Investigation of Tragic Floatplane Accident Ongoing

The National Transportation Safety Board (NTSB), U.S. Navy, and personnel from Washington State’s Island County have undertaken the recovery of N725TH, the 1967 de Havilland DHC-3 Otter that went down in Western Washington’s Puget Sound on 4 September. Work will be conducted in 12-hour shifts around the clock.

Two barges arrived off the southwestern shore of Whidbey Island on 26 September and took up station in proximity to the crash-site. One barge is outfitted with a crane by dint of which the salvage team will endeavor to lift the aircraft’s wreckage from approximately 190-feet of cold, turbid water; the second barge carries and facilitates operation of a U.S. Navy work-class Remotely Piloted Vehicle (ROV) appositely christened Deep Drone 8000.

The ROV, upon confirming the wreckage is indeed that of N725TH, will conduct a survey of the adjacent seafloor for purpose of visualizing the debris-field, and documenting the condition and position of the main and extrinsic aircraft wreckage. Thereafter, the salvage team will determine the optimal means and order of operations by which to recover the aircraft and artifacts germane to the NTSB’s investigation of the accident.

The lifting process may be accomplished by attaching crane cables to the wreckage and hoisting it directly aboard the barge. Alternately, the ROV may be tasked with recovering small sections of wreckage and placing them in baskets to be raised individually to the surface.

The Deep Drone 8000 system comprises a 4,100-pound submersible vehicle, an umbilical cable, a motion-compensated handling system, a deck hydraulic power unit, a generator, an operations van, and a maintenance van. The ROV, which is navigated via an acoustic tracking system, is capable of detecting small targets to a distance of two-thousand-feet. The contraption’s remarkable sensitivity is attributable to its Continuous Tone Frequency Modulation (CTFM) sonar—an advanced detection and ranging technology that accounts for relative motion between sensor and target by analyzing and accounting for Doppler effect during the interrogator wave’s propagation process. The moniker Deep Drone 8000 denotes the submersible’s eight-thousand-foot maximum depth.

Deep Drone uses both electric and hydraulic thrusters for propulsion, and sports a pair of seven-function manipulators adept in the handling of tools, cables, and rigging. To gather the photographic documentation essential to the NTSB’s ongoing investigation, the ROV is kitted-out with a digital still camera and both black-and-white and color television cameras.

In the days following the accident, the NTSB—in cooperation with the Washington State Department of Fish and Wildlife, the National Oceanic and Atmospheric Administration’s (NOAA) Navigation Response Team, and the University of Washington’s Applied Physics Laboratory—located N725TH’s wreckage using side-scan sonar, multibeam sonar, and various 3D imaging instruments.

The NTSB’s preliminary report on the investigation is available for download on the agency’s CAROL database.

FMI: www.ntsb.gov