What is factor of safety and what values are typical in aircraft design?

Factor of safety (FoS) is the ratio of ultimate load to design limit load, providing a margin against material variations, manufacturing tolerances, and unforeseen loads. For aircraft structures, FAR/CS regulations require FoS of 1.5 for metallic structures, meaning the structure must withstand 1.5 times the maximum expected load without failure. Limit load must be sustained without permanent deformation, while ultimate load must be sustained for 3 seconds without failure. Lower FoS than other industries is acceptable due to stringent quality control and inspection requirements.