Upon landing, what function do spoilers serve?

Spoilers serve a critical role upon landing by disrupting the airflow over the wings, which in turn helps to decrease lift. This increases the weight on the wheels, enhancing tire-to-ground friction and improving braking effectiveness. When the spoilers are deployed during the landing roll, they help to ensure that the aircraft can decelerate more quickly by allowing the weight to transfer onto the landing gear, thus maximizing the efficiency of the brakes.

By increasing tire-to-ground friction, spoilers are essential for ensuring that the aircraft can stop safely and effectively on the runway. This function is particularly important in preventing the aircraft from continuing to roll after touchdown and assists in achieving stability during rollout.

The other options, while touching on various aspects of aircraft operation, do not accurately describe the primary purpose of spoilers in the context of landing. For instance, spoilers should indeed be deployed at the appropriate time in relation to thrust reversers, and they do not increase lift during potential landings; rather, they were designed to decrease lift. Also, they do not decrease directional stability on the rollout but rather assist in stabilizing the aircraft by enhancing braking performance.