What happened
On January 21, 2024, a Boeing 737-800, registration SP-ENW, operated by Enter Air, was performing a scheduled flight from EGNJ to EFET. Prior to departure, the pilot requested a specific fuel load of 10,600 kg. To accommodate this, a second operational flight plan (OFP) was generated that excluded "tankering" fuel, resulting in a total planned fuel load of 10,653 kg.
During the flight, the crew monitored weather conditions at the destination and two alternate aerodromes (ENBO and ESN and ESNU). Due to low cloud bases at EFET and reported wind shear at ENBO, the crew prepared to divert to ESNU if a landing at the destination was not possible. During the approach to EFET, the crew was forced to execute a missed approach due to visibility issues.
As the aircraft proceeded to the alternate airport, the crew realized that actual fuel consumption was higher than predicted. Approximately 10 minutes before landing at ESNU, the Flight Management System (FMS) calculated that the remaining fuel upon arrival would be approximately 1,100 kg, which was below the required reserve of 1,150 kg. Consequently, the pilot declared a MAYDAY, FUEL emergency. The aircraft landed at ESNU with 993 kg of fuel remaining.
The investigation
The investigation examined the fuel consumption of SP-ENW and found that engine number 1 consumed, on average, 30 kg more fuel per hour than engine number 2. A broader analysis of 50 random flights from the same operator revealed that actual fuel consumption was, on average, approximately 3% higher than the values predicted in the OFP. Furthermore, the investigation found that the OFP generation software had not been optimized for the operator's specific fleet and did not account for real-world consumption variations. The investigation also identified that the layout of the first page of the OFP was inconsistent with the airline's Operations Manual (OM A), which could lead to crew misinterpretation of required fuel values.
Findings
- The operational flight plan contained an underestimated fuel requirement for the leg between EFET and ESNU.
- The fuel calculations failed to account for critical operational factors, specifically the fuel required for a missed approach at the destination and the impact of unfavorable headwind components at cruise altitude.
- Actual aircraft fuel consumption was consistently higher than the values predicted by the OFP software.
- The presentation of fuel requirements on the first page of the OFP was non-compliant with the operator's Operations Manual, creating a risk of pilot error.