What happened
On March 29, 2019, a Jetstar Airways Boeing 787-8, registration VHVKJ, was performing a scheduled flight from Cairns to Kansai International Airport. While descending at approximately 4,900 meters, the aircraft's right engine began displaying unstable parameters, accompanied by several EICAS messages, including an engine fuel split valve alert. Shortly after, at an altitude of about 3,600 meters, the left engine also experienced a temporary drop in RPM, falling below idle for eight seconds. The right engine followed a similar pattern, with its RPM dropping below idle for 81 seconds. The crew responded by disengaging the right auto-throttle and setting the thrust lever to idle to manage the unstable parameters. The aircraft landed safely at Kansai International Airport without injuries or damage.
The investigation
The JTSB investigation focused on the engine fuel components, which were sent to a laboratory in the United States. Investigators discovered residue in the fuel filters and various engine spools, including the fuel metering valve (FMV) and fuel splitting valve (FSV). This residue was primarily composed of magnesium salts. The investigation also reviewed maintenance records, revealing that a biocide treatment using Kathon FP1.5 had been performed on the aircraft two days prior to the incident. Laboratory tests confirmed that magnesium salts from the biocide can precipitate and accumulate in engine components when they encounter water in the fuel. Furthermore, the investigation examined the mixing process of the biocide-treated fuel within the aircraft's tanks.
Findings
- The engine RPM oscillations and subsequent drops below idle were caused by magnesium salt residue accumulating in the fuel metering and splitting valve spools, which restricted their movement.
- The accumulation of this residue was linked to the biocide treatment conducted two days before the incident.
- It is probable that the fuel containing a higher concentration of biocide did not mix uniformly with the existing fuel in the wing tanks, leading to the delivery of high-concentration biocide to the engines.
- While the maintenance staff intended to reach a 100 ppm concentration, the lack of recorded calculations for the dosage in the left and right tanks meant that the actual concentration in those tanks likely reached much higher levels (approximately 250–285 ppm).