A321, vicinity London Gatwick, UK 2020

A321, vicinity London Gatwick, UK 2020


On 26 February 2020, after a difficult Airbus A321 left engine first flight of the day start, the same happened on the third sector with en-route engine abnormalities then affecting both engines. With no fault found during post flight maintenance inspections and despite similar engine starting problems, both engines then malfunctioned after takeoff from Gatwick. A MAYDAY return followed. Investigation found that the cause was fuel system contamination by addition of approximately 38 times the correct quantity of biocide during earlier scheduled maintenance and that the release of the aircraft to service for the flight had followed inadequate troubleshooting action.

Event Details
Event Type
Flight Conditions
Flight Details
Type of Flight
Public Transport (Non Revenue)
Flight Origin
Intended Destination
Actual Destination
Take-off Commenced
Flight Airborne
Flight Completed
Phase of Flight
Location - Airport
Air Turnback
Procedural non compliance
Loss of Engine Power
MAYDAY declaration
Engine Fuel and Control
Maintenance Error (valid guidance available), Inadequate Maintenance Inspection, Dispatch of Unserviceable Aircraft
Damage or injury
Non-aircraft damage
Non-occupant Casualties
Off Airport Landing
Causal Factor Group(s)
Aircraft Technical
Safety Recommendation(s)
Aircraft Airworthiness
Investigation Type


On 26 February 2020, an Airbus A321 (G-POWN) being operated by Titan Airways on a non revenue positioning flight from London Gatwick to London Stansted in night VMC took off just after midnight and almost immediately after it became airborne experienced successive malfunctions of both engines. Following declaration of a MAYDAY the flight crew was able to complete a return and landed back at Gatwick 11 minutes later. 


A Field Investigation was carried out by the UK Air Accident Investigation Branch (AAIB).

Relevant data was successfully downloaded from both the SSFDR and the 2 hour SSCVR. ECAM messaging relevant to the aircraft engines and systems which had been presented to the pilots of the final and previous flights was also examined and ATC radar and Gatwick Airport CCTV recordings were also available as was the maintenance history of the aircraft. Since early findings of the Investigation were conclusive as to the underlying cause of the engine malfunction, a Special Bulletin covering this progress was published on 21 April 2020.   

It was noted that the 28 year-old Captain, who had been PF for the flight, had a total of 5,059 hours flying experience of which all but 204 hours were on type. The 38 year-old First Officer had a total of 1,245 hours flying experience of which all but 162 hours were on A320 series aircraft. 

The 52 year-old Engineer whose actions during a period of scheduled base maintenance at Larnaca Cyprus prior to the series of flights during which engine abnormalities due to fuel system contamination occurred was found to hold a EASA Part 66 Category B1, B2 and C Licence without limitations. He had accumulated a total of 24 years experience which included A320 series aircraft with both permitted engine types (CFM 56 and IAE V2500) installed. He had been working for the MRO (maintenance, repair and overhaul company) involved for four months and his recent duty pattern had been Monday-Friday with weekends off and the weeks alternating between early and late daytime shifts. His first language was not English and although he had “communicated effectively in conversation with the AAIB”, he had spoken predominantly in the present tense even when describing events in the past. He claimed that he had “no difficulty working with procedures written in English”.

The 47 year-old Line Engineer who inspected the aircraft at Gatwick and found no fault prior to its release to service for the investigated flight held a Category A and B1 Licence and had a total of 30 years experience which included A320 series aircraft with both permitted engine types. He had been working for the MRO involved for four months and worked four 12 hour shifts followed by three days off each week. The first two shifts were always days and the second two were always nights. The shift during which he unintentionally released the un-airworthy aircraft to service was his second night shift and therefore his last shift prior to three days off. However, a sleep and work history compiled for the Investigation “did not indicate any sleep or task related risk factors for fatigue”.   

What Happened

It was established that during the 24 hours preceding this Serious Incident, during which three flights had been conducted, the aircraft had experienced abnormalities in engine performance which had eventually resulted in maintenance action which found no fault and had therefore resulted in it being released to service. 

It was noted that the flight crew operating the investigated flight had also operated the inbound non-revenue positioning flight from the operator’s base at London Stansted to London Gatwick in the early hours of the previous day and had then spent the day at a hotel near the airport whilst a second fight crew used the aircraft to operate international passenger flights to Krakow and back. 

After successfully starting the No 2 engine prior to the flight from London Stansted to London Gatwick, the first crew had problems starting the No 1 engine. It was reported that “an engineer, who was assisting with the engine starts via an external headset, advised them to attempt another start on the No 1 engine”, which was successful and the performance of both engines thereafter was normal. 

The engines also functioned normally on the outbound flight to Krakow operated by the second crew but on departure from Krakow for the return flight, the No 1 engine again “required more than one attempt” to achieve a successful start. After getting airborne from Krakow, the Captain sent a datalink message to the aircraft operator notifying them of the starting problem. Later on in the flight, an ECAM ‘ENG 2 STALL’ message was displayed momentarily on two occasions during which the crew reported having felt airframe vibration. On the second occasion, the message occurred during the descent with N1 at about 66%. There was no concurrent change in engine control indications, but having “perceived” that the vibration had been less at lower thrust settings, they attempted to maintain the N1 at below 50%. 

Following arrival at Gatwick, the Captain telephoned the operator’s Technical Control Office and reported the No 2 engine stall event and made a corresponding defect entry in the aircraft Technical Log. He then spoke to the Captain who had brought the aircraft to Gatwick earlier in the day and would be taking it back to Stansted to update him on the situation.

The attending engineer could find no fault which would explain the defect entry made and so cleared the defect entry and released the aircraft to service. The oncoming Captain “agreed with Technical Control that he would accelerate the engines to 50% N1 for longer than usual" before takeoff to “check the engine control indications”. For the third time, the No 2 engine started normally but the No 1 did not and the Captain decided to speak again to Technical Control who “suggested that the No 1 engine’s abnormalities were only associated with starting and to attempt another start”. As on the previous occasions, this was successful and after normal engine indications with the engines at 50% N1, takeoff was commenced.

The Captain reported that climbing through about 500 feet agl, the No 1 engine had begun “banging and surging” with corresponding fluctuations in its control indications but no  accompanying ECAM message. FDR data showed that at this time, the No 1 engine N1 had dropped below 40% for a period of approximately 25 seconds with the thrust levers remaining set to FLEX/MCT. A number of the staff passengers (cabin crew) reported having seen flames coming from the No 1 engine tailpipe and had attempted to contact the flight deck by interphone. The Captain made a MAYDAY call and requested a return and then spoke to the passengers. He disengaged the AP and turned right onto a downwind leg (see the illustration below) and then set the No 1 engine thrust lever to idle.  

An annotated radar-derived flight path for the investigated flight. [Reproduced from the Official Report]

Subsequently, he “recalled seeing the No 2 engine control indications begin to fluctuate” and just after beginning descent, the ECAM ‘ENG 2 STALL’ message appeared “three times in quick succession” which prompted the Captain to increase thrust on the No 1 engine. He subsequently stated that both engines had appeared more stable when the thrust was reduced while descending and he had decided to keep the N1 of both engines at “around 49%”. The aircraft was then positioned for a 9 nm final approach to runway 27L and intentionally flown slightly above the ILS GS “so he could glide the aircraft to the runway if the engine problems worsened”. The approach and landing were completed without further problems and after touchdown following eleven minutes airborne, it was noted that reverse thrust appeared to operate normally. 

Chemical analysis of the small amount of debris present in both engines’ fuel filters indicated unusually high levels of magnesium, a constituent element present in the biocide product trade name ‘Kathon’. Both engines were subjected to an initial visual inspection by borescope on the day after the flight under investigation. The condition of both engines was similar. There was apparently no significant damage to the fan, LP compressor or HP compressor with minor defects observed being within AMM tolerances. However, the combustion chambers, HPT blades and LPT blades of both engines “were coated in a thin layer of white material that was observed on the turbine blades’ convex surfaces” as were the HPT nozzle guide vanes but as the engines were not disassembled, it was not possible to obtain a sample of this deposit. A subsequent borescope inspection did not find any damage to the No 1 engine which exceeded AMM limits but 16 blades of the Stage 3 HP compressor of the No 2 engine and one Stage 7 HP compressor blade were found to have smooth impact depressions on the blade root radius which were outside AMM limits. It was not possible to tell if this damage was present before the investigated flight departed. Both engines’ Hydro Mechanical Units were removed and sent to the OEM where they were found to be contaminated with a sticky brown residue which infrared analysis showed was a mixture of dipropylene glycol, nitrate salt and water which was consistent with the chemical composition of Kathon biocide. Fuel removed from the aircraft was also found to indicate contamination with undesolved Kathon.

Relevant Maintenance Actions

It was found that prior to the four flights which had culminated in the one under investigation, the aircraft involved had undergone scheduled heavy maintenance at a Cyprus-based MRO. The work package for this maintenance input included biocidal treatment of the fuel tanks which had been added after prior testing had found “moderate” contamination. The AMO Work Card for this task did not designate this as a ‘Critical’ maintenance task. The work was required to be carried out in accordance with a detailed AMM procedure in which fuel is mixed with Kathon biocide at a concentration of 100 ppm by volume and the aircraft is then pressure-refuelled using the onboard automatic control function. 

The AMOs base engineer was unfamiliar with the term ‘ppm’ and it was not explained in the AMM. He stated that he had therefore “searched the internet for a definition and conversion calculator” and using the one he found had calculated that he needed 30 kg of Kathon for the 6,200 kg of fuel which would be uplifted to each wing tank. However, taking the slightly different Specific Gravities of Kathon and fuel into account, the correct quantity of Kathon was 0.799kg per wing tank, a dramatically different figure to 30kg. He was unable to remember what website he had used or how he performed the calculation but had concluded that 60 kg of the AMM-required ‘Kathon’ biocide would be required for each of two Airbus A320 aircraft due to be treated during January/February 2019 and had made a corresponding ‘material requisition’ for this quantity in good time so that it was available in the stores when the first aircraft was scheduled to receive the treatment. 

A different engineer, who was also unsure what ‘ppm’ meant, was assigned to carry out the treatment on the other aircraft ahead of G-POWN and having consulted the engineer who had ordered the biocide for advice then added it at a rate of 3814 ppm instead of the specified 100 ppm. However, the engines of this other aircraft were not subsequently started and still had not been started when information about the G-POWN, which had subsequently received the same overdose, were received. It was noted that the AMM included two options for mixing the biocide and fuel, either prior to the mixed fuel being uplifted to the aircraft or using a metering rig during the refuelling process. However, there was no access to a metering rig and there were no instructions on how to mix the additive and the fuel prior to uplift so despite the fact that it was not an approved AMM process, the overwing refuel aperture was used to add the biocide to each wing tank. These two biocide treatments were only the second and third such treatments which the MRO involved had carried out since its first some eight months earlier. This first treatment had been overseen by a similarly licensed engineer but he also had a degree in Mechanical Engineering and so was familiar with the term ‘ppm’. Following completion of the biocide treatment, the aircraft was positioned from Larnaca to the operator’s base at London Stansted for a night stop before it was further positioned to London Gatwick the following morning to undertake the revenue flight to Krakow and back.    

The action taken at Gatwick in response to the Captain’s Technical Log defect entry was performed by a licensed engineer contracted by the operator to provide line maintenance services for them at Gatwick. He acted in accordance with instructions provided by the operator’s Technical Control Office. The applicable A320 Trouble Shooting Manual (TSM) was provided using the Airbus online system (Airbus World) which at the time was available in two versions, a ‘legacy’ one (AirN@v) and an updated version (airnavX). The engineer was obliged to use the ‘legacy’ version since the aircraft operator had only granted the AMO providing the engineer with access to that version. This legacy version of the Manual did not require the user to identify which of the available data was applicable to a specific aircraft or engine type before beginning the trouble shooting process although the latter was possible and Airbus “recommended that the TSM must always be filtered for a specific aircraft registration, fleet serial number or effectivity and that it should be accessed using the ‘Start Troubleshooting’ function”.

Having not first identified the appropriate aircraft-specific procedure, the engineer then printed out and followed the TSM procedure for “Stall above idle on engine 1(2)” for the V2500 engine instead of the “Stall of engine 1 or 2 in flight” for the CFM engines fitted to G-POWN. The latter “required an extensive examination of the engine, including borescope inspections of the high pressure and Stage 1 low pressure turbine blades” whereas the incorrect procedure followed resulted in no fault being found and the aircraft being released to service, a very significant consequence. 

A survey of AMOs with EASA 145 approval was undertaken to find out how often they carried out biocide fuel tank treatments and led to the finding that although a few carried out a lot of them, over half only performed them relatively infrequently which indicated that this task was “generally not a routine or common procedure”. Also, few of the AMOs responding to the survey classified the dosing task as a critical maintenance task in accordance with EASA Part M.A.402(h) and most did not provide specific training for it with those who did stating that this mainly related to the use of metered injection rigs.

It was found that the Cyprus MRO where the biocide dosing error occurred had only recently begun trading as such with considerable evidence indicating that the level of work contracted was stretching capability beyond levels general conducive to safety. In the case of the Gatwick line engineer’s failure to identify the correct inspection response for the documented defect recorded in the aircraft Technical Log, the same scenario presented to other engineers unfamiliar with the occurrence did not result in all of them using an approved TSM process although most did do so and as a result identified the correct procedure. It was noted that the most routine competency assessment for the engineer who had responded to the defect which had recently been performed by the organisation’s station manager had not documented evidence of his method of using the TSM or his confirmation that maintenance data printed was “correct for the task and had the correct effectivity”. Overall there were potentially significant concerns about the way both training and competency assessment appeared to be being conducted at the AMO involved.     

Other Similar Serious Incidents

It was noted that the investigated event was not the first in-flight event involving the malfunction of both engines which the subsequent independent investigation had attributed to incorrect biocide dosing. The 2019 event to a Boeing 787-8 in Japan was published by the Japan Transport Safety Board whilst the Gatwick event Investigation was being carried out.

Safety Action taken as a result of this event whilst the Investigation was in progress and known to it was noted as having included, but not been limited to, the following: 

  • Within a month of the occurrence, both the EASA and the FAA issued Information Bulletins to alert aircraft operators and their aircraft maintenance organisations of the importance of effective risk management in relation to the use of biocide fuel additives.
  • The manufacturers of Kathon, the biocide involved, almost immediately discontinued the use of the product for all aviation fuel applications and both CFM and GE recommended suspending the use of Kathon during aircraft fuel system biocide treatments.
  • Airbus has begun revising the AMMs for all its aircraft types to replace ‘ppm’ with the term ‘ml/1,000ltrs’ and notified its intention to include a definition of ‘ppm’ in the AMM glossary in cases where this term is used elsewhere. 
  • The IATA Technical Fuel Group established an informal Biocide Task Force to facilitate sharing of industry experience and best practices between airlines, AMOs and OEMs and support both the development of an equipment standard for biocide metered injection systems and research into alternative biocide products.

Five Safety Recommendations were made as a result of the Investigation as follows:

  • that the European Union Aviation Safety Agency amend the Acceptable Means of Compliance AMC2(a)(3) for regulation Part-145.A.48(b), Performance of Maintenance, to include the treatment of aircraft fuel systems with biocide additives as an example task that is to be considered as a critical maintenance task. [2020-018]
  • that the European Union Aviation Safety Agency amend the Acceptable Means of Compliance AMC1(c) for regulation M.A.402(h), Performance of Maintenance, to include the treatment of aircraft fuel systems with biocide additives as an example task that is to be considered as a critical maintenance task. [2020-019]
  • that the European Union Aviation Safety Agency (EASA) conduct safety promotion with the National Aviation Authorities of EASA Member States to promote the classification of biocide treatment of aircraft fuel systems as a critical maintenance task. [2020-020]
  • that the European Union Aviation Safety Agency, during future audits of Continued Airworthiness Management Organisations and Approved Maintenance Organisations for which it is the Competent Authority, include a check that consideration has been given to the classification of biocide treatment of aircraft fuel systems as a critical maintenance task. [2020-021]
  • the Civil Aviation Authority (CAA), during future audits of CAA-approved Continued Airworthiness Management Organisations and Approved Maintenance Organisations, include a check that consideration has been given to the classification of biocide treatment of aircraft fuel systems as a critical maintenance task. [2020-022]

The Final Report of the Investigation was published on 4 May 2021.

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