FL400 refers to a flight level of 40,000 feet above sea level, commonly used in aviation. This altitude is typically associated with commercial jetliners, which cruise at high altitudes to optimize fuel efficiency and reduce air resistance. At FL400, aircraft are above most weather disturbances and turbulence, providing a smoother flying experience. Pilots use altimeters to maintain this altitude, ensuring safe separation from other aircraft and compliance with air traffic control regulations. This level is part of the standard flight levels used in aviation, which are measured in hundreds of feet.