Engine failure caused Qantas flight to make emergency landing at ...

Urgent investigations are underway after a Qantas flight experienced engine failure on take-off, forcing it to turn back to Sydney Airport.

Pilots made what is known as a "'pan-pan" call — a international distress signal less serious than a "mayday" — shortly after taking to the skies on Friday.

They managed to turn the plane around and land back at the airport safely without injury to passengers or crew.

The failure sparked a grassfire to break out next to one of Sydney Airport's runways, and caused flight delays.

Here is what we know so far.

Emergency services were on stand-by after QF520 was forced to make an emergency landing. (ABC News)

Just after take-off, as QF520's wheels left the tarmac, passengers heard a "loud bang".

"It wasn't the bang, or the shudder, that made my blood run cold," wrote passenger and ABC journalist, Mark Willacy.

"It was the plane's urgent struggle to get altitude," he said.

Qantas confirmed the flight to Brisbane experienced a "contained engine failure".

Professor Ron Bartsch is a leading expert in aviation safety and chairman of Avlaw — a respected international aviation consulting company.

"Engine failure of this type is very, very rare and it can lead to catastrophic situations," he said.

"Numerous things could have caused the occurrence, such as a turbine blade could have detached, or a compressor blade could have detached.

"That would have caused a catastrophic failure of the engine," Professor Bartsch explained.

"If a blade detaches, it essentially just destroys the engine by going through the latter part of it."

Professor Ron Bartsch said that plane engine failure was "very rare". (Supplied)

Professor Bartsch praised the flight crew's actions.

"Records prove that more fatalities happen due to engine failure during take-off phase," he said.

"Bear in mind, this is when engines are spooled up, to develop take-off thrust.

"The fact it happened during this time means the crew were able to handle the situation very well."

He said the aircraft would have passed what's known as "V1" or "decision speed"

After V1, a pilot is committed to take-off, with too much speed and not enough runway to abort.

"The most critical phase of flight for an engine failure to occur is at or near V1," Professor Bartsch said.

"If it's at a speed beyond V1, then the aircraft has got sufficient power with the remaining engines to be able to climb out.

"What passengers probably experienced is if [the plane is] only climbing out on one engine, it's going to be a reduced rate of climb," he said, when asked about the struggle for altitude.

The aircraft involved was a Boeing 737 — the workhorse of Qantas' fleet.

It flies domestically and to destinations in New Zealand, Asia, and the Pacific.

"[Boeing] 737-800s are a very common aircraft used throughout the world and extensively in Australia," Professor Bartsch said.

"As with all modern jet aircraft, they're very reliable."

The aircraft involved in the incident was a Boeing 737. (AAP: Dan Himbrechts)

The aircraft is made by American plane manufacturer Boeing, which has experienced a safety crisis in recent years.

Boeing's woes have been caused by a newer generation of aircraft based on a similar plane design called the 737 MAX.

The 737 MAX, fitted with trademark split winglets, has been involved in two major disasters — Lion Air Flight 610 in 2018 and Ethiopian Airlines Flight 302 in 2019, killing a combined 346 people.

In January, the model was involved in another emergency when Alaska Airlines Flight 1282 had a door blow out mid-flight. In that incident no-one was killed.

The ABC is not suggesting there are links between safety issues involving the 737 MAX and the engine failure on QF520.

"Boeing has not had a very good record in the past, particularly with all the failures with the Boeing MAX," Professor Bartsch said.

"But it's a completely different situation compared to a 737-800 — which is a proven and reliable aircraft.

"So there really are different issues involved."

A Qantas 737 experienced an engine failure on a flight from Auckland to Sydney in January 2023.

A subsequent investigation found it was caused when a component moved during operation, shutting down the engine mid-flight.

It safely landed at Sydney Airport about an hour later.

Professor Bartsch, former head of safety at the national airline, said Qantas enjoys a very high record of safety.

"I don't think Qantas' safety record has ever come under criticism," he said.

"This is something that Qantas should be very proud of, the fact that their crew has responded in the appropriate way."

Footage of QF520's landing has been broadcast around the world.

It showed the plane making its final approach, while a large grassfire burned close to the runway from where the aircraft had just taken off.

Airservices Australia runs the firefighting teams which operate at Sydney Airport.

It confirmed the blaze was caused by the engine failure which was soon extinguished.

The Australian Transport Safety Bureau is investigating what happened on QF520. (ABC News)

Professor Bartsch said it is possible mechanical components being ejected from the engine could be to blame.

"The temperatures of the turbine blades, or the compressor blades, would be in the vicinity of 800 degrees Celsius.

"If any of them were to, you know, land on grass or whatever, that could cause an immediate fire."

The Australian Transport Safety Bureau is investigating exactly what happened on QF520.

It will take some months, or even years, for the agency to hand down its final report.