Class A airspace is a type of controlled airspace that extends from 18,000 feet up to 60,000 feet above sea level. It is primarily used by commercial airliners and other high-altitude aircraft. In this airspace, all flights must operate under Instrument Flight Rules (IFR), which means pilots must rely on instruments for navigation and communication. To enter Class A airspace, pilots must have an IFR flight plan and be in contact with air traffic control. This ensures safe separation between aircraft and helps manage the high volume of traffic in this busy airspace.