Aircraft Accident Report AAR 1/2021 - Airbus A321-211, G-POWN, 26 February 2020
Engine malfunction after takeoff from London Gatwick Airport.
Introduction
The Air Accidents Investigation Branch (AAIB) became aware of this serious incident on 26 February 2020. In exercise of his powers, the Chief Inspector of Air Accidents ordered an investigation to be carried out in accordance with the provisions of Regulation (EU) 996/2010 (as amended) and the UK Civil Aviation (Investigation of Air Accidents and Incidents) Regulations 2018.
The sole objective of the investigation of an accident or serious incident under these regulations is the prevention of accidents and serious incidents. It shall not be the purpose of such an investigation to apportion blame or liability.
In accordance with established international arrangements, the following safety investigation authorities appointed Accredited Representatives to the investigation: the Bureau d’Enquêtes et d’Analyses pour la Sécurité de l’Aviation Civile (BEA) in France, representing the State of Design and Manufacture of the aircraft; the National Transportation Safety Board (NTSB) in the USA alongside the BEA, representing the State of Design and Manufacture of the engines; and the Aircraft Accident and Incident Investigation Board of Cyprus. The aircraft operator, various maintenance organisations, the European Union Aviation Safety Agency (EASA), and the UK Civil Aviation Authority (CAA) also assisted with the investigation.
Summary
As part of scheduled maintenance overseas, G-POWN underwent a biocide shock treatment on its fuel system, using Kathon biocide, to treat microbial contamination. The aircraft returned to the UK on 24 February 2020, once the maintenance was complete.
In the 24 hours preceding this serious incident, there were abnormalities with the operation of both engines across four flights. On the flight before the fourth (event) flight, the crew reported momentary indications of a No 2 (right) engine stall. After the aircraft landed, this was investigated using an inappropriate procedure obtained from an aircraft troubleshooting manual not applicable to G-POWN, but no fault was found.
The aircraft took off from London Gatwick Airport Runway 26L at 0009 hrs on 26 February 2020 but, at around 500 ft agl, the No 1 (left) engine began to surge. The commander declared a MAYDAY and turned right downwind for an immediate return to the airport but, shortly afterwards, the crew received indications that the No 2 engine had stalled. The crew established that the engines were more stable at low thrust settings and the thrust available at those settings was sufficient to maintain a safe flightpath. They continued the approach and the aircraft landed at 0020 hrs.
The investigation identified the following causal factors:
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G-POWN’s fuel tanks were treated with approximately 38 times the recommended concentration of Kathon.
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The excessive Kathon level in the aircraft’s fuel system caused contamination of the engine Hydro Mechanical Units (HMU) resulting in a loss of correct HMU regulation of the aircraft’s engines.
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A troubleshooting procedure was used for the engine No 2 stall that applied to LEAP-1A32 engines, but G-POWN was fitted with CFM56 5B3/3 engines. The procedure for CFM56-5B3/3 engines required additional steps that would have precluded G-POWN’s departure on the incident flight.
The investigation identified the following contributory factors:
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The Aircraft Maintenance Manual (AMM) procedure did not provide enough information to enable maintenance engineers to reliably calculate the quantity of Kathon required, and the specific gravity value of Kathon was not readily available.
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There were no independent checking procedures in place at the base maintenance Approved Maintenance Organisation (Base AMO) to prevent, or reduce the likelihood of, calculating and administering an incorrect quantity of biocide.
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There were organisational factors at the Base AMO that contributed to the incorrect Kathon quantity calculations. In particular, the workload was high for the available facilities and personnel, and there was no internal technical support function for engineers to consult when they were uncertain.
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The manufacturer’s recommended method of searching the troubleshooting manual was not used to find the applicable procedure relating to the engine No 2 stall.
Following this serious incident, Safety Action was taken by regulators, the International Air Transport Association, the manufacturers of the aircraft, engines and biocide, the AMOs involved, and the operator. The specific action taken is detailed in Section 4.2 of this report.
Redundancy in safety critical systems is one of the principles supporting the safety of commercial air transport but fuel contamination undermines that redundancy because it can affect all engines simultaneously. It is essential that maintenance systems are resilient to errors that can lead to fuel system contamination. Therefore, five Safety Recommendations have been made in this report to promote the classification of biocide treatment of aircraft fuel systems as a critical maintenance task, which would ensure that an error-capturing method is included as part of the task.
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Special Bulletins:
This Special Bulletin, published on 21 April 2020, contains preliminary information about the investigation and is intended to highlight the importance of using correct procedures when dosing fuel with biocide to combat microbial contamination.
Safety Recommendation document: