PILATUS PC12

Full Narrative

On November 6, 2020, about 1520 Pacific standard time, a Pilatus PC-12, N400PW, was substantially damaged when it was ditched in the Pacific Ocean about 1,000 miles east of Hilo, Hawaii, following a total loss of engine power. The two pilots sustained no injuries. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 ferry flight.
According to the PIC, he and another pilot were ferrying a new airplane from California to Australia. The first transoceanic leg was planned for 10 hours, from Santa Maria Airport (KSMX), Santa Maria, California, to Hilo Airport (PHTO), Hilo, Hawaii. A subsidiary of the airframe manufacturer had an auxiliary ferry fuel line and check valve installed in the left wing after delivery of the aircraft to the United States. About 1 month before the trip, the PIC hired a ferry company to install an internal temporary ferry fuel system for the trip. The pilots attempted the first transoceanic flight on November 2, but the ferry fuel system did not transfer properly, so they diverted.
The system was modified with the addition of two 30-psi fuel transfer pumps that could overcome the airplane’s ejector fuel pump pressure and the ferry system check valve. The final system consisted of two aluminum tanks, two transfer pumps, transfer and tank valves, and associated fuel lines and fittings. 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 installed system was ground- and flight-checked before the trip.
According to Federal Aviation Administration (FAA) automatic dependent surveillance broadcast (ADS-B) data, the airplane departed KSMX about 1000. For the accident flight, the main fuel tanks were full with 402 gallons, the No. 1 ferry tank contained 100 gallons, and the No. 2 ferry tank contained 60 gallons. The fuel utilization procedure for the flight was:
1) Use main tanks until their quantity decreased to 300 gallons;
2) Transfer half of ferry tank number two or when the main tanks reach 350 gallons;
3) Use main tanks until their quantity decreased to 300 gallons;
4) Transfer half of ferry tank number one or when the main tanks reach 350 gallons;
5) Repeat until ferry tanks are empty.
The first halves of the ferry tanks were transferred without any issues except for the occasional illumination of the FUEL IMBALANCE caution light, but that was expected as excess transfer fuel was sent back to the main tanks after passing through the engine. Then the ferry system was used to transfer the remaining fuel from the No. 2 ferry tank. The airframe manufacturer noted that this ferry tank utilization procedure did not give the flight crew a reliable situational awareness of the airplane’s fuel state, as it relied on information the crew did not have (motive flow rate) and on fuel quantity indications on the tanks that were not accessible to the crew while transferring fuel.
About 3.5 to 4 hours into the flight, the airplane was light enough to climb from FL 200 to FL 280. About 5 hours into the flight, the No. 2 ferry tank was almost empty so the pilots prepared to stop transferring fuel from that tank. The ignition switch was placed ON. After the transfer pump was turned OFF, with fuel still visible in the supply line, the FUEL LOW PRESSURE light illuminated.
The PIC had already placed the ignition switch to ON during the ferry transfer, and now set the two in-aircraft boost pumps to the ON position for the end of the transfer process and confirmed the pumps were on with the green L PUMP and R PUMP lights on the Fuel System Status Window and green IGNITION message on the Primary Flight Display (PFD) Engine Window. About 5 seconds after the low-pressure light illuminated, the engine surged and then completely shut down and feathered. The pilots estimated that the engine lost power about 20 seconds after turning the transfer pump off. The fuel quantity in the main tanks and No. 1 ferry tank was about 450 gallons at the time. The fuel temperature in the wing tanks was unknown.
The engine stopped while the aircraft was at FL 280. Although the pilots could not recall what altitude the air start procedures were performed, they knew that 20,000 ft was the maximum altitude for restart according to the Pilot’s Operating Handbook. The pilots used the checklist to perform an air start and the engine started and the propeller unfeathered; however, the engine never reached idle rpm and manipulation of the power control lever did not affect the engine. The engine did not fully start. They shut off the engine per the checklist and then attempted another air start.
During the next start sequence, the engine made a loud grinding noise and then a loud catastrophic “bang.” There was no evidence of smoke or flames from the exhaust on either side of the aircraft. The CAS panel had numerous messages. At some point, the Engine and Propeller Electric Control System (EPECS) FAIL light illuminated, but the pilots could not recall exactly when. As the airplane descended, they attempted multiple air starts, including the procedures for when the EPECS FAIL light was on. The propeller never moved and the engine never started. About 8,000 ft, the pilots committed to ditching the airplane and they commenced the ditching checklist.
After preparing the survival gear, donning life vests, and making mayday calls on VHF 121.5, the PIC performed a full-flaps, gear-up landing at an angle to the sea swells and into the wind. He estimated that the swells were 5 to 10 ft high with crests 20 feet apart. During the landing, the PIC held back elevator pressure for as long as possible and the airplane landed upright. The pilots evacuated through the right over-wing exit and boarded the six-person covered life raft. A photograph of the airplane revealed that the bottom of the rudder was substantially damaged. The airplane remained afloat after ditching.
The pilots used a satellite phone to communicate with Oakland Center. The United States Coast Guard (USCG) coordinated a rescue mission with a nearby oil tanker, the M/V Ariel, for rescue of the crew. According to the pilots, during the night, many rescue attempts were made by the M/V Ariel; however, the ship was too fast for them to grab lines and the seas were too rough. After a night of high seas, the M/V Ariel attempted rescue again; however, they were unsuccessful. That afternoon, a container ship in the area, the M/V Horizon Reliance, successfully maneuvered slowly to the raft, then the ship’s crew shot rope cannons that propelled lines to the raft, and they were able to assist the pilots onboard. They had been in the raft for about 22 hours. The airplane was lost at sea.
Aircraft Information
On April 15, 2020, the FAA issued a Standard Certificate of Airworthiness for the accident airplane. On this model Pilatus PC-12, the aircraft was certified without an air separator in the engine fuel feed line. An air separator in the engine fuel feed line was included on previous models of the PC-12. In addition, the production fuel system design of the accident airplane was such that a Fuel System Icing Inhibitor (FSII) was not required.
On June 9, 2020, the ferry fuel system provisions were designed and installed by the aircraft manufacturer as a major alteration per FAA Form 337. The FAA Form 337, in Section 2.0, stated “The ferry tank provisions feed directly into the engine's fuel supply line.” It also stated, “Ferry tank installations should ensure that no air is introduced into the fuel system.” In addition, the drawings and schematics showed the ferry system connected directly to the engine's fuel supply line.
The FAA Form 337 submitted by the company that installed the ferry system fuel tanks on October 28, 2020, stated in Section 3.0, “The ferry fuel feed is directly to the left main tank.”
During the positioning flight on November 1, 2020, and an attempted ferry flight on November 2, 2020, the ferry fuel system would not transfer any fuel from the ferry tanks. The ferry fuel system was further modified on November 2-5, 2020, by installing two pumps to provide enough fuel pressure (30 psi) to overcome the aircraft’s delivery fuel ejector pump pressure (10 psi) and supply fuel to the engine fuel supply line. The pumps were installed so that either pump could transfer fuel from either ferry tank. Due to the additional changes made to the airplane’s ferry fuel system, a new FAA Form 337 should have been submitted to the FAA before the flight.
On November 6, 2020, the airplane was returned to service. The pilots flew from Merced Airport (KMCE), Merced, California, to KSMX and tested the ferry fuel transfer process with both the front and rear internal tanks and both transfer pumps up to an altitude of 17,500 ft. The system worked as tested. There were no further tests conducted of the ferry fuel system. They refueled at KSMX and departed for Hawaii about 1000.
Fuel System
The installed ferry fuel system altered the fuel flow characteristics of the airplane as compared to the production fuel distribution system. As originally designed, the fuel distribution system transferred fuel from the left and right main wing tanks into the respective collector tanks through one-way valves located between the two fuel tanks. The transfer was facilitated by a transfer ejector pump located in each main tank. Fuel was fed from the collector tanks, through a common manifold, toward the engine primarily via delivery ejector pumps. The nominal output pressure of the delivery ejector pumps was 10 psi. The ejectors were energized by heated, high pressure, regulated motive flow from the engine fuel system. The delivery ejector pumps have a flap valve installed in the outlet to prevent reverse flow through the delivery ejector pumps. An electric fuel boost pump, located in each collector tank, was used to provide fuel if either of the delivery ejector pumps could not supply the required fuel pressure. The nominal output pressure of the boost pumps was 31 psi. The boost pumps are also used to provide fuel pressure for engine start, and to laterally balance the fuel load. From the wing tanks, fuel flowed forward through a firewall shutoff valve, a low-pressure engine-driven pump, an oil/fuel heat exchanger, a fuel filter, and a high-pressure engine-driven pump to the fuel control unit. Unused fuel is returned to the wing tanks through the motive flow line.
Unlike the production fuel system, the ferry fuel system moved fuel to the engine feed line from ferry tanks through a check valve. 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. The ferry system provided fuel to the engine through a firewall shutoff valve, a low-pressure engine-driven pump, an oil/fuel heat exchanger, a fuel filter, and a high-pressure engine-driven pump to the fuel control unit. The excess fuel was then returned to the wing tanks through the motive flow circuit. Unlike the production fuel system, the motive flow fuel going to the delivery ejector pumps would flow out the pump inlet because the flap valve was closed. The fuel in the motive flow line refilled the wing fuel tanks.
FAA Advisory Circular 23-10
The FAA Advisory Circular 23-10 “Auxiliary Fuel Systems for Reciprocating and Turbine Powered Part 23 Airplanes” states the following:
“The requirements for a direct feed auxiliary fuel system are considerably more stringent than those for a transfer auxiliary fuel system. In general, these requirements ensure that an uninterrupted flow of fuel at the required pressure and flow rate is provided to each engine for all operating conditions of the airplane. For turbine engine airplanes, these provisions should be automatic to meet the requirements of §23.955(f)(2). These requirements also address altitude performance effects and low and high temperature fuel aspects as well as providing fuel system independence in at least one configuration. Failure Mode and Effects Analyses (FMEA) are needed to ensure that no hazardous conditions exist due to a failure of the auxiliary system. Continuous engine operation should be verified when the auxiliary tank system is depleted of fuel in order to prevent engine flameout or other unacceptable operating conditions.” The Advisory Circular 23-10 also stated that the auxiliary tank depletion characteristics should also be evaluated to ensure that air entrainment, etc., do not alter main tank performance.
After the ferry fuel system was installed on the airplane, there were no tests or evaluations that 1) addressed altitude performance effects and low and high temperature fuel aspects on the production system due to the ferry system operation; 2) completed an FMEA to ensure that no hazardous conditions existed due to a failure of the auxiliary system; or 3) verified continuous engine operation when the auxiliary tank system was depleted of fuel in order to prevent engine flameout or other unacceptable operating conditions as required in Advisory Circular 23-10.
There was no evaluation of 1) the ferry system’s impact on the production fuel system operating temperature; 2) whether an FSII should be required; and 3) if not having an air separator in the engine fuel feed line would impact the system. Although such an evaluation was not required and was advisory, the underlying certification provisions are required to ensure continuous operation of the fuel system.