Pilatus PC12 accident near Pacific Ocean, PO, November 6, 2020

The NTSB's narrative final · quoted from the NTSB record

The new production airplane was ditched in the ocean about 1,000 miles from its destination following a total loss of engine power during its first 10-hour transoceanic leg. The two pilots sustained no injuries, and the airplane was lost at sea. A subsidiary of the aircraft manufacturer installed an auxiliary ferry fuel line and check valve in the left wing as a major alteration (per FAA Form 337) that stated, “The ferry tank provisions feed directly into the engine’s fuel supply line.” It also stated that “ferry tank installations should ensure that no air is introduced into the fuel system.” Another company installed the ferry fuel system that initially consisted of two aluminum tanks, transfer and tank valves, and associated fuel lines and fittings. The company submitted a FAA Form 337 that stated, “The ferry fuel feed is directly to the left main tank.” The ferry fuel supply line was connected to the newly installed ferry fuel line fitting at the left-wing bulkhead, which then fed directly to the main fuel line through a check valve and directly to the engine fuel system. The pilots’ first attempt at the transoceanic flight failed because the ferry fuel system did not transfer any fuel. The system was further modified with the addition of two 30 pounds-per-square-inch (psi) fuel pumps that could overcome aircraft’s ejector fuel pump pressure (10 psi) and the ferry system’s check valve. The airplane was returned to service. The pilots flew a positioning flight and tested the ferry fuel transfer process, with both the front (No. 1) and rear (No. 2) internal tanks and both transfer pumps, up to an altitude of 17,500 ft. The system worked as tested and there were no further tests conducted of the ferry fuel system. The pilots departed on the 10-hour flight and the ferry fuel system worked initially as they used the operating procedures that were supplied by the installer. About 3.5 to 4 hours into the flight, the airplane was light enough to climb from flight level (FL) 200 to FL 280. About 5 hours into the flight, the No. 2 ferry tank was almost empty, and the No. 1 tank was about 1/2 full. The pilots were concerned about introducing air into the engine as they emptied the No. 2 ferry tank, so the pilot in command (PIC) placed the ignition switch to ON. The non-flying pilot then turned the ferry tank fuel transfer pump to off and soon after the engine surged and flamed out. The pilots commenced the pilot operating handbook’s emergency checklist procedures for emergency descent and then loss of engine power in flight. They attempted multiple engine air starts without success. About 8,000 ft mean sea level, the pilots committed to ditching and performed an emergency landing in the ocean. The pilots evacuated through the right over-wing exit, boarded the covered life raft, and were rescued about 22 hours later. The installed ferry fuel system altered the fuel flow characteristics of the airplane when it was used to transfer fuel from the ferry fuel tanks. The delivery ejector pumps had a flap valve installed in the outlet to prevent reverse flow. However, the ferry system transfer pumps provided fuel at a higher pressure than the delivery ejector pumps, which closed the flap valve in the delivery ejector pumps. Also, the unused fuel returned to the wing tanks through the motive flow line would flow out the delivery ejector pumps’ inlet because the delivery ejector pumps’ flap valve was closed. It is possible that the loss of engine power was due to air being introduced into the fuel line from the ferry system, although the boost pumps, if operating properly, should have compressed the air and forced it through the fuel line. It is also possible that ice built up in the aircraft fuel tanks during the fuel transfer operations, and when the ferry system was turned OFF, fuel flow to the engine stopped or was restricted because 1) the left- and right-wing fuel was too viscous; and/or 2) the ejector flap valves were stuck closed. The aircraft was certified without an air separator in the engine fuel feed line. In addition, the production fuel system design of the accident airplane was such that a Fuel System Icing Inhibitor (FSII) was not required. Although not required, neither the aircraft manufacturer nor the company that installed the ferry fuel system evaluated 1) the ferry system’s impact on the production fuel system operating temperature; 2) if an FSII should be required; and 3) if not having an air separator in the engine fuel feed line would impact the system. The loss of engine power likely was caused by fuel starvation as a result of 1) air in the fuel line from operating on the ferry fuel system; or 2) a build-up of ice in the production fuel system due to operating on the ferry fuel system. However, because the airplane was lost at sea and was not available for postaccident examination, the exact cause of the fuel starvation could not be determined.

The complete narrative as the NTSB published it. The NTSB's docket holds the report as a PDF and any photographs, statements and other documents from the investigation.