Phugoid

Phugoid: When an airplane rises and falls. The apparent derivation is from the Greek word for flight: phuge. However, this is the Greek word is the word for fleeing, not flying.

It's just what an aeroplane does when its controls are neutral. The plane will start going down. But when it goes down beyond a certain angle, its internal axis will correct itself and the plane will start going up again. Then it will go down again.