An Air Canada 767 flight made an emergency landing at Toronto's Pearson International Airport Thursday after the pilot was alerted to a mechanical problem.

The plane, which left Toronto for Buenos Aires late Wednesday, landed back at Pearson at about 4:15 a.m. Thursday.

Air Canada spokesperson Peter Fitzpatrick said there were 129 people on board.

Fitzpatrick told Thursday that the pilot reported an issue with the automatic pilot system.

"As a precaution, they decided to return to get that checked out," Fitzpatrick said.

He said it is "standard operating procedure" and that the aircraft landed normally without incident.

"Aircraft have so many redundant systems that it's really quite safe," Fitzpatrick said.

Scott Armstrong, a spokesperson for Greater Toronto Airports Authority, said three fire trucks were standing by on the ground as a precaution.

"The passengers were taken to Terminal 1 and they're waiting to go out on a flight later this morning," Armstrong told on Thursday.

He said mechanical issues with aircraft do pop up occasionally and that it's not something travellers should be too concerned about.

Fitzpatrick said there were 129 people on board.