An American Airlines flight departed from St. Louis Lambert International Airport on Oct. 17, bound for Philadelphia, before turning back to the airport.

Gary Leff, a Texas-based aviation expert who founded the blog “View from the Wing,” told Fox News Digital that engine oil entering an aircraft’s air supply could potentially explain the odor, although the cause of the incident has not yet been confirmed.

“When that happens, you can get smoke, you can get vibration, you can get loud noises. It can also wind up making people ill,” Leff said.

FlightAware data reviewed by Fox News Digital showed that the plane was in the air for 34 minutes before landing back at St. Louis Lambert International Airport.

In a statement to Fox News Digital, American Airlines confirmed that the flight returned to St. Louis shortly after takeoff following a reported odor on board.

The airline said the aircraft landed safely and was taken out of service for inspection by its maintenance team.

The flight had already been delayed roughly two hours following a reported engine issue before passengers experienced trouble in the air, according to a couple who spoke to local outlet First Alert 4.

The passengers — Tessa and Mark Springer of Waterloo, Illinois — were traveling to Pennsylvania for work when they heard a troubling noise coming from the back of the plane before takeoff.

“We heard just this really horrible noise coming from the back of the plane. It was like … real loud buzzing, kind of [a] grinding sound,” Mark Springer said.

Mark Springer said the pilot then announced there was a mechanical issue.

“The pilot comes back on and says, ‘Uh, there’s a problem with the engine on the airplane,’” the passenger added. “‘We can’t get it started.’”

After the delay, the plane finally departed.

But about 15 minutes into the flight, passengers experienced a sudden jolt that sent them lunging forward, according to Mark Springer.

“All of a sudden, the plane just felt like it hit a … like in a car, when you slam on your brakes. … Everybody kind of lunges forward like this,” he said.

“And then, maybe a minute later, the plane just kind of goes boom and drops. … All of the oxygen masks fall. And we just kind of looked at each other like, is this serious? Is this real?” added Mark Springer.

Tessa Springer recalled fearing for her life as the plane turned back toward the airport, telling First Alert 4 she was terrified she would die.

She texted her children, “Problem on our plane, love you guys,” per the outlet.

“I was like, we’re going to die,” Tessa Springer said.

The couple said that once they landed, they evacuated the plane by jumping from its wing, with fellow passengers helping catch one another as they made their way to the ground.

“I don’t think I’ll ever be able to get on another airplane ever again,” Mark Springer said.

Despite the frightening ordeal, Leff of “View from the Wing” said a lengthy delay due to a mechanical issue does not necessarily indicate that a flight is unsafe.

“Part of that delay is diagnosing, part of it’s addressing it and part of it’s getting all the paperwork right and just doing all the final readbacks and rechecks to make sure that everything is safe for departure,” Leff said.

“A long delay in and of itself is not a safety concern,” he added.

Leff also noted that aircraft are equipped with multiple safety systems and emergency procedures designed to protect passengers when problems arise.

“There are tremendous redundancies of systems and emergency procedures in place, such that the cockpit crew immediately reacted, responded and got everyone on the ground and off the aircraft to safety,” Leff said.

“It’s certainly not something that you want to go through — but even when something goes wrong, that doesn’t mean that things end badly at all,” he added.