Accident Overview
On September 4, 2017, about 1430 Alaska daylight time, a Cessna U206 airplane equipped with amphibious floats, registration N16GP, sustained substantial damage while attempting to depart from the water at Trident Basin Seaplane Base (T44) in Kodiak, Alaska. The aircraft was operated by the pilot under 14 Code of Federal Regulations Part 91 as a visual flight rules flight. The airline transport pilot and three passengers were not injured. Marginal visual meteorological conditions existed, and no flight plan was filed.
Sequence of Events
According to the pilot, during the takeoff run, the airplane encountered small swells and began to lose speed while simultaneously pitching forward. In response, the pilot applied full aft elevator. Shortly thereafter, he aborted the takeoff, but the aircraft continued to pitch forward and was struck by a larger swell. The right forward float strut fractured, causing the airplane to roll to the right, resulting in substantial damage to the right wing's lift strut. An initial examination by the pilot revealed that the left nose wheel was partially deployed with the landing gear handle in the up position.
Witness Accounts
A pilot-rated witness reported that the accident pilot had consulted with several pilots before boarding, asking "what do you think?" and "which way to go?" One option discussed was using "The Cut," an easterly departure through a break between islands. The witness stated that this departure route worked as long as the aircraft was airborne before leaving the protected area, as ocean swells were encountered on the other side of the islands.
Another pilot-rated witness observed the water run from his office, noting that the airplane seemed "must be really heavy" because it took a long time to accelerate onto the step. As the airplane disappeared through "The Cut," he saw the wings rocking as it encountered ocean swells.
A third witness reported that the airplane was in the plow for an estimated 1,100 feet before accelerating onto the step. The airplane then passed through "The Cut" and encountered about 8-foot-tall ocean swells. He heard the engine operating at full power for about 15 seconds before it abruptly stopped. He then motored through "The Cut" to assist.
A fourth witness, who aided in recovery, reported wind from the east at 15 to 20 knots, seas of 1 to 1½ feet with 6 to 8-inch swells, and no evidence of hydraulic oil on the water during towing.
Neither the pilot nor witnesses reported the airplane yawing about its vertical axis during the takeoff run.
Weather and Conditions
The coastal waters forecast for Chiniak Bay on September 4 called for seas of 9 feet, wind from the southeast at 25 knots, rain, and a Small Craft Advisory had been issued. The closest METAR, from Kodiak Airport (PADQ) at 1428, reported wind 100° at 15 knots, visibility 4 statute miles with mist, scattered clouds at 1,000 feet, overcast at 3,100 feet, temperature 54°F, dew point 54°F, and altimeter 29.59 inHg.
Aircraft Examination
The airplane was equipped with EDO 696-3500 amphibious floats. The mechanic who removed the aircraft from the water reported that all three right float struts were completely separated from their attach points, and the hydraulic lines for the landing wheels were severed. A postaccident examination revealed that the left and right mechanical landing gear retraction system was not rigged correctly, and the amphibious float cable loops were set to about a 17-pound tension load. The EDO Model 696-3500 Service and Maintenance Manual specifies a cable loop tension of 75 to 125 pounds. No evidence of a hydraulic leak was found inside the floats or fuselage.
Guidance from Manufacturer
The book "How To Fly Floats" by EDO Corporation, in the section on rough water take-offs, states: "When possible, take-offs in rough water should be avoided. Before attempting a rough water take-off, do some reconnaissance of the area first... Pay special attention to any severe swell conditions that may exist... There is a very good possibility that severe damage can be done to either the floats, the attachment gear or the aircraft in big swell conditions."