What causes clear air turbulence? Singapore Airlines passengers injured by rare phenomenon

A commercial airplane flies through storm clouds. Clear air turbulence comes out of the blue.

Severe turbulence hospitalised two New Zealanders and killed one passenger overnight, aboard Singapore Airlines Flight SQ321.

The service was diverted to Bangkok with dozens requiring medical attention, as the sudden drop reportedly took many off guard. But what is clear air turbulence and how much warning do pilots and passengers have?

Passenger Andrew Davies told Sky News there was little warning before the cabin seatbelt sign was illuminated, and no time for crew to take seats.

In aviation unexpected rough airflow is described as clear air turbulence as it comes out of what was forecast to be clear, predictable, air currents.

Flying at high altitude and over large bodies of water or mountain ranges can cause orthographic turbulence. However, airlines and meteorological reports can normally forecast patches of rough flying and warn the pilots.

Clear air turbulence comes out of the blue.

What causes clear air turbulence?

Clear air turbulence is one of several kinds of atmospheric turbulence that interfere with aviation says New Zealand Met Servie.

“There are four main types of atmospheric turbulence that occur, all of which we include information about in charts and warnings we produce for the aviation sector,” says Met meteorologist Daniel Corrigan.

Along with mechanical, mountain and convection patterns, meteorology servies try to provide clear air turbulence forcasts. Though, by their nature, this is a challenge and must be inferred by calculations from known known wind sheer patterns.

High altitude flights may encounter strong “jet stream” air currents, at above 10,000m. Typically the cruising altitude for jetliners.

“Clear air turbulence occurs at heights well above the terrain, and is the product of wind shear,” says Corrigan. “One place this is common is near the edge of jet streams - fast flowing air currents at typical jet aircraft cruising altitudes.”

Changes in air temperature mixed with fast-moving currents can lead to unpredictable, “strong” or “severe” clear air turbulence.

Forecasters often have to rely on reports from planes to update their turbulence charts, so they can warn other pilots.

Rising clouds and thunderstorms can also provide unpredictable currents for aircraft passing overhead, and pilots often choose to fly around areas with storms forecast.

“For the Singapore Airlines flight SQ321, it is likely that a thunderstorm caused the rapid changes in vertical speed they experienced,” says Corrigan.

What warning do pilots have of clear air turbulence?

It would appear there was little warning for passengers and cabin crew, says New Zealand Air Line Pilots’ Association (NZALPA) president, Andrew McKeen.

Although he said it was too early to speculate about what the pilots knew.

“While equipment on the flightdeck assists in real time to avoid turbulence associated with certain cloud formations, weather forecasting in general has limitations as it relies on computer modelling using estimates and assumptions to predict future weather, so turbulence events can occur without warning,” he told the Herald, extending throughts and condolences to the family of the deceased passenger.

This effectively “unseeable” turbulence is identified as one of the biggest challenges for airlines.

For the past five years the International Air Transport Administration (IATA) has been trying to predict routes and areas where such turbulence might arise using airline data.

“Turbulence is the leading cause of injuries to cabin crew and passengers in non-fatal accidents,” said a statement from the IATA Turbulence Aware Programme.

More disturbing still, the phenomenon is predicted to grow and become harder to forecast with a warming, less predictable, climate.

Earlier this year IATA warned that climate change was making the jobs of pilots and airlines more difficult, and was investing in making more weather data available to partner airlines.

“Managing turbulence is expected to remain challenging, as climate change continues to impact weather patterns. This has implications for both safety and efficiency of flight.”

As well as passenger comfort, better forecasts mean association airlines can minimise disruption of routes.

At present, more than 2000 daily aircraft from 23 airlines provide data to help forecast air turbulence patterns, mapping 380 million annual observations a year.

Severe injuries caused by turbulence are still rare on commercial aircraft. Deaths are even more infrequent. However, it is normal for airlines to advise passengers to keep seatbelts fastened while seated.

“It is a matter of life and death,” says Sara Nelson, international president for flight attendants association CWA.

“As our climate changes, severe and clear air turbulence instances are on the rise. Always follow crew instructions and wear your seatbelt whenever seated.”

A report into turbulence-related injuries by the United States Federal Aviation Administration showed that 80 per cent are to crew. That’s mainly because they are most likely to be out of their jumpseats, pursuing their duties in the cabin.

“Unfortunately, incidents occur where passengers and crew are injured due to severe turbulence. Our advice is to always keep your seatbelt fastened while you are seated even if the seatbelt sign is off,” said McKeen.