Titan A321 at London on Feb 26th 2020, left engine surged, engine stall indications for right engine
Last Update: May 4, 2021 / 14:18:10 GMT/Zulu time
The UK AAIB reported the occurrence was rated a serious incident, the AAIB is investigating. The aircraft sustained minor damage.
On Mar 30th 2020 The Aviation Herald received information that overdoses of biocides contributed to the occurrence deteriorating the performance of both engines as well as the APU, requiring the change of both engines and the APU. CFM and Boeing released Alert Service Bulletins requiring the suspension of the use of Kathon FP 1.5 Biocide Treatment, the FAA followed up with a SAIB.
On Mar 25th 2020 the FAA released a Special Airworthiness Information Bulletin (SAIB) NE-20-04 reasoning:
In engines and aircraft where biocides are approved for use, the manufacturers provide procedures in their AMMs for the application of these biocides into the aircraft fuel tanks. Several recent events have been documented showing adverse engine effects on the ground and in-flight after application of a biocide treatment of the aircraft. Two of these events were the result of overdosing the fuel system beyond the recommended dosage, however, one event has shown no evidence of misapplication. While lack of clarity of the AMM procedures, or lack of adherence to those procedures by the maintenance personnel, may have contributed to the overdosing events, evidence suggests that some engine models are more sensitive to Kathon FP1.5 concentration than others.
On Apr 21st 2020 the AAIB released a special bulletin reporting the aircraft had already experienced trouble upon starting the left hand engine for the previous three flights, with the #1 engine requiring repeated attempts to start. In each case maintenance checked the engine with no findings and recommended another start attempted which was successful.
On the 4th flight the aircraft had again experienced problems with the left hand engine start up, a second start attempt was successful. The AAIB then described the sequence of events:
Crew A reported accelerating the engines to 50% N1 against the footbrakes on Runway 26L (Rwy 26L). The engine control indications appeared normal, so they commenced the takeoff at 0009 hrs.
At around 500 ft agl, the No 1 engine began banging and surging. Commander A recalled that the engine’s control indications were fluctuating, and the aircraft was “yawing… and fishtailing… all over the place”. There was no accompanying ECAM message. Data recorded on the flight data recorder subsequently showed that the No 1 engine N1 reduced below 40% for a period of approximately 25 seconds despite the thrust levers remaining in the FLEX/MCT detent.
A number of cabin crew saw flames coming from the No 1 engine’s tailpipe and attempted to contact crew A using the interphone.
Commander A transmitted a MAYDAY call, requesting a return to Rwy 26L and issued an alert call to the cabin crew. He disengaged the autopilot and turned right, downwind. He moved the No 1 engine’s thrust lever to idle. At one stage after doing so, he recalled seeing the No 2 engine’s control indications begin to fluctuate.
Just after commencing descent from around 3,600 ft agl, the ECAM message eng 2 stall was displayed three times in quick succession. This prompted commander A to move the No 1 engine’s thrust lever forward out of idle. He commented that both engines appeared more stable when the thrust was reduced while descending, and he aimed to maintain each engine’s N1 at around 49%.
Co-pilot A prepared the aircraft’s flight management guidance system for a return to Rwy 26L, and commander A positioned the aircraft on a 9 nm final approach. He opted to fly slightly above the glidepath in order to minimise the thrust required by the engines, and so he could glide the aircraft to the runway if the engine problems worsened. The aircraft landed at 0020 hrs, with the reverse thrust appearing to function normally.
The aircraft had been scheduled for extensive maintenance in late January 2020. As prerequisite fuel samples were taken from the fuel tanks for tests for microbiological contamination, moderate contamination was found. A second test confirmed the contamination, which triggered biocidal treatment.
The AAIB wrote:
The operator’s work card called for biocidal shock treatment for moderate contamination with fuel mixed with Kathon FP1.5 biocide (Kathon) in accordance with AMM Task 28-11-00-600-008-A01, Biocidal Shock Treatment for Moderate Contamination - With Fuel Mixed with Kathon Biocide. The biocidal treatment was not designated a ‘Critical’ maintenance task by the AMO. The task states that fuel should be mixed with Kathon biocide at a concentration of 100 parts per million (ppm) by volume and then the aircraft pressure-refuelled using the onboard automatic control functionality in accordance with AMM Task 12-11-28-650-003-A, Pressure Refuel with Automatic Control. The Kathon-dosed fuel should remain in the aircraft fuel tanks for 24 hours.
The EASA Part-66 B1 licensed AMO engineer was not familiar with the term ‘ppm’. It was not written in expanded form anywhere in the AMM Task or glossary, and the AMM task did not provide instructions about how to perform the calculation of how much Kathon to use. He therefore searched the internet for a definition and conversion calculator. The AMO engineer knew that he would be uploading 6,200 kg of fuel into each wing tank and, using an internet calculator, he calculated a quantity of 30 kg of Kathon for each wing tank. There was 150 kg of Kathon available in the AMO stores and so he made a material requisition for 60 kg of Kathon.
To achieve a concentration of 100 ppm by volume, the following calculation should be made:
Fuel uplifted: 6,200 kg with a Specific Gravity of 0.80812 = 7,678 litres
100 ppm = 0.0001
7,678 x 0.0001 = 0.768 litres of Kathon
Using a Kathon Specific Gravity of 1.04 = 0.799 kg per wing tank.
In the AMM there are four tasks for the shock treatment for moderate contamination: two tasks with Kathon biocide; and two with Biobor JF (Biobor), which is not currently registered for use in the European Union. For each biocide there is a task for mixing it with fuel prior to the mixed fuel then being uplifted to the aircraft, and a second, alternative, task for adding the biocide via a metering rig during the refuelling process. The manufacturer does not provide instructions for a method of mixing the biocide with fuel. The biocide dosing task was to be combined with the fuel tank leak check, and the AMO engineer responsible for the task used the overwing refuel aperture to add the Kathon13. The AMO engineer added 30 kg of Kathon to the left wing tank through the overwing aperture and a further 30 kg of Kathon to the right wing, also through its overwing aperture, whilst he uplifted 6,200 kg of fuel into each wing tank. The Kathon-dosed fuel, at 3,750 ppm (by volume), approximately 37 times the recommended dose, was left in situ for 24 hours in accordance with the AMM task, and the engine and APU fuel filters where changed.
The next day 6,400 kg of fuel was transferred from the wing tanks to the centre fuel tank and again left in situ for 24 hours. After this time the task card was stamped as complete, with a further task opened to perform a biological contamination check within 10 days but after at least 5 flights. The aircraft departed the AMO and returned to the operator’s base on 24 February 2020.
Examination of the fuel tanks following the serious incident revealed:
Following the serious incident, fuel samples were taken from the left and right wing fuel tank water drain valves and were subjected to laboratory analysis. When the fuel was tested it was found not to comply with the JET A-1 specification requirements14 for appearance and water separation characteristics (MSEP15). The fuel samples, once the contents had settled out under gravity, contained a separate brown liquid layer beneath the main fuel layer. Trace element results of the fuel and the bottom brown layer showed similar spectra to a reference Kathon sample, but with a higher water content. The laboratory that conducted the fuel testing commented that:
‘The results indicate contamination with undissolved Kathon. It was noted that the bottom layer that is mostly Kathon plus some unknown products and water, suspected to be causing the darker colour than the reference Kathon sample. This is likely due to the glycol type solvent used in Kathon product dissolving polar materials from the fuel and fuel tank surfaces. This may be analogous to observations with another similar glycol additive, FSII (Fuel System Icing Inhibitor), which is used in military jet fuels. It is colourless but forms brown additive/water layer in tank bottoms.’
The engines’ fuel filter and filter bowl fuel samples were also analysed. The fuel filters were clean in appearance and generally free from debris, however chemical analysis of the small amount of filter debris present indicated unusually high levels of magnesium, a constituent element present in Kathon.
The AAIB discussed:
The excessive level of Kathon in the aircraft’s fuel system is suspected to have caused the subsequent problems with the aircraft’s engines, including those experienced during the incident flight. The AAIB is also aware of other events where engine performance was affected by over-dosing of fuel with biocide. Visual inspection confirmed the presence of abnormal deposits within both engines downstream of the fuel spray nozzles. The influence of the over-dosed fuel on the engines’ HMUs and other fuel system components is subject to the ongoing AAIB investigation.
Before the incident flight, there were start-up difficulties with the No 1 engine and momentary eng 2 stall messages associated with the No 2 engine on descent into Gatwick. An engineer was tasked with troubleshooting the engine stall messages. This intervention was a potential opportunity to detect the abnormal deposits on the high pressure and low pressure turbine blades. It is considered likely that a borescope inspection would have detected these deposits and, had it done so, it is unlikely that the aircraft would have been released to service. The engineer was not tasked with investigating any issues with the No 1 engine. The symptoms presented by each engine were different and no one considered there to be a possible common cause.
On May 4th 2021 the AAIB released their final report concluding the probable cause of the serious incident was:
The investigation identified the following causal factors:
- G-POWN’s fuel tanks were treated with approximately 38 times the recommended concentration of Kathon.
- The excessive Kathon level in the aircraft’s fuel system caused contamination of the engine HMUs resulting in a loss of correct HMU regulation of the aircraft’s engines.
- 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:
- 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.
- 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.
- 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.
- 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.
This article is published under license from Avherald.com. © of text by Avherald.com.
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