance to moisture and temperature variations, casein glue demands more vigilance to ensure safety and reliability in aviation applications.