What could be a reason for not using cotter pins?

Cotter pins are commonly utilized in aviation and mechanical applications for securing fasteners. However, they may not be appropriate in situations requiring extra strength. The primary reason a technician might avoid using cotter pins in these scenarios is that they may not provide the durability and holding power needed for high-stress applications. Alternative fastening methods, such as lock nuts, double-nut combinations, or locking washers, might be better suited for maintaining a secure hold under significant loads or vibration.

In contrast, the other options relate to situational constraints or material considerations rather than inherent weaknesses of cotter pins. If cotter pins are unavailable, that is a logistic issue, not a performance one. Lightweight components might not inherently benefit from alternative fasteners if the cotter pin is adequate for the required load. Similarly, while soft materials could sometimes limit the effectiveness of a fastening method, it does not directly indicate that cotter pins are inappropriate for all applications involving softer materials.