US investigators believe a Pilatus PC-12 turboprop with a modified fuel system ditched in the Pacific Ocean after a build-up of ice, or the emergence of air, led to fuel starvation and a loss of engine power.

The newly-built aircraft’s fuel system had been modified by Flight Contract Services in order to conduct a transoceanic ferry flight, from California to Australia via Hawaii.

This modification – which updated an earlier installation that had not functioned correctly – featured two aluminium tanks fitted in the cabin, transfer pumps and valves, and fuel lines to provide additional fuel capacity for the flight.

The supply line for the ferry fuel fed directly into the main fuel line through a check valve.

While the system was ground- and flight-checked, an advisory circular from the US FAA points out that requirements for a direct-feed auxiliary fuel system are “considerably more stringent” than those for a transfer auxiliary fuel system.

These requirements ensure that the fuel-flow is uninterrupted, and takes place at the necessary pressure and flow-rate for each engine under all operating conditions. They also address performance effects from altitude and changing fuel temperature.

But after the ferry fuel system was installed on the single-engined PC-12, there were “no tests or evaluations” that addressed potential altitude or temperature effects, states the US National Transportation Safety Board.

Nor was analysis carried out on hazardous conditions – including engine flame-out – arising from auxiliary tank failure or fuel depletion.

The inquiry says that the pilots flew a positioning flight, and tested the ferry fuel transfer process up to an altitude of 17,500ft. No further tests were conducted.

Ditched PC-12-c-NTSB

Source: NTSB

Both pilots survived after the PC-12 ditched in the Pacific Ocean

Investigators state that the aircraft (N400PW) departed Santa Maria airport, on a 10h leg to Hilo in Hawaii, on 6 November 2020.

Some 3.5-4h into the flight the aircraft was light enough to climb from 20,000ft to 28,000ft. About 1h later, one of the two ferry tanks was nearly empty so the pilots prepared to stop transferring fuel from that tank.

But during this procedure – about 20s after turning off the transfer pump – the engine surged, then completely shut down with its propeller feathered.

The aircraft still had about 450USgal of fuel in the main tank and remaining ferry tank. The fuel temperature in the wing tanks was unknown.

Attempts to restart the engine failed, and culminated in a “loud grinding noise” and then a “loud, catastrophic ‘bang’,” says the inquiry. As the aircraft descended through 8,000ft the pilots committed to ditching. Both pilot survived the emergency landing but spent 22h in a life-raft before being rescued.

The aircraft, barely six months old, was subsequently “lost at sea”, says the inquiry, and the exact cause of the fuel starvation could not be determined.

But it points out that the modified fuel system “altered the fuel flow characteristics” of the aircraft.

The loss of engine power could have resulted from air being introduced to the fuel line from the ferry system – although boost pumps installed as part of the modification should have compressed the air and forced it through the fuel line.

Investigators also theorise that ice built up inside the fuel tanks during the fuel-transfer operations – restricting fuel flow to the engine, either because the wing-tank fuel was too viscous or a valve became stuck closed.

The inquiry says the aircraft was certified without an air-separator in the engine fuel feed line, nor did its fuel system require an icing inhibitor.

Neither the aircraft manufacturer, nor the company that installed the ferry fuel system, evaluated the impact of the system on operating temperature, or whether an inhibitor might be needed, or any possible effect of not having an air-separator.