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The Brutal Physics Of An Aircraft Stall Are Emerging As The Likely Key Factor In The AirAsia Disaster

An Indonesian military pilot looks out over the sea in the search for AirAsia flight 8501. Photo: Getty

The official investigation into the loss of AirAsia flight QZ8501, as is typical in these cases, is likely to take months.

The black boxes haven’t yet been recovered and the wreckage of the Airbus A320, which was en route from Indonesia to Singapore, is still being inspected on the sea bed off Borneo. But there are some strong pieces of evidence which, so far, consistently point to the plane stalling, triggering a steep descent which led to the plane crashing into the sea, killing all 162 people on board.

First, there’s this radar map, tweeted soon after the crash by aviation commentator Gerry Soejatman, showing the plane flying at 353 knots, at flight level 363, or 36,300ft. (I’ve circled the data for the AirAsia Airbus.)

Soon after the crash on December 28th, the aviation community was immediately talking about the airspeed of 353 knots. While this might sound fast, it turns out it’s actually very slow for that altitude, where the air is thinner. (In the middle of that radar grab above you can also see UAE409 – Emirates flight 409 from Melbourne to Kuala Lumpur – flying at 36,000ft but at a more comfortable speed of 503 knots.)

As a plane flies higher, it needs to fly faster and faster because the wing can’t generate lift as efficiently as it can with the denser airflow at lower altitudes. Below a certain speed and at a certain angle, the wing stops generating enough lift to keep the plane flying. With the wings no longer doing their job, the plane suddenly becomes a train carriage in the sky, and starts falling.

Dealing with a stall is something all pilots learn in training. But in commercial airliners, stalls are extremely hard to recover from, and bad weather makes it even more difficult. Veteran aviation analyst David Learmount, in an article about the AirAsia crash for the UK Telegraph, has expressed concern that pilots are relying too heavily on the technology in modern airliners and aren’t sufficiently able to recover control in emergencies.

But assuming the data in the radar grab is accurate – there’s no reason yet to doubt it – we have the following facts:

  • The plane was flying at 32,000ft, and asked for clearance to climb to 38,000ft to avoid heavy weather;
  • It was caught on radar four thousand feet above that its 32,000-foot approved altitude, and flying much slower than it should have been travelling, and
  • Clearance was eventually given to climb to 36,000 feet, minutes after the request, but air traffic controllers didn’t hear from the crew again.

There are also reports today that AirAsia flight 8501 entered a baffling sudden climb of up to 9000 ft per minute – around three times faster than what a pilot in control of the Airbus A320 would be able to achieve in normal conditions. (Experts have cast doubt on the plane’s ability to physically climb at this rate, regardless of whether it was under control.) Reuters also reports investigators talking about an “unbelievably” steep climb shortly before the crash.

When you put it together it looks like the plane was climbing quickly, and flying too slowly to maintain lift. It’s the perfect recipe for a stall.

Soejatman also posted this leaked data from the investigation team, which purports to show flight data with the plane falling at a terrifying 11,000ft per minute and moving forward at only 65 knots.

This again is consistent with the plane suddenly losing power and falling straight down – exactly what would be expected after a stall.

Learmount notes there are similarities – at least on the facts currently available – between this accident and other previous tragedies where pilots lost control of their aircraft in bad weather. He notes that the 2009 Air France flight 447 disaster, in which 228 people died, involved the pilots getting confused about their airspeed while manoeuvring to avoid a storm, leading to a stall. In that case, it was found that icing in the instrumentation led to false airspeed data being relayed to the pilots.

The flight data recorder and the cockpit voice recorder, assuming they are found, should quickly confirm if something similar happened on QZ8501. Critically, it will show how the pilots responded and whether weather and the brutal physics of the simple aircraft wing were indeed the central players in the disaster or if the aviation community needs to look again at how pilots are trained to respond in emergencies.

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