Which glue is NOT used in modern aircraft construction?

• A. Resorcinol glue
• B. Phenol-formaldehyde glue
• C. Epoxy resin glue
• D. Casein glue

Answer: (D)

Adhesives in modern aircraft are chosen for high strength, durability, and resistance to moisture, heat, and chemicals. Casein glue is an older natural adhesive made from milk proteins; its performance degrades with humidity and temperature, and it lacks long-term durability in demanding airframe environments. That makes it unsuitable for contemporary aircraft construction. In contrast, resorcinol-formaldehyde and phenol-formaldehyde resins provide excellent moisture resistance and strong bonding for wood laminates and exterior joints, while epoxy resins are widely used for bonding metals and composites due to their strong adhesion and toughness. So the glue not used in modern aircraft construction is casein glue.