What are two reasons for installing a lightening hole in a sheet metal wing rib?

Installing a lightening hole in a sheet metal wing rib serves primarily to reduce weight and increase stiffness. By creating a cutout in the rib, the overall mass of the rib is reduced, which contributes to the aircraft's overall weight savings. This is crucial in aircraft design, where every ounce counts toward achieving efficiency and performance.

At the same time, the design of the rib, particularly when incorporating lightening holes, can enhance its structural stiffness. The remaining material around the hole is often optimized to provide the necessary support while allowing for a lighter structure. This combination of reduced weight without sacrificing strength is a key consideration in aerospace engineering, as it promotes improved fuel efficiency and payload capacity.

This reasoning aligns with fundamental principles of aircraft fabrication, where balancing weight, performance, and structural integrity is essential for safety and efficiency in flight.