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A321, vicinity London Gatwick, UK 2020

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On 26 February 2020, after difficulty starting an Airbus A321 left engine for the first flight of the day, the same difficulty recurred on the third flight followed by subsequent en-route abnormalities affecting both engines. After no fault was found during post flight maintenance investigation, similar problems occurred starting the left engine and after takeoff from Gatwick, both engines malfunctioned and a MAYDAY return followed. The continuing Investigation has already found that the engine problems were attributable to fuel system contamination following the addition of 37 times the maximum permitted dosage of Kathon biocide during prior scheduled maintenance work.
Event Details
When February 2020
Actual or Potential
Event Type
Airworthiness, Loss of Control
Day/Night Night
Flight Conditions VMC
Flight Details
Aircraft AIRBUS A-321
Operator Titan Airways
Domicile United Kingdom
Type of Flight Public Transport (Non Revenue)
Origin London Gatwick Airport
Intended Destination London Stansted Airport
Actual Destination London Gatwick Airport
Take off Commenced Yes
Flight Airborne Yes
Flight Completed Yes
Flight Phase Climb
Location - Airport
Airport vicinity London Gatwick Airport
Tag(s) Air Turnback
Tag(s) Loss of Engine Power
Tag(s) MAYDAY declaration
System(s) Engine Fuel and Control
Contributor(s) Maintenance Error (valid guidance available),
Inadequate Maintenance Inspection
Damage or injury No
Causal Factor Group(s)
Group(s) Aircraft Technical
Investigation Type
Type Independent


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. The Investigation is ongoing but the root cause of the malfunctions has been identified as fuel system contamination.


A Field Investigation is being carried out by the UK Air Accident Investigation Branch (AAIB). Recorded flight data is available for all the flights conducted the previous day as well as the flight under investigation and the maintenance history of the aircraft has also been obtained. 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 A320 series aircraft.

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 for the flight from Stansted to 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 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, it was noted that reverse thrust appeared to operate normally.

Relevant Maintenance Actions

It was found that in the weeks before the flight sequence which culminated in the investigated event, the aircraft involved had undergone scheduled heavy maintenance during which prior testing had indicated the need for biocidal treatment of the fuel tanks. The AMO (approved maintenance organization) 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 licensed 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.

The AMM includes two options for mixing the two, either prior to the mixed fuel being uplifted to the aircraft or using a metering rig during the refuelling process. 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 engineer used the overwing refuel aperture to add the quantity of Kathon he had calculated was required to each wing tank. This quantity was approximately 37 times the recommended dose. The aircraft subsequently returned to the operator’s base on 24 February 2020.

The action taken at Gatwick in response to the Captain’s Technical Log defect entry was performed by a licensed engineer staff employed by an AMO which provided line maintenance for the operator at Gatwick. He acted in accordance with instructions provided by the operator’s Technical Control Office. The applicable (online) Trouble Shooting Manual (TSM) was provided using the Airbus online system which at the time was available in two versions, a ‘legacy’ one and its intended replacement. 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. The version used did not require the user to filter data to a specific aircraft beginning the troubleshooting process although 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”.

Instead, the engineer printed and followed the TSM procedure for “Stall above idle on engine 1(2)” which applied to a different engine to the ones fitted to G-POWN for which the procedure was “Stall of engine 1 or 2 in flight” which “required an extensive examination of the engine, including borescope inspections of the high pressure and Stage 1 low pressure turbine blades”. The incorrect procedure followed resulted in no fault being found during the troubleshooting, so the aircraft was released to service, although at this stage it is not known whether this would have had any relevant consequence. However, it is considered at this stage that had a borescope inspection taken place during the Gatwick trouble shooting action, it is “likely that it would have detected these deposits and, had it done so, it is unlikely that the aircraft would have been released to service”. It is noted that the engineer “was not tasked by aircraft operator with investigating any issues with the No 1 engine [and that] the symptoms presented by each engine were different and no one considered there to be a possible common cause”.

Given the finding in respect of the gross overdosing of the fuel with Kathon and the results from fuel sample testing which indicated significant contamination on the fuel with un-dissolved Kathon, both engines were borescoped and the same observations were made for both engines. The combustion chambers and the convex surfaces of both high-pressure and low-pressure turbine blades were coated in a thin layer of white material which was also present on the high-pressure turbine nozzle guide vanes. In addition, “significant deposits of a brown material were evident on all the combustion chamber swirl cups adjacent to the fuel spray nozzles”. Further examination of the engine fuel system components will be undertaken.

Safety Action taken so far and known to the Investigation has included the following:

  • Both the EASA and the FAA have formally notified affected stakeholders of recent air safety-related events involving Kathon biocide and reminded aircraft owners and operators to ensure that the correct method and dosage is used for approved biocide treatment of aircraft fuel systems.
  • The manufacturer of Kathon has discontinued the use of its product for aviation fuel applications.
  • CFM, the manufacturer of the CFM56-5B engines installed in G-POWN, has issued an ABS 73-A0296 recommending that operators of these engines suspend the use of Kathon during aircraft fuel system biocide treatments as well as similar instructions for other variants of the CFM56 and all General Electric turbofan engines.
  • The AMO that performed the biocide treatment on G-POWN has introduced a new role of ‘technical engineer’ who will be available to assist other licensed engineers and mechanics with technical queries, such as calculations and intends to make it impossible for its personnel to withdraw chemicals in quantities that significantly exceed the maximum permitted for their use.

A Special Bulletin S1/2020 which reports initial progress with the Investigation was published by the AAIB on 21 April 2020.

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