What is the phugoid mode and what are its characteristics?

The phugoid is a long-period longitudinal oscillation involving exchange between kinetic and potential energy at nearly constant angle of attack. The aircraft slowly pitches up, climbs while losing speed, then pitches down and dives while accelerating, continuing this cycle. Phugoid period is typically 30-60 seconds with light damping. It is usually easily controlled by the pilot due to its slow nature. Phugoid frequency is approximately sqrt(2)*g/V and is lightly damped in most aircraft. Unlike the short period mode, the phugoid involves significant altitude and speed changes.