Tuninter Flight 1153

Investigation

The investigation revealed several factors leading to the crash. First, the investigation examined how the incorrect fuel quantity indicator (FQI) came to be installed on the plane.

The final report on the crash notes that during a flight the day before the incident, the captain (who was also flying during the incident) became aware that the FQI on the aircraft dashboard was not working correctly and reported the problem. That evening, the FQI was replaced, but with an FQI that was intended for the ATR 42, a different model of aircraft. The correct FQI was not found because its part number had been entered into the database in a different format than was searched for, and the inventory database mistakenly indicated that the ATR 42 part could be used on both models of aircraft. The FQI for the two aircraft models have different markings on the faceplate, though the difference was not noticed.

Second, the investigation examined the fueling of the plane. On the day of the incident, the aircraft was fueled for the flights from Tunis to Bari and Bari to Djerba based on calculations using incorrect readings from the FQI. Because of differences in the shape of the fuel tanks, the incorrect FQI indicated a larger volume of fuel than the tanks held. When fueling the aircraft in Tunis, neither the refueling operator nor the flight engineer noticed the difference between the amount of fuel loaded and the change in reading on the FQI.

The investigation also found that when departing from Tunis on the Tunis-Bari route taken before Flight 1153, the captain noticed by reading his cockpit displays that the aircraft's fuel level seemed to have increased overnight, but did not find a corresponding refueling slip. However, he accepted the explanation that the slip had been kept by a previous crew, though the aircraft had not been flown nor refueled since the day before. Because the aircraft had been fueled for two flights, the flight from Tunis to Bari was uneventful. In Bari, the plane took additional fuel to a level where the incorrect indicator showed 2,700 kilograms. The correct indicator would have shown that there were just 540 kilograms in the tanks when departing for the Bari-Djerba route, insufficient to reach the destination.

Finally, the investigation examined the flight after the engines had failed. During an engine flameout, crews must feather the propellers to reduce the drag on the blades so the plane can glide a farther distance. While the propellers of the aircraft were found fully feathered after the crash, the crew did not feather them, because they were attempting to restart the engines (as per the checklist at the time). Furthermore, as a result of their efforts to restart the engines, the crew did not glide the aircraft at the optimum speed to extend the gliding distance.

Simulation results suggested that, handled optimally, the ATR could have reached Palermo with the tailwind of that day. Two crews flew a simulator at ATR's facility in France from the same starting conditions. By feathering the propellers and reducing the speed to the optimal gliding speed, one made a landing at Palermo, and the other one ditched a mile short of the runway. The fundamental difference was that the simulator pilots knew there was no fuel available and responded accordingly. In contrast, the captain of the Tuninter ATR – whose FQI showed sufficient fuel remained – focused initially on trying to restart the engines in the hope they would respond. When the engines could not be restarted, the captain then focused on selecting a place to ditch the aircraft. Unlike the simulator pilots, Gharbi had a lack of instruments and experienced radio interruptions. The final investigative report suggested that airlines train their pilots to deal with unusual situations.