‘Surge and stall’: What pilots think caused Qantas jet to shoot out flames

Former pilots say a jarring incident in which flames were seen spouting from a Qantas jet in New Zealand could have been a case of “surge and stall” caused by hitting birds or other airflow problems with the engine.

On Saturday, Qantas flight QF122 suffered a suspected engine issue just after taking off from Queenstown. Flight Aware data showed the Boeing 737 – bound for Sydney – instead landed 52 minutes after take-off in Christchurch.

Qantas said the cause of the incident was still being investigated. The Civil Aviation Authority of New Zealand said that following an engine issue or similar incident, “aircraft undergo thorough inspections and any necessary maintenance before being returned to service”.

Former pilots on Monday played down safety concerns over the incident, though they also said these types of failures were extremely rare and would have alarmed passengers.

Former Qantas captain David Evans, who has previously written for this masthead, said that, from the video footage, the “flames and ‘popping’ sounds heard indicate disrupted airflow through the jet engine”.

This is known as a “surge and stall”, which can be caused by many things, including “bird strike of the turbine” or when airflow doesn’t hug the engine’s turbine blades, hampering the unit’s ability to generate thrust.

While it’s known to create flames and a small explosion in an engine, which can be disconcerting to passengers, it’s not the most worrying scenario, Evans said.

Qantas, in fact, said that the plane did not make an emergency landing and noted that it was operating with both engines for the unscheduled landing at Christchurch.

“One of our flights to Sydney experienced a suspected engine issue after take-off from Queenstown on Saturday afternoon and the aircraft diverted to Christchurch,” the airline said. “Our pilots are highly trained to manage situations like this and the aircraft landed safely.”

Evans said that on a scale of 1 to 10 for severity, from his knowledge of the New Zealand incident, he would rate it a five. During Evans’ career, which involved flying 28,000 hours on 747s and A380s, he encountered engine issues on only about five occasions.

“An engine shutdown in flight was more of an ‘inconvenience’ than a full-on emergency, although passengers and cabin crew would probably disagree!”

Evans, who helped bring Qantas QF32 to land safely in 2010 after a catastrophic engine failure in Singapore, said that after an engine failure or surge and stall incident, it’s normal to shut the engine down.

An engine can continue to run at idle, so it can support various services powered by it, he said.

“However, you just don’t know what damage is lurking within an engine after a surge and stall, so it’s ‘normal’ to shut it down,” he said. Modern passenger planes are certified to continue flying safely on a single engine, he said.

A surge and stall occurs when the airflow into an engine falters. Fuel in the turbine can ignite, or pressure can force air out of the front of the jet.

Former commercial pilot Keith Tonkin said, “It’s kind of like when a car backfires.”

Tonkin, managing director of Aviation Projects, said of QF122: “Something happened to disrupt the normal airflow through the engine”.

“It could have been caused by disrupted intake air, mechanical damage/failure of a part of the engine or nacelle [the casing of the engine] or a bird.”

Australian and International Pilots Association president Andrew Marshall said: “Events like this are remarkably rare, but Qantas pilots train rigorously for these unexpected situations.”

“The crew on board QF122 demonstrated exceptional professionalism, composure and skill in ensuring everyone landed safely.”

Another former pilot, with extensive experience in commercial aircraft, said there could be many reasons for the QF122 incident, such as an engine worn to tolerance or an engine that plays up when used at high power.

There could be fuel control unit problems, internal damage to compressors, bird strike, a failed blade or damage to blades from foreign objects such as ingesting rocks and rubbish on the runway or tarmac.

He said that the incident looked dramatic, but the aircraft was unlikely to have been in danger.

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