Summary
On 21 January 2024, an Enter Air Boeing 737-800, registration SP-ENW, was operating a scheduled flight from EGNJ to EFET. Due to low cloud at the destination and wind shear at the primary alternate, the crew planned to divert to the secondary alternate, ESNU. During the approach to EFET, the crew executed a missed approach and proceeded to ESNU. Approximately 10 minutes before landing, the Flight Management System indicated the estimated fuel on arrival would be about 1100 kg, below the required reserve of 1150 kg. The captain declared a MAYDAY FUEL emergency. The aircraft landed safely at ESNU with 993 kg of fuel remaining.
Incident Details
Before departure, the captain requested a revised operational flight plan (OFP) that excluded tankering fuel. The first OFP showed a total fuel requirement of 11500 kg, but the revised OFP (OFP No. 2) showed 10653 kg. The crew loaded 10600 kg. During the flight, the crew analyzed weather at EFET, ENBO, and ESNU. With low clouds at EFET and wind shear at ENBO, they decided to use ESNU as the diversion airport if missed approach occurred. They were aware they had fuel for one approach at EFET, but unaware that the OFP fuel for the alternate did not account for the missed approach procedure or the headwind component at cruise altitude.
Analysis
The investigation found that the actual fuel consumption of SP-ENW was higher than planned: engine 1 consumed about 30 kg/hour more than engine 2. A review of 50 random flights showed average fuel consumption was about 3% higher than OFP predictions. The OFP generation software did not account for real-world fuel consumption and was not optimized for the carrier's fleet. Additionally, the first page of the OFP did not include taxi fuel in the REQD FUEL value, contrary to the Operations Manual Part A (OM A 8.1.7.2), which could mislead crews.
Cause and Contributing Factors
Probable Cause: The operational flight plan (OFP) contained an underestimated fuel requirement for the segment from the destination airport (EFET) to the alternate airport (ESNU). These calculations did not account for significant operational factors, such as the missed approach procedure or the impact of an adverse headwind component at cruise altitude, which consequently led to an underestimation of actual fuel consumption.
Contributing Factors: Analysis showed that actual aircraft fuel consumption was on average about 3% higher than values predicted in OFPs. Additionally, the layout of the first page of the OFP was inconsistent with the provisions of Operations Manual Part A, which could lead to incorrect interpretation by crews.
Preventive Actions
The operator planned to replace engine 1 on SP-ENW due to higher fuel consumption compared to engine 2. Simultaneously, actions were taken to adjust the layout of the operational flight plan to comply with the Operations Manual requirements. No safety recommendations were issued.