It was shortly after takeoff from Toronto's Pearson airport, when their airplane was still climbing, that passengers aboard the Air Canada jetliner heard very loud bangs.
Some saw red sparks and flames coming from the left engine of the plane.
The plane was able to return safely to Pearson on its own power, but the Friday evening incident added disruption to the normally routine Air Canada daily service from Toronto to Ottawa.
The aircraft, a twin-engine Boeing 767 manufactured in 1989, was carrying 175 passengers.
According to Transport Canada's Civil Aviation Daily Occurrence Reporting System (CADORS), the problem is believed to have been a surge – a malfunction of the compressor – in the left engine.
At night, engine surging can be spectacular because of the noise and flash but the passengers were not in harm's way, aviation consultant Jock Williams said in an interview.
"It's not a huge danger but the pilot preferred taking every precaution," he said.
To passengers, it was dramatic. One woman, who was travelling with her three-year-old and eight-month-old, told The Globe and Mail that she was sure the plane was going to crash.
"All I could think was my poor babies will never get to experience life," said the woman, who didn't want her name published.
Lynda Judd, an Ottawa-area resident, was also aboard the flight, AC476. In an account she gave to her husband, Peter, she recalled that the plane stopped climbing after the banging sounds.
The Boeing started turning toward Lake Ontario then back toward Pearson. "No announcements were made for a long time but everyone knew something was wrong," Mr. Judd told The Globe.
"We definitely heard and felt the engine surging," another passenger, Canadian musician Joshua Bartholomew, tweeted.
Mr. Bartholomew and his wife, Lisa Harriton, who are a Grammy-nominated song-writing and music producing team, were both travelling aboard Flight AC476. He tweeted that it was a "scary" experience.
Mr. Judd said he was told that the pilot eventually announced that they had lost the left engine but were cleared for an emergency landing.
On the ground, several residents in the Toronto area reported hearing blast-like sounds and seeing flames as the plane returned.
According to Ms. Judd, there were many fire trucks with flashing lights waiting for the plane on the runway. It sat for about 20 minutes while the firefighters inspected it, before it could proceed to the gate.
Normally, Flight AC476 would have landed in Ottawa around 8:31 p.m., then the Boeing would have proceeded across the Atlantic to London's Heathrow airport, as Flight AC888.
The Transportation Safety Board of Canada didn't immediately deploy investigators, but spokesman Chris Krepski said the board would follow up with the carrier to find out what happened.
According to the preliminary CADORS information, the Boeing had left Pearson's runway 06L when, around 7:55 p.m., its left engine failed.
"Encountered a surge in the left engine and as a precaution returned to Toronto," the report said.
Surges happen when the compressor, which provides pressurized air to burn the aircraft's fuel, stalls, possibly because of prior damage, hitting a bird or air backing up, Mr. Williams said.
The passengers were transferred to another aircraft and eventually arrived in Ottawa shortly after midnight.