If a pilot license or rating expires, the pilot cannot legally operate an aircraft until it is renewed, and depending on how long it has been inactive, they may need to complete refresher training, flight reviews, simulator checks, and pass medical certification again; if the lapse is short, revalidation is usually straightforward through an instructor check or exam, but if the expiry is long, aviation authorities may require partial or full retraining similar to re-earning the rating, making the process more time-consuming and costly to return to active flying status.