What type of glue requires careful inspection in older wooden aircraft due to deterioration?

Casein glue is derived from milk and was commonly used in older wooden aircraft during the time when synthetic adhesives were not widely available. Over time, casein glue can deteriorate due to exposure to moisture and temperature fluctuations, leading to potential failure in structural integrity.

In contrast, epoxy resin glue, phenol-formaldehyde glue, and resorcinol glue are synthetic adhesives that, while they may also require some form of inspection for effectiveness, are generally more resistant to environmental degradation compared to casein glue. The natural composition of casein glue makes it particularly vulnerable to issues like mold growth and loss of bonding strength, making careful inspection crucial in maintaining the safety and airworthiness of older wooden aircraft.