During longitudinal phugoid oscillations, if airspeed increases and decreases, the aircraft is likely:

During longitudinal phugoid oscillations, the aircraft's behavior is characterized by a cyclical variation in airspeed and altitude, where the changes in airspeed occur while maintaining a nearly constant angle of attack. In a stable flight condition, when the aircraft encounters these oscillations, it typically achieves a balance where the lift generated remains adequate for the weight being supported, allowing the angle of attack to remain fairly constant throughout the oscillation cycle.

This maintenance of a constant angle of attack is crucial because it indicates that the aircraft is not entering a stall condition; instead, it remains within a safe operational envelope, allowing for periodic increases and decreases in speed as it moves up and down in altitude. Thus, the aircraft continues to function effectively, despite the fluctuations in airspeed, showing the inherent stability in its design and aerodynamics.

Understanding this concept is important for safe flying and instructional practices, as it helps pilots recognize and manage phugoid oscillations without misinterpreting them as indications of performance issues or unsafe flight conditions.