InvestigationThe Portuguese Aviation Accidents Prevention and Investigation Department (GPIAA) investigated the incident along with Canadian and French authorities.[2]
The investigation revealed that the cause of the incident was a fuel leak in the number two engine, caused by an incorrect part installed in the hydraulics system by Air Transat maintenance staff. The part, which was adapted from a similar engine, did not maintain adequate clearance between the hydraulic lines and the fuel line. This allowed vibration in the hydraulic lines to degrade the fuel line and cause the leak
Human error, be it by design (Airbus) or by action (Air Transat maintenance technicians) caused the engines to shut down.