An Air Canada Boeing 777-300, registration C-FNNU performing flight AC-890 from Toronto,ON (Canada) to Rome Fiumicino (Italy) with 465 people on board, was enroute at FL330 about 40nm north of Prince Edward Island,PE (Canada) when the crew noticed the fuel quantity of the center fuel tank was decreasing. The crew transferred the remaining fuel from the center tank to the wings and decided to return to Toronto as precaution. The crew did not request priority however requested emergency services on standby for the landing. The aircraft landed safely on Toronto's runway 15L about 105 minutes later.

The Canadian TSB reported maitenance identified the left center fuel discharge check valve as faulty and replaced the unit.

https://flightaware.com/live/flight/ACA890/history/20180901/0120Z/CYYZ/LIRF