What happened
On 26 February 2020, an Airbus A321-211, registration G-POWN, departed London Gatwick Airport for London Stansted Airport. Shortly after takeoff, at an altitude of approximately 500 feet, the No 1 engine began surging, accompanied by loud noises and visible flames from the tailpipe. The crew declared a MAYDAY and initiated a return to the airport. During the maneuver, the No 2 engine also began to show fluctuating parameters and experienced a stall indication. The aircraft successfully landed at 0020 hrs with no injuries to the seven crew members.
The investigation
The investigation revealed that in the days leading up to the incident, the aircraft had experienced several engine abnormalities. During scheduled maintenance, an engineer attempted to treat the fuel tanks for microbial contamination. Using an internet calculator to determine the required amount of Kathon biocide, the engineer mistakenly added 30 kg of the chemical to each wing tank. This resulted in a concentration of approximately 3,750 ppm, which was roughly 37 times the recommended dosage.
Furthermore, when the aircraft experienced engine stall indications prior to the incident flight, a maintenance engineer at Gatwick attempted to troubleshoot the issue. However, because the engineer did not filter the troubleshooting manual for the specific aircraft registration, they applied a procedure intended for a different engine type (the LEAP-1A engine) rather than the CFM56 engines fitted to G-POWN. This incorrect procedure did not require the borescope inspections that likely would have identified the engine deposits.
Findings
- The primary cause of the engine malfunctions was the extreme over-dosing of Kathon biocide into the fuel system.
- Laboratory analysis of fuel samples confirmed the presence of undissolved Kathon and a brown liquid layer, indicating the fuel did not meet standard specifications.
- Borescope inspections of both engines revealed abnormal brown and white deposits on the combustion chamber swirl cups and turbine blades.
- The maintenance engineer's calculation error was compounded by the lack of a clear definition for 'ppm' within the aircraft maintenance manual.
- The use of an incorrect troubleshooting procedure prevented the early detection of engine contamination.