Incident Overview
On 2 October 2024, an ATR-72 aircraft, registration SP-SPN, operated by SprintAir SA, was performing a flight from ENTC to ENGM. During the climb phase, the crew observed a fuel imbalance of 230 kg less fuel in the left tank compared to the right tank. They executed the non-normal checklist procedure for fuel imbalance (NNC 1 from the QRH). The flight continued to the destination airport, where fuel consumption was monitored and no excessive consumption by either engine was noted.
After landing at the destination airport, an inspection of the upper wing surface revealed that the gravity fuel filler cap on the left wing was missing.
Investigation
The Polish State Commission on Aircraft Accident Investigation (PKBWL) analyzed the incident report and determined that the information was sufficient to conclude the investigation without further inquiry.
Probable Cause
The probable cause of the incident was determined to be improper installation of the fuel filler cap after the last gravity refueling or incorrect assembly of the panel/cap after a scheduled fuel tank inspection, resulting in its damage.
Contributing Factors
A contributing factor was the improper action of the rebound spring of the cap.
Safety Actions
The investigating entity proposed and the Commission accepted the following preventive actions: 1. A technical recommendation was issued to check the correct installation of gravity fuel filler caps across the entire ATR fleet. 2. An additional maintenance task was added to the Maintenance Program to verify proper cap installation at intervals of 500 flight hours. 3. A training program for technical personnel on the installation and removal of gravity fuel filler caps was planned.