Concentration error led to Gatwick aircraft stall and MAYDAY

by Callum Tennant
The affected aircraft

A significant maintenance error was responsible for an aircraft’s engine stalling, and an ensuing MAYDAY call at Gatwick on 26 February 2020, a formal report by the UK’s Air Accident Investigation Branch (AAIB) has found. The mistake led to the aircraft’s fuel tanks being treated with a bio-cleaning product that was 38 times above the recommended concentration level.

Flight plan

The sequence of events on 26 February 2020 © AAIB

From runway to MAYDAY

At 23:51 on 25 February, an A321-211 aircraft, operated by Titan Airways, pushed back from its stand at Gatwick Airport. As the pilots started Engine 2, everything seemed normal. However, when the pilots attempted to start Engine 1, the co-pilot reported an ECAM warning of an ENGINE 1 START FAULT and ENG 1 IGNITION FAULT. The pilot tried once again to start Engine 1: ENG 1 FAIL. The crew had been informed that the same warnings also occurred on the previous flight.

The pilot then rung technical control, they advised to try again. At this stage, the pilot had decided that if the engine didn’t start on the third attempt, the crew would return to the stand and reconsider whether the plane could depart. On the third attempt, a brief ENG 1 FAIL ECAM warning flashed up, before automatically extinguishing. With engine performance appearing normal, the commanding officer rung technical control for a final time. They advised that the issue would likely be resolved once the engines were running; the pilot was given the all clear and pushed back.

At 00:09 on 26 February, the A321 powered down runway 27L at Gatwick Airport, before taking off. However, just 15 seconds into the flight, at 500ft, the aircraft’s left engine started banging and surging, with the thrust falling below 40%. The aircraft began “yawing and fishtailing all over the place”, with cabin crew seeing flames coming out the left-hand engine.

A MAYDAY call was issued, and the pilot reduced engine 1 to idle. The aircraft began turning around, as the crew prepared for an immediate return to Gatwick, however, less than two minutes later, instruments indicated that the right-hand engine had stalled. The pilots brought the left-hand engine out of idle, continuing the return with reduced power to the engines, finding that this made the aircraft’s performance more stable.

The crippled jet managed to make it back to Gatwick, touching down just 11 minutes after it had first taken off. Emergency services, which attended the aircraft after it landed, reported hearing unusual engine noises, leading to them being shut down on a nearby taxiway. Inspection of the engines found multiple compressor blades in the right-hand engine showed signs of damage.

Inner engine

Deposit build up in Engine 2 © AAIB

What went wrong?

A subsequent investigation found that the aircraft’s fuel tanks were treated with 38 times the recommended concentration of an antimicrobial agent, called Kathon. Kathon is used to prevent the growth of microorganisms in aircraft fuel tanks, a build up of microorganisms can lead to a reduction of fuel quality and even structural damage.

The high concentration of Kathon in the fuel tanks caused contamination of the engine’s hydro mechanical units (HMU), which determine the level of fuel supply being fed to an aircraft’s engines.  Loss of HMU regulation led to the surge in the aircraft’s left engine and a stall in its right hand-side engine.

Warning signs missed

Alarmingly, earlier warning signs and unusual aircraft behaviour had been observed in the 24 hours before the incident.  On the flight before the incident, it also took three attempts to start the left-hand engine before the aircraft pushed back, with alarms for Start Fault and Engine Stall all being triggered. On final approach to Gatwick, the crew reported feeling the vibrations in the aircraft, with fleeting intermittent warnings of a stall in the right-hand engine. They agreed that if any further ECAM warnings were heard, the aircraft would declare a MAYDAY and ask to go straight into Gatwick.

Following this earlier incident, an investigation by an engineer into the right-hand engine’s irregular performance was done using the wrong manual. An engineer used a trouble-shooting manual that was for a LEAP-1A32 engine, not the CFM56-5B3/3 engines which the aircraft had . Consequently, no fault was found and the engineer and flight crew signed off on the next flight.

Learning from past mistakes

Crispin Orr, Chief Inspector of Air Accidents said:

This was a very serious incident that in different circumstances could have had a catastrophic outcome. The safety of commercial air transport aircraft depends in part on redundancy of safety critical systems such as engines.  However, fuel contamination can affect all engines simultaneously and so it is essential that maintenance regimes are resilient to errors that could lead to fuel system contamination”.

The AAIB has now issued safety recommendations, leading to safety action being taken by regulators, the International Air Transport Association, the manufacturers of the aircraft, engines and biocide, the maintenance organisations involved, and the operator.

Were you surprised that the aircraft was allowed to carry on flying after previous incidents? Let us know in the comments below

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