ANC93LA136

Pilot of lake amphibian airplane encountered porpoising loss of control while step taxiing in choppy lake conditions. Porpoising progressed to wingfloat pitching and collision with water. The airplane sank.

History of flight

On August 6, 1993, at approximately 1930 Alaska daylight time, a Lake Amphibian LA04 airplane, N6669L, porpoised, dragged a wing and waterlooped while conducting a high speed on-the-step taxi. The airplane sank on Blair Lake, 30 miles SE of Fairbanks. The flight had originated at Wrights Field, North Pole, Alaska at 1730 on a personal flight in visual meteorological conditions, under 14 CFR Part 91. No flight plan was filed. The commercial pilot and two passengers escaped without injury and the aircraft was substantially damaged.

In a statement to the NTSB the pilot said that the airplane developed a porpoise while taxiing on the step at 60 MPH which progressed to wing oscillations and loss of control.

Damage to aircraft

The aircraft sustained hull damage and loss of wing floats, consistent with sideloads experienced in a waterloop.

Aircraft information

In boat hull seaplanes, a porpoise is a pitch into and out of the sea by the bow of the aircraft about the lateral axis. The magnitude of the oscillations relate to the energy needed to displace the water under the bow and the resultant upward force by the water on the hull. The frequency of the oscillation relates to aircraft speed and period of the sea (distance between swell crests). The oscillations can be pilot-induced or sea-condition-induced. Typically, this porpoise is followed by rolls about the longitudinal axis which, as the wingtip floats alternately enters the water, and exacerbates the rolling moment by porpoising themselves. A final turn (waterloop) is typically a result of the hydrodynamic center of drag becoming forward of the aerodynamic center of gravity, leading to a destabilized loss of control and sudden sideload on the forward hull. Damage to cantilever mounted wing floats by these sideloads is typical.