Qantas and aviation investigators are putting the pieces together to discover what caused a plane carrying 145 passengers to lose an engine yesterday.
QF144 landed safely in Sydney about 3:30pm on Wednesday, after its pilot had issued a rare mayday call.
Today, the airline experienced another issue with QF101 forced to turn back to Sydney as a precaution.
Maydays are normally reserved for the most urgent incidents — its use yesterday sent emergency services rushing to Sydney Airport and caused a media frenzy.
How often do engines fail?
Vice president of the Australian and International Pilots Association, Mark Hofmeyer, backed the decision, saying the pilots on board were "well within their rights" to make the call.
Mr Hofmeyer said he knew of "maybe three" engine shutdowns on Qantas aircraft in the past 15 years.
"So we're talking millions of flying hours," he said.
"These aircraft or these engines, we almost joke they're bullet proof. Very rarely does this occur."
Angus Mitchell, the Australian Transport Safety Bureau's (ATSB) chief commissioner, said up to 8,000 reports were made to the organisation each year.
He said minor incidents on passenger airlines occurred "on a regular basis" but this type of mid-air failure was rare.
"Losing an engine, and that's the initial reports, is something that's quite serious, Mr Mitchell said.
"It doesn't happen that often."
How does an engine fail?
Engine failure is particularly uncommon in the circumstances reported by QF144, according to pilot and aviation expert Keith Tonkin.
A former Qantas pilot, he did not want to speculate on what occurred, but said many things can cause an engine to fail.
It could be part of the engine breaking, an electrical fault, or even a bird or volcanic dust being sucked into the turbine.
What is most unusual about QF144, Mr Tonkin said, was that the failure was reported while the plane was cruising several hours into its flight.
"The aircraft was in normal flight conditions," he said. "It usually happens on the take-off."
Mr Tonkin, however, said planes were certified to fly on one engine for lengthy periods of time and pilots are trained in what to do in the event of a failure.
Some of the wide-body aircraft used in international travel - such as the A330 or Boeing 787 — can fly for hours on one engine.
Mr Hofmeyer, a Qantas first officer with 20 years of experience, said pilots would move down from a cruising height of 38,000ft to 20,000ft to manage the situation.
'Mayday' or 'pan-pan'
Mayday calls are more grave than another international distress signal, called a "pan-pan".
A pan-pan call — derived from the French word for "breakdown" — signifies an urgent situation that does not pose immediate threat to the vessel or life.
In 2010, the two Australian pilots of Cathay Pacific flight CX780 successfully landed a plane with two stalled engines at Hong Kong International Airport.
Captain Malcolm Waters and First Officer David Hayhoe initially declared a pan-pan after losing the first engine on the descent.
They didn't make a mayday call until the second one failed a few minutes later.
Several of the 322 people on board were injured when the plane landed at twice the normal speed, but there were no deaths and the aircraft remained intact.
Investigations found contaminated fuel taken on at Juanda International Airport, in Indonesia, damaged the engines.
Mr Hofmeyer praised QF144's crew, who later downgraded their distress call to a pan-pan, saying it was "fair and reasonable" to do so.
"When they've levelled off at 20,000 feet, they can then reassess their options work out what the nature of the problem was," he said.
"The decision that the pilot made, we back that because they were in the flight deck.
"They were there and they had all the information in front of them to make that decision."
What happens after a mayday call?
Maydays and pan-pans alert air traffic control and emergency services of an issue on board.
Mr Tonkin said they informed crews on the ground the aircraft needs priority, which could be in the form of clearing the plan's path for the fastest landing.
He said maydays are usually only issued when the pilots think they are unable to properly control the aircraft.
But either way, the flight crew are the only people who can ensure the plane lands safely.
"They can't get help until they're on the ground," Mr Tonkin said.
Qantas said it was still investigating the cause of QF144's engine failure, as is the ATSB.
