What happened
On October 2, 2024, an ATR-72, registration SP-SPN, operated by SprintAir SA, was performing a flight from ENTC to ENGM. During the climb phase of the flight, the crew identified a fuel imbalance between the wing tanks, noting that the left tank contained 230 kg less fuel than the right tank.
In response to the discrepancy, the crew initiated the Non-Normal Checklist (NNC 1) procedure for fuel unbalance as outlined in the Quick Reference Handbook (QRH). The flight proceeded to the destination airport, with the crew closely monitoring fuel consumption. No excessive fuel flow was detected from either engine during the flight. Upon landing at ENGM, a physical inspection of the upper wing surface was conducted, which revealed that the gravity fuel filler cap on the left wing was missing.
The investigation
The PKBWL examined the circumstances surrounding the fuel loss and the state of the aircraft following the landing. The investigation focused on the period following the last refueling and maintenance activities to determine how the cap became dislodged or lost during flight.
Findings
- The probable cause of the incident was the improper installation of the gravity fuel filler cap following the most recent gravity refueling, or the incorrect reassembly of the cap/panel following a scheduled fuel tank inspection, which resulted in its failure.
- A contributing factor was the improper functioning of the cap's spring-loaded retaining mechanism.
Safety action
Following the investigation, several technical and maintenance actions were implemented:
- A technical recommendation was developed regarding the verification of gravity fuel filler cap installation across the entire ATR fleet.
- An additional maintenance task was added to the Maintenance Program to check the correct installation of these caps at 500-hour intervals.
- A training program for technical personnel is being developed to improve procedures for the installation and removal of gravity fuel filler caps.