What happened
On a scheduled flight from Toronto to Montréal, an Air Canada Airbus A3/330-343, registration C-GFAH, experienced a series of fuel system irregularities. During the flight, several fuel pump low-pressure warnings appeared, prompting the crew to switch off the affected pumps according to established procedures. As the aircraft descended toward Montréal, additional low-pressure warnings occurred on the remaining functional pumps, leading the crew to deactivate them and rely on gravity fuel feeding.
While level at 11,000 feet, the left engine experienced a rollback below idle, resulting in an engine stall and an engine failure message. The crew responded by switching all fuel pumps back on, which restored power to the left engine. An emergency was declared, and the aircraft landed safely without further incident.
The investigation
The investigation examined the fuel system, the cause of the initial warnings, and the mechanics of the engine rollback. Investigators found that the initial low-pressure warnings on the main pumps were likely erroneous, caused by moisture freezing in the pressure switch sensing lines because the fuel tanks had not been drained according to the 7-day schedule.
However, the investigation established that the subsequent low-pressure warnings on the standby pumps were authentic. The investigation focused on the A330 fuel system design, specifically how the standby pump inlet port is positioned. It was determined that when main pumps are inactive, a "whirlpool" effect can occur at the standby pump inlet port, leading to air ingestion when fuel levels are relatively high.
Findings
- The initial main pump warnings were false indications caused by freezing moisture in the pressure switches.
- The engine rollback was caused by air ingestion into the fuel line through the standby pump inlet port.
- The A330 fuel system design allows for air ingestion during gravity fuel feeding when inner tank quantities are below approximately 2500 kg.
- The aircraft's fuel low-level sensors are positioned below the standby pump inlet, meaning the crew receives no advance warning of imminent starvation if the main pumps are inoperative.
- The airline's anticipated low fuel ECAM (ALFE) procedure contained instructions that potentially conflicted with Airbus-recommended procedures, increasing the risk of confusion.