On May 20, 2023, at about 1354 Pacific daylight time, a de Havilland DHC-6-400, N153QS, was substantially damaged when it impacted the Pacific Ocean near Half Moon Bay, California, and sank. The pilot and one passenger onboard were fatally injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 ferry flight. The U.S. Coast Guard responded to the accident site. The operator reported that the airplane had a ferry fuel tank system installed for the trans-Pacific flight. The two aircraft main tanks were filled with fuel, then the two forward ferry bladder tanks and the one smaller bladder took on fuel. Fueling documentation revealed that the fixed-based operator at the Charles M Schulz-Sonoma County Airport (STS), Santa Rosa, California, added 1,189 gallons of fuel the day prior to the flight. The airplane’s amphibious floats were removed for the flight. The flight departed STS and was destined for Daniel K Inouye International Airport (HNL), Honolulu, Hawaii. Satellite flight tracking data showed the airplane departed to the southwest over the ocean and climbed to an altitude of about 15,500 ft mean sea level (msl). About 4 hours into the flight, the crew contacted ATC and reported that they were having a fuel transfer problem and were thinking of turning around. The crew then reported they were declaring an emergency and had 10 hours of fuel remaining but could only access about 2 hours of fuel. About 356 miles from the California coast, the flight track reversed course to the northeast, towards STS. About 132 miles from the coast the flight track showed a decrease in altitude to about 4,000 ft msl. ATC advised the crew to expect a USCG helicopter to respond to their last known position and that “we will come and get you.” The pilot acknowledged by thanking him. This was the last communication from the crew. The last few minutes of the data shows an altitude drop from about 3,600 to about 240 ft msl. The last track data point was located about 33 miles off the California coast. The USCG responded to the scene minutes after the accident. Examination of the wreckage in the water revealed that the fuselage was inverted, and the wings and engines had separated from the fuselage. Both occupants were visible and remained restrained in their respective seats, unresponsive. Due to safety concerns, a USCG swimmer made a limited but unsuccessful attempt to recover the occupants. The horizontal stabilizer remained partially attached to the empennage and the forward fuselage had impact damage. Wreckage recovery was not performed at this time Aerial and maritime sub-surface search efforts resulted in finding the nose landing gear, right wing and right engine. A fuel bladder tank had washed ashore in southern California. The occupants were not found. A mechanic reported that he had been instructed to prepare the ferry fuel tank system for installation in the accident airplane. He did not complete the installation due to not knowing what day the flight would take place. To complete the installation, the fuel bladders would have to be filled and the logbook entry would need to be completed. He stated that it would have been unsafe to have the bladders contain fuel days before the departure date. After completing the preparation of the ferry fuel tank system, he contacted the copilot, who was also one of the only individuals authorized to sign off on the installation of the ferry fuel tank system. The mechanic stated that he understood that the copilot would be responsible for the final installation of the ferry fuel tank system and completion of the appropriate logbook entries. The mechanic further stated that he was not present when the copilot completed the installation on May 19, 2023, and was not sure if the copilot had signed off on the installation. The examination of the recovered components revealed no evidence of any preimpact mechanical malfunctions or failures that would have precluded normal operation. The recovered 400-gallon fuel bladder was removed from the shipping container. Jet fuel was present during the examination. The flat tube and cap were attached to the bladder and were undamaged. The top plate was damaged on one side between fasteners. No leaks of the bladder were noted. The outlet fitting attached to the top plate was undamaged and uncapped. The three longitudinal strap loops were damaged, and the straps were missing. The five tie-down lift points remained attached to the bladder on one side. Both forward and aft center tie-down lift points separated. Two tie-down lift points on the other side separated with the one remaining attached. The fuel cap was removed from the filler neck and sand was present in the flat tube. The top plate was removed from the bladder and the inside of the bladder revealed the presence of fuel and was clear of debris. The outlet fitting was unobstructed. A review of the aircraft maintenance documents shows that on February 20, 2022, a ferry fuel tank system was installed on the airplane and then removed on March 12, 2022. These logbook entries were signed by the mechanic who performed the preparation for the installation of the tanks for the accident flight. The most recent entries were not available during the investigation and were likely onboard the airplane during the flight.