Which of the following is NOT one of the three types of glue used in modern aircraft construction?

Casein glue is not commonly used in modern aircraft construction, making it the correct choice for this question. In aircraft manufacturing, adhesives must provide superior strength, durability, and resistance to environmental factors, which is why epoxy resin glue, phenol-formaldehyde glue, and resorcinol glue are preferred.

Epoxy resin glue is popular due to its exceptional bonding capabilities and resistance to heat and moisture, making it ideal for applications that require structural integrity. Phenol-formaldehyde glue is also favored in the industry because of its strong adhesive properties and moisture resistance, especially in applications where wood is utilized. Resorcinol glue is another adhesive used mainly for bonding wood components and offers good resistance to water and heat.

In contrast, casein glue, derived from milk proteins, was more commonly used in the past but lacks the performance characteristics required for modern aircraft applications. Its susceptibility to moisture and weaker bonding capabilities compared to synthetic adhesives make it unsuitable for the rigorous demands of aircraft construction today.