Flight delays could be caused by a variety of factors, such as bad weather, maintenance issues, air traffic control issues and shortages of personnel or aircraft. Poor weather can delay take-off or cause safety concerns. Aircraft maintenance issues can cause delays. Air traffic control delays may be caused by large groups of aircraft leaving congested airports or arriving at the same time. Personnel shortages can cause delays, as well as last-minute aircraft swaps due to technical difficulties. Some delays may be unavoidable, but many can be minimized through careful planning.