Aircraft undergo a series of rigorous tests before they are certified for commercial use. These tests include ground tests, such as engine runs and taxi tests, as well as flight tests, which are conducted in various weather conditions and at different altitudes and speeds. The flight tests are designed to test the aircraft's performance, stability, and handling characteristics, as well as its compliance with safety regulations. The testing process is overseen by regulatory bodies such as the Federal Aviation Administration (FAA) in the United States, the European Aviation Safety Agency (EASA) in Europe, and the Civil Aviation Authority (CAA) in the United Kingdom, among others.