Accident Overview
On May 20, 2023, at approximately 1354 Pacific daylight time, a de Havilland DHC-6-400, registration 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.
Flight Details
According to the operator, the airplane was equipped with a ferry fuel tank system for the trans-Pacific flight. The two main tanks were filled with fuel, followed by the two forward ferry bladder tanks and one smaller bladder tank. Fueling documentation showed that the fixed-based operator at Charles M Schulz-Sonoma County Airport (STS), Santa Rosa, California, added 1,189 gallons of fuel the day prior to the flight. The aircraft's amphibious floats had been removed for the flight. The flight departed STS destined for Daniel K Inouye International Airport (HNL), Honolulu, Hawaii. Satellite flight tracking data indicated that the airplane departed to the southwest over the ocean and climbed to an altitude of about 15,500 ft mean sea level (msl).
Emergency and Last Communication
About four hours into the flight, the crew contacted air traffic control (ATC) and reported a fuel transfer problem, considering turning around. Subsequently, they declared an emergency, stating they had 10 hours of fuel remaining but could only access about 2 hours of fuel. At approximately 356 miles from the California coast, the flight track reversed to the northeast, toward STS. About 132 miles from the coast, the flight track showed a descent to about 4,000 ft msl. ATC advised the crew to expect a U.S. Coast Guard (USCG) helicopter response to their last known position, stating, “we will come and get you.” The pilot acknowledged with thanks, which was the last communication from the crew. The final minutes of data showed an altitude drop from about 3,600 to about 240 ft msl, with the last track data point approximately 33 miles off the California coast.
Response and Wreckage Examination
The USCG responded to the scene shortly after the accident. Examination of the wreckage in the water revealed the fuselage inverted, with the wings and engines separated from the fuselage. Both occupants were visible, restrained in their seats, and 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 showed impact damage. Wreckage recovery was not performed at that time.
Subsequent aerial and maritime sub-surface searches located the nose landing gear, right wing, and right engine. A fuel bladder tank washed ashore in southern California. The occupants were not found.
Fuel System Installation
A mechanic reported that he had been instructed to prepare the ferry fuel tank system for installation on the accident airplane but did not complete the installation due to uncertainty about the departure date. He stated that to complete the installation, the fuel bladders would need to be filled and a logbook entry completed, and that it would be unsafe to have the bladders contain fuel days before departure. After preparing the system, he contacted the copilot, who was one of the few individuals authorized to sign off on the installation. The mechanic understood that the copilot would be responsible for the final installation and logbook entries. He was not present when the copilot completed the installation on May 19, 2023, and was unsure if the copilot signed off.
Examination of Recovered Components
Examination of recovered components found no evidence of preimpact mechanical malfunctions or failures that would have precluded normal operation. The recovered 400-gallon fuel bladder was removed from its shipping container; jet fuel was present during examination. The flat tube and cap were attached and undamaged. The top plate was damaged on one side between fasteners, but no leaks were noted. The outlet fitting was undamaged and uncapped. The three longitudinal strap loops were damaged, and the straps missing. Five tie-down lift points remained attached on one side; both forward and aft center tie-down lift points separated; two tie-down lift points on the other side separated, with one remaining attached. The fuel cap was removed, and sand was present in the flat tube. The top plate was removed, revealing fuel inside clear of debris. The outlet fitting was unobstructed.
Maintenance Records
A review of aircraft maintenance documents showed that on February 20, 2022, a ferry fuel tank system was installed and then removed on March 12, 2022. These logbook entries were signed by the mechanic who performed the preparation for the accident flight installation. The most recent entries were not available during the investigation and were likely onboard the airplane during the flight.