Final answer:
The phugoid mode is a dynamic longitudinal stability mode characterized by a nearly constant angle of attack with lightly damped, long-period oscillations in speed and flight path angle, often experienced in aircrafts.
Step-by-step explanation:
The dynamic longitudinal stability mode characterized by almost constant angle of attack but lightly damped, relatively long-period oscillations in speed and flight path angle is known as phugoid mode. The phugoid mode involves longitudinal oscillations in an aircraft's flight path where the speed oscillates around a mean value, with corresponding changes in the flight path angle and altitude. This motion is a form of damped harmonic motion where the system is underdamped, leading to oscillations through the equilibrium position. A phugoid can be sustained due to the complex interplay of gravitational, lift, drag, and thrust forces acting on the aircraft. These oscillations can be undesirable as they might affect passenger comfort and flight safety.