How an internet search helped cause dual A321 engine stall

Categorised as a “serious incident” by investigators, the simple mistake by engineers caused the powerplants to begin surging at just 500ft after take-off

A Titan Airways Airbus A321 experienced a dual engine stall shortly after take-off from London/Gatwick last year as a result of a miscalculation during a fuel tank treatment procedure, a report by the UK’s Air Accidents Investigation Branch has found. 

Titan Airways A321
The incident aircraft is owned by ACMI carrier Titan Airways. It was leased to Jet2 every summer from 2016 until 2019. Aviation Image Network/Bailey

The jet, G-POWN (c/n 3830), had recently returned from an overseas base maintenance organisation where it had undergone biocide shock treatment designed to eliminate microbial contamination in the fuel tanks. 

The flight 

Just after midnight on February 26, 2020, the narrowbody departed the London hub and at approximately 500ft, engine number one began “banging and surging” which the commander recalled, caused the aircraft to start “yawing… and fishtailing all over the place”.  

No indications on the electronic centralised aircraft monitor (ECAM) displays were seen at this time, but post-flight analysis of the flight data recorder (FDR) showed the powerplant’s N1 rotation had reduced below 40% despite the thrust levers being in the FLEX/MCT detent. 

Aviation Image Network/Simon Gregory
G-POWN was originally delivered to Austrian low-cost airline Niki in 2009 before it was acquired by Titan in 2016. Aviation Image Network/Simon Gregory

According to the report, a number of cabin crew saw flames coming from the turbofan’s tailpipe and attempted to contact the pilots using the interphone. 

Approximately 30 seconds after the surging began, the captain transmitted a MAYDAY call requesting an immediate turn to runway 26L. A right turn was commenced while the autopilot was disengaged. The power lever for the affected engine was also moved to idle. 

Upon reaching the downwind leg, the jet levelled at 3,600ft. When adjacent to the approach end of 26L, a descent was initiated shortly after which, an ECAM message indicated an engine number two stall.  

AAIB flight track
The aircraft touched down at Gatwick just 11 minutes after taking off. AAIB

Engine No 1’s thrust lever was brought out of idle at this point. The commander reported that both engines appeared more stable when the thrust was reduced while descending, so he aimed to maintain around 49% N1 on the pair. 

The narrowbody was position on a 9nm final approach for runway 26L. The commander flew slightly above the glide path to minimise engine thrust and so he could reach the landing area if the engine problems worsened. 

At 0020hrs – around 11 minutes after it departed – the A321 landing back on the ground safely. 

Incident analysis 

Whiskey November was powered by two CFM International CFM56-5B3/3 turbofan engines, both of which had around 28,000 flight hours and 14,000 cycles. Airbus offers two engine choices with the type which also include the IAE V2500. 

Six days before the incident, the jet arrived at the overseas base maintenance organisation for its scheduled maintenance. Following preparation by the operator in November 2019, it was determined that a biocide shock treatment was required so this was added to the work package. 

The job required a chemical called Kathon to be added to the fuel tanks so it could eradicate microbes in the fuel system. An engineer at the overseas MRO facility studied the treatment procedure in December 2019 in preparation for similar work on another A321. 

According to the report, he did not understand the term “PPM” – which stands for “parts per million”. This initial calculation was used as the basis for Kathon orders for both jets. By the time Whiskey November needed treating, there was 150kg of the chemical available in the firm’s stores. 

The engineer had not done a biocide treatment before and expected it to be an “easy job”. While not being familiar with the term “PPM”, he attempted to find it in the aircraft maintenance manual glossary but could not find it.  

He also could not find any engineer at the facility who knew how to calculate the biocide quantity, so he searched the internet and found an online calculator. The end result was determined to be 30kg of Kathon for each wing tank. 

The chemical was administered directly into the overwing port at the same time as the tanks were also filled with fuel. On completion, the Kathon-dosed fuel had a concentration of 3,814ppm by volume, which was approximately 38 times the recommended strength of 100ppm. The correct dosage was determined by investigators to be 0.799kg per tank. 

AAIB fuel picture
Fuel samples drained from both the left and right wing tanks following the incident. AAIB

The end result of this incorrect procedure was the blockage of hydro-mechanical units (HMUs) resulting in a loss of correct HMU regulation of the aircraft’s powerplants.  

Prior to departure from Gatwick, the jet’s number two engine was troubleshot for engine surging issues that had occurred on the previous flight. The AAIB concluded that the third causal factor to the incident – in addition to incorrect Kathon dosing and resulting HMU clogging – was the use of a troubleshooting procedure which applied exclusively to the CFM International LEAP-1A32 engines – which are found on the A321neo – and not the CFM56 powerplants on Whiskey November. 

If the correct procedure had been applied, it would have prevented the departure from taking place. 

Following the “serious incident”, safety action was taken by regulators, IATA, manufacturers of the aircraft, engines and biocide, the maintenance organisations involved, and the operator.