Marine Helicopter
Many think of AFO as a specific aircraft, but it’s a designation. Any aircraft carrying the president is referred to as AFO. However, it’s worth noting that the president doesn’t exclusively travel by plane. When aboard an Army aircraft, it’s referred to as Army One, and if the mode of transport is a helicopter, it’s called Marine One. Therefore, Marine One serves as the radio call sign for a Marine Corps helicopter tasked with transporting the president. Lockheed is constructing six new helicopters for Marine One, with the initial one anticipated operational by 2020. These helicopters within the Marine One fleet are equipped with features like ballistic armor and anti-missile defenses. The skilled pilots responsible for these missions are Marine Helicopter Squadron One members, often called the Nighthawks.
nytimes.com