According to PD 1570, which term is defined as any contrivance for navigation or flight in the air?

The term described in PD 1570 as any contrivance for navigation or flight in the air is "Aircraft." This definition is broad and encompasses all types of flying vehicles, including airplanes, helicopters, drones, and any other means of flight. The importance of this definition lies in its inclusivity, allowing it to cover various forms of air travel and transport under one term, which is critical for regulatory and legal purposes.

The other terms, while also related to air travel, refer to more specific categories or types of aircraft. For example, an airplane is a specific kind of aircraft that typically has fixed wings and is driven by engines. A helicopter is another specific type that uses rotating blades for lift. Rotorcraft is a broader category but still confines itself to aircraft using rotor blades for flight. Therefore, the correct term that encompasses all of these and aligns with the definition in PD 1570 is "Aircraft." This comprehensive definition is significant for legal references in aviation regulations, safety protocols, and other related fields.