The mechanics of turbulence; How Singapore Airlines incident occurred

The sudden turbulence occurred over the Irrawaddy Basin in Myanmar about 10 hours into the flight, the airline said. The pilot declared a medical emergency and diverted the aircraft to Bangkok, it said.

Aircraft tracking provider FlightRadar 24 said at about 0749 GMT the flight encountered “a rapid change in vertical rate, consistent with a sudden turbulence event”, based on flight tracking data.

“There were thunderstorms, some severe, in the area at the time,” it said.

Rapid movements

Altitude data from FlightRadar24 shows how the aircraft was thrust up and down over the course of a minute, before returning to its original cruising altitude of 37,000 ft.

During the event, the plane’s vertical speed, the rate by which the plane ascends or descends, quickly surged to 1,664 feet per minute before plummeting to -1,536 feet per minute in a matter of three seconds.

What is Turbulence?

Turbulence or pockets of disturbed air can have many causes, most obviously the unstable weather patterns that trigger storms, according to an industry briefing by European planemaker Airbus.

The resulting water particles can be detected by weather radar. Crews plan ahead by studying turbulence and other weather forecasts, which have improved over the years, loading extra fuel when needed and monitoring weather radar during flight.

Clear Air Turbulence (CAT) is a sudden and severe swirl that causes violent buffeting of a plane even where there are no clouds, according to the U.S. Federal Aviation Administration.

Such pockets of air can occur without warning and are hard to predict. Mark Prosser of the University of Reading said Tuesday’s event most likely involved convective or storm-related turbulence, but stressed it was too early to be certain.

Other factors can disrupt the smooth flow of air around an aircraft. Frontal turbulence happens when a plane flies through a weather front, where different air masses meet. This mixing of air causes unstable conditions and bumpy flights. Flying through the wake of other aircraft can also cause disruption.

Other forms of turbulence can come from airflow disruptions from ground obstacles like buildings and mountains, common during takeoff and landing. On hot, sunny days over land, rising warm air and descending cooler air can also cause turbulence.

Buckling up

When an aircraft encounters turbulence and suddenly moves, anything not secured, such as passengers, can continue moving in the original direction, while the plane moves in another. Injuries from turbulence can occur as passengers are thrust towards the ceiling of the aircraft.

Photographs from the plane’s interior showed large gashes in the overhead cabin panels, gas masks and panels hanging from the ceiling and items of hand luggage strewn around. A passenger said some people’s heads had slammed into the lights above the seats and punctured the panels.

Unions representing U.S. airline pilots and flight attendants told Reuters the incident highlights the importance of following crew instructions and wearing seatbelt whenever seated.

But they cautioned that leaving the seatbelt sign on all the time could erode its significance, causing passengers to start ignoring it.

Among the most exposed to injury are crew who must tour the cabin to check seatbelts have been fastened when signs go on.

All modern commercial jets are designed and built to cope with forces many times those experienced in-flight, Hayes said.

But according to Swedish turbulence-forecasting website Turbli, the feeling experienced by passengers varies from plane to plane and seat to seat.

Long planes can feel most turbulent at the back and the ideal spot is around the centre of gravity, typically just ahead of the wings.

How common is it?

Turbulence-related incidents are common, according to a 2021 study by the U.S. National Transportation Safety Board.

From 2009 through 2018, it found that turbulence accounted for more than a third of reported events and most resulted in one or more serious injuries, but no aircraft damage.

Yet fatal turbulence in air travel remains extremely rare.

“It is a very unusual and rare event. As far as I can tell it is over 25 years since a passenger was killed by commercial airliner turbulence,” said Paul Hayes, director of safety at UK-based aviation data group Cirium Ascend.

The last fatal turbulence-related accident on Cirium's database involved a United Airlines Boeing 747 in 1997.