What causes a porpoising landing error?

A porpoising landing error primarily occurs when there is an incorrect (flat) landing attitude combined with inappropriate airspeed during touchdown. When the aircraft makes contact with the runway at a flat attitude, it can bounce off the surface due to the angle of contact. If the airspeed is too high at this moment, the aircraft can become airborne again, causing it to pitch up and then down in a cyclical manner, resulting in a porpoising motion. The aircraft will continue to lose altitude and gain lift alternately, creating a series of bounces and potentially leading to a loss of control.

In contrast, excessive pitch down during the approach can lead to a different kind of landing problem, but it does not specifically cause porpoising. A delay in throttle adjustment is more associated with inadequate power management rather than directly causing the porpoise effect. Likewise, insufficient flap deployment affects the lift characteristics and landing speed required but does not directly result in a porpoising motion upon touchdown. Understanding the correct landing attitude and approach speed is crucial for executing a smooth landing and avoiding porpoising.