Which designation indicates that an aircraft is in the early stages of planning?

The designation that indicates an aircraft is in the early stages of planning is referred to as "Early stages of planning." This term encompasses the initial phases of aircraft development, where broad concepts, mission requirements, and design parameters are being defined. At this stage, engineers and designers are focusing on understanding the objectives and essential features the aircraft must fulfill before moving on to more detailed design and testing phases.

In contrast, the other designations refer to different phases of aircraft development. "Modification" typically involves changes made to an existing aircraft to enhance its performance or capabilities. "Prototype" refers to a preliminary model built to test and validate design concepts, showing that it has moved beyond just planning. "Research/experimental" indicates that an aircraft is being developed or tested with innovative technologies or theories, often involving significant experimentation beyond the initial planning stages.