Fuel Filter Bypass Incident Involving Enter Air Boeing 737-800 (SP-ENT)

Incident Overview and Immediate Response On January 16, 2020, a Boeing 737-800 with registration SP-ENT operated by Enter Air encountered a fuel system anomaly during its climb after departing from Katowice (EPKT). The crew received a master caution warning indicating a fuel filter bypass on engine number 1. Following the non-normal checklist for a fuel filter bypass, the flight crew coordinated with the airline's operations department and decided to return to the departure airport. The aircraft landed with a weight of 71.215 tonnes, which exceeded the maximum allowable landing mass, but the landing was completed without further consequences. ## Investigation and Contamination Analysis The incident was part of a broader pattern of similar issues within the Enter Air fleet. Between January 9 and January 21, 2020, three aircraft in the fleet experienced the same fuel filter bypass indication during regular operations. In each case, replacing the fuel filter resolved the issue. Over a longer period, the operator recorded 20 such incidents, with seven occurring in 2018, ten in 2019, and three in 2020. Following the third incident in January 2020, a urgent analysis was initiated to identify the root cause. Fuel filters removed from the affected aircraft were tested for microbial contamination, with preliminary results showing no biological contamination exceeding acceptable limits. However, after flushing and filtering, brown dust was found in the filters. Laboratory analysis of the samples revealed an organic composition with high levels of carbon, oxygen, sulfur, and calcium, suggesting water contaminated with organic matter. Each sample also contained sodium, which could indicate the presence of superabsorbent polymers (SAP), though not in quantities sufficient to be the primary cause of the blockage. The aircraft manufacturer reviewed the chemical composition of the samples and definitively ruled out that the contamination originated from the aircraft's fuel system. To determine the scale of the issue, the operator's technical department conducted a widespread replacement of fuel filters across the fleet. In 33 out of 51 filter replacements, accumulated material similar to that found in the initial incidents was discovered. Further laboratory analysis confirmed the chemical similarity of all tested samples. ## Contributing Factors and Safety Recommendations The investigation pointed to an external source of fuel system contamination as the most likely cause. An analysis of the airports where the affected aircraft operated did not reveal any irregularities in the documentation provided. The investigation also reviewed EASA Special Information Bulletin (SIB) No. 2018-10 and IATA documents regarding SAP in fuel. These documents indicated that the primary factor causing filter contamination by SAP is non-compliance with fueling recommendations, particularly failing to maintain proper fueling pressure, which should not exceed 3 bar (15 psi). The operation and maintenance of the Enter Air fleet at smaller airports, where fuel quality and fueling equipment may be less robust than at major international hubs, was identified as a contributing factor. The probable cause of the incident was determined to be non-compliance with the fueling recommendations contained in EASA and IATA documents. No specific contributing circumstances were formally identified beyond the general factors noted. The investigation committee accepted several preventive recommendations proposed by the investigating entity: 1. All fuel filters in the Enter Air fleet were replaced, regardless of flight hours. 2. The contents of the fleet's 'Fly Away Kit' were expanded to include components necessary for filter replacement in the event of a fuel filter bypass indication. 3. The fuel filter replacement interval was reduced from 6,000 flight hours to 2,000 flight hours, although the manufacturer's current recommended interval is 12,000 flight hours. 4. The operator's personnel responsible for overseeing fueling service providers were instructed to pay special attention to compliance with the requirements of SIB 2018-10. The committee did not formulate any additional safety recommendations.