One returning pilot lost control of an aircraft during landing and skidded off the runway into a ditch. Another just returning from furlough forgot to activate a critical anti-icing system designed to prevent hazards in cold weather. Several others flew at the wrong altitudes, which they attributed to distractions and lapses in communication.

In all of these incidents, which were recorded on NASA’s Aviation Safety Reporting System, a database of commercial aviation mistakes that are anonymously reported by pilots and other airline crew, the pilots involved blamed the same thing for their mistakes: a lack of practice flying during the pandemic.

In 2020, global air passenger traffic experienced the largest year-on-year decline in aviation history, falling 65.9 percent compared with 2019, according to the International Air Transport Association. Flights were grounded, schedules reduced and thousands of pilots were laid off or put on furlough for up to 12 months.

As vaccination programs pick up speed across some parts of the world and travel starts to rebound, airlines are beginning to reactivate their fleets and summoning pilots back as they prepare to expand their schedules for the summer. On Thursday, the T.S.A. recorded the second-highest day of airport screenings since the pandemic hit: more than 1.64 million.

But returning pilots can’t just pick up where they left off.

“It’s not quite like riding a bike,” said Joe Townshend, a former pilot for Titan Airways, a British charter airline, who was laid off when the pandemic hit in March last year.

“You can probably go 10 years without flying a plane and still get it off the ground,” he said, “but what fades is the operational side of things.”