Fuel Microbiological Contamination Treatment

Microbiological contamination refers to the presence of microorganisms such as bacteria, yeast, or fungi. It can be seen as deposits, which vary in color from translucent to dark, with a viscosity that ranges from oily gel to solid.

(fig.1) Examples of fuel tank contamination

Microorganisms need a food source, water, and warm temperatures to grow. A fuel tank provides the perfect conditions for microorganisms to develop, because they feed on the fuel hydrocarbons, and water is always present in a fuel tank. This water comes from two main sources: dissolved water in uplifted fuel and condensation due to vent air, which enters the fuel tank during normal operations, especially during descent from dry, cold air in cruise to warmer wetter air at lower altitudes. This is the reason why the risk of contamination from microorganisms increases in locations with hot and humid weather conditions.

Erroneous fuel quantity measurement

Fuel microbiological contamination can affect the measurement of fuel quantity on board the aircraft. This can result in the flight crew observing false fuel quantity indications, which are often overreads. In extreme cases, these can lead to the total loss of fuel quantity indication for one or several fuel tanks.

Fuel filter clogging, pump failure, and engine malfunction

Deposits from contamination can clog fuel pumps and engine fuel filters, leading to fuel pump failures and degradation in the engine fuel supply.

Corrosion

Microorganisms can sometimes emit acid that causes corrosion, affects coatings, and deteriorates sealants in the fuel tank, which can eventually lead to fuel leaks.

As stated above, microbiological contamination can cause significant operational disruptions, which can affect the safety and efficiency of operations. An effective maintenance strategy needs to be developed to prevent contamination. Regular drainage of water from fuel tanks is necessary, even if the fuel system on Airbus aircraft is designed to manage and reduce the quantity of water in the fuel tanks. Regular tests to check for the presence of microorganisms are also crucial. If contamination is detected, curative treatment must be used.

Before the event flight

A moderate level of fuel microbiological contamination was detected during scheduled maintenance checks of an Airbus A321 aircraft. As recommended in the Aircraft Maintenance Manual (AMM), a biocide curative treatment was performed using Kathon® FP 1.5 biocide.

Following the biocide treatment, there were four flights before the event occurred on the fifth flight (fig.1). The first flight was uneventful. ① On the second flight, the ENG 1 HP FUEL VALVE ECAM alert triggered, but ENG 1 start was successful on the second attempt. The third flight was uneventful. ② On the fourth flight, it took four attempts to successfully start ENG 1 with the reappearance of the ENG 1 HP FUEL VALVE ECAM alert and the ENG 1 START FAULT alert. This led to an automatic restart. ③ The ENG 2 STALL ECAM alert triggered twice during the descent and the flight crew felt airframe vibrations. They reduced N1 below 50 % and safely landed the aircraft.

④ Line maintenance performed the troubleshooting actions related to the ENG 2 STALL ECAM alert, but they could not confirm the fault. They released the aircraft back into service.

The event on the fifth flight

⑤ During ENG 1 start, the ENG 1 START FAULT ECAM alert triggered. The second start attempt generated the ENG 1 FAIL ECAM alert. This ENG 1 FAIL alert briefly appeared on the third attempt, but quickly disappeared and the ENG 1 start was finally successful. All engine parameters were normal during the taxi. At the runway holding point, the flight crew accelerated the engines twice for more than 10 seconds. All engine parameters were normal and the flight crew decided to take off.

⑥ ENG 1 began to surge at 500 ft RA and the engine parameters were fluctuating. N1 decreased below 40 % for 25 seconds. The Captain made a MAYDAY call and asked for an immediate return to the runway. He switched off the AP and noticed that the engine parameters of ENG 2 were also beginning to fluctuate. ⑦ During the level-off at 3 600 ft, the ENG 2 STALL ECAM alert triggered three times. The Captain reduced the thrust on both engines and was prepared to glide the aircraft, if necessary. The aircraft eventually landed safely. It was observed that the engine parameters had returned to normal when the aircraft came to a complete stop. The flight crew reported that they shut down both engines on the taxiway when they heard unusual noises from the engines.

(fig.2) Flight chronology: from the biocide treatment until the event flight

The fuel from the aircraft showed contamination and an amount of undissolved Kathon® FP 1.5 biocide. An inspection of the engine found the presence of viscous, gelatinous deposits on the engine parts.

(fig.3) Brown deposits found in Engine 2 during borescope inspection

Biocide overdose

The maintenance crew who performed the biocide curative treatment, was not familiar with the use of the “parts per million” (ppm) unit, which was used in the associated AMM task. They incorrectly used an online conversion tool and this led to a concentration of Kathon® FP 1.5 biocide that was more than 37 times the correct dosage.

Biocide not correctly mixed with the fuel

The maintenance engineer used the overwing refuel aperture to deliver the biocide into the aircraft fuel tanks, which prevented the biocide from correctly mixing with the fuel. It remained at the bottom of the tank and migrated along the wing towards the fuel pump.

The AMM task provided two options to introduce the biocide into the aircraft fuel tank. One method was to premix the fuel and biocide and then uplift the mixture into the fuel tank using the normal refueling procedure, but not using the overwing refuel aperture. The other method was to use an adjustable metered injection rig to inject the biocide and correctly mix the fuel during a normal refueling procedure.

Wrong troubleshooting procedure

Before the event flight, the maintenance crew used the TroubleShooting Manual (TSM) to perform the troubleshooting actions associated with the ENG 2 STALL ECAM alert. However, they referred to tasks that were applicable to CFM LEAP-1A32 engines, but the A321 aircraft from the event was equipped with CFM56 engines. Therefore, the troubleshooting actions did not correctly guide the maintenance crew to identify the cause of the ECAM alert and the aircraft was released for flight. If the crew had applied the CFM56 troubleshooting actions, it is likely that they would have identified the fuel contamination issue.

When fuel microbiological contamination is confirmed, curative treatment based on fuel additives with antimicrobial properties, or biocides, is used to arrest and remove the contamination. If biocide treatment is not available, the fuel tanks must be thoroughly cleaned, which is a long process and not always 100 % effective. That is why the use of biocide is the preferred method to treat microbiological contamination.

There were two widely-used biocides approved for use in the aviation industry: Kathon® FP 1.5 and Biobor® JF. Further to in-service experience with Kathon® FP 1.5, including the above event and another case of dual loss of thrust control, GE took the proactive decision to remove the approval for Kathon® FP 1.5 as an approved additive for use in their engines, including CFM and Engine Alliance (EA). At the same time, the manufacturer of Kathon® FP 1.5 stopped producing its biocide for aviation applications to avoid any further risk of misuse.

In addition, currently the use of Biobor® JF is not approved in the European Union (EU) except under specific derogations that airlines and their local authorities discuss on a case-by-case basis.

Only Biobor® JF biocide can be used on Airbus aircraft equipped with GE, CFM, or EA engines. For other engines, such as Rolls-Royce (RR) and Pratt & Whitney (PW), Biobor® JF and any existing stock of Kathon® FP 1.5 can be used. When maintenance is performed in EU countries, Biobor® JF can be used if derogations are obtained.

Production of Kathon® FP 1.5 for aviation applications was stopped in 2020. Kathon® FP 1.5 has a shelf life of only 2 years and this means that any existing stock should be used this year (2022). Only Biobor® JF biocide will remain available for fuel contamination treatment. New biocides are under study, but are not expected to be available for several years due to the high cost and complexity of the approval process.

Unit standardization

The dosage of biocide required was previously provided in ppm by weight for Biobor® JF and by ppm by volume for Kathon® FP 1.5. There is now industry-wide agreement to use mL/L unit for both Biobor® JF and Kathon® FP 1.5 biocides. Airbus maintenance procedures for all Airbus aircraft, the AMM for A300/A310/A320/A330/A340/A380, MP for A350 aircraft and AMP for A220 aircraft, were updated accordingly by removing the use of the ppm unit.

Table of maximum biocide quantities

To prevent any future case of biocide overdose, Airbus maintenance procedures for biocide treatment now include a table, which provides the maximum quantity of biocide that can be uplifted in the aircraft for different fuel quantities. Maintenance crews should use this table as a guide to check if their computed quantity of biocide is correct before uplifting it into the aircraft.

As another lesson learned from the previous case study, a metered injection rig must be used to correctly mix the biocide with the fuel and uplift it into the aircraft fuel tanks. All Airbus AMM/MP/AMP procedures were updated to reflect this change and there is no alternative method if a metered injection rig is not available.

Lessons learned from previous events involving incorrect use or overdose of biocide helped to define new curative treatment procedures that reduce the risk of incorrect mixing or overdose errors. The main steps of the procedure for efficient curative biocide treatment are:

Drain water from the fuel tank

If water remains in the tank it may cause crystallization inside the fuel tank following biocide treatment.

Defuel the aircraft

Contaminated fuel must be removed from the aircraft.

Clean the inside of the fuel tank

Only in the case of heavy contamination, thorough cleaning of the fuel tank must be performed to remove all deposits.

Compute the quantity of biocide needed

There is a table in the AMM/MP/AMP procedure that provides maximum biocide quantities depending on fuel quantities that should be used to avoid any risk of overdose.

Uplift the mix fuel/biocide inside the fuel tank using a metered injection rig

The only allowable method to uplift the biocide inside the fuel tank is by using a metered injection rig. The fuel tank should be full, because this will ensure that the contamination is treated in all areas, including on the upper surface of the tank.

Final crosscheck computation

Perform a new computation to crosscheck and confirm that the uplifted biocide quantity is correct and minimize any risk of biocide overdose.

Wait for 72 hours (Biobor® JF) or 24 hours (Kathon® FP 1.5)

Soak time is necessary before any engine operations.

Replace fuel filters

Fuel filters and engine filters that were in contact with the contaminated fuel must be replaced.

Burn fuel or defuel within 48 hours

The biocide is a corrosive product for the tank and must be removed after treatment, either by being burnt during normal engine operations or by defueling.