A flight departing Toronto last month could have ended in disaster for over 150 passengers if not for the flight crew and air traffic controllers' calm handling of the situation.

The Air Canada Airbus A321 departed Toronto Pearson International Airport shortly after 10:00 a.m. on July 23, bound for Calgary International Airport, however, it would not arrive as scheduled.

Flight AC139 was in the process of climbing out of Pearson to cruising altitude when pilots experienced the first signs of a problem, and contacted the Toronto control tower with a request to stop their climb at 9,000 feet to sort out an apparent mechanical issue.

A recording of the flight crew and air traffic controllers' conversation was shared on YouTube, where the pilot can be heard stating, "We'd like to stay at 9,000 [feet] for now. We just need to sort out a checklist here."

The pilot then requests the tower to "keep [the aircraft] on vectors to stay in the vicinity of the airport," foreshadowing the flight's return to Pearson.

Moments later, the crew declared PAN-PAN, an international urgency signal used when a ship, aircraft, or other type of vehicle is in a state of urgency and requires assistance — but short of the more critical "Mayday" distress call.

"Toronto, it's Air Canada 139, PAN-PAN, PAN-PAN, we've had a hydraulic system failure. We're gonna be requesting return to Toronto."

Hydraulics are a critical component in commercial aircraft, such as the A321 in this incident, and are used to move the control surfaces that steer the plane.

The deadliest instance of a plane losing hydraulic functions happened in 1985 with the Japan Air Lines Flight 123 tragedy, where 520 passengers and crew were killed after a structural failure cut the hydraulic lines of a Boeing 747 and rendered the aircraft uncontrollable.

Luckily, the hydraulic issue threatening Air Canada Flight 139 last month proved merely a leak, and the plane was able to safely land back at Pearson Airport with 169 passengers and crew all unharmed.

Emergency services responding following the plane's touchdown reported leaking hydraulic fluid under the aircraft's fuselage — a problem that could have resulted in catastrophe if gone unchecked.