Featured image: Michael Rodeback/Airways

Flight SQ321: The Facts Around Air Turbulence

Airways Staff

DALLAS — Turbulence in flight is the phenomenon involving the airplane's irregular, annoying and unpredictable motion causing it to fly in an undesired attitude, altitude, and direction.

If this situation is severe and uncontrolled, it can cause injuries to passengers and crew and damage the airframe.

Singapore Airlines (SQ) Flight SQ321, departing from London Heathrow Airport (LHR) on May 20, 2024, encountered severe turbulence during stormy weather and was forced to make an emergency landing at Suvarnabhumi International Airport (BKK) in Bangkok.

The SQ Boeing 777 aircraft, carrying 211 passengers and 18 crew members, experienced a rapid plunge while flying at approximately 37,000 feet. Tragically, at least one passenger, a British citizen, has died as a result of the turbulence, and several others have been injured.

The incident occurred amid stormy weather conditions. What are other causes and how do pilots deal with turbulence?

Stormy skies surround this departing JetBlue Airbus A321. Photo: Matt Calise/Airways

Types of Turbulence

There are four main types of turbulence:

  1. Clear-air turbulence: This type of turbulence occurs when large air masses of high or low pressure collide or when encountering high-level jet streams. As we'll see, clear-air turbulence incidents have increased in the last four decades, and avoiding them by planning flight routes that steer clear of these areas is never easy.
  2. Wake turbulence: Flying in the wake of a preceding aircraft that has disturbed the air generates wake turbulence. Consider the visible wake left by a large boat. To avoid wake turbulence, aircraft maintain lateral separation, fly at different altitudes, or offset their tracks.
  3. Mechanical turbulence: Strong winds passing over mountains cause mechanical turbulence. This generates waves of turbulence that can reach up to five times the height of the mountains and extend for hundreds of miles, depending on wind strength. Planning flight routes around mountainous areas or regions prone to this type of turbulence can prevent encountering it.
  4. Weather-related turbulence: Weather-related turbulence occurs in the presence of cumulonimbus clouds, commonly known as thunderclouds. These large, white, puffy clouds can grow in width and height, often surpassing the normal cruising altitude of airliners. To avoid weather-related turbulence, pilots choose to fly around these clouds or delay take-offs and landings if they directly impact the airport.

Regarding Flight SQ321, the incident involved clear-air turbulence, which can be challenging to anticipate as it does not appear on the weather radar in the flight deck.

Conducting a comprehensive analysis of the meteorological conditions and the specific type of turbulence that led to the sudden drop of the aircraft will require further investigation.

Photo: Fabrizio Spicuglia/Airways

Causes of Turbulence

Various factors cause turbulence, including:

Thermal heating on the earth's surface by solar radiation on a hot sunny day warms the air, making it less dense and rising vertically into the atmosphere. The rising air mixes up with the air above, leading to a turbulent airflow/wind, which can alter an aircraft's flight path in severe cases.

Wake Vortex that trails from the wing tips of larger, heavier, and low-speed airplanes causes turbulent air, which poses a danger to any lighter aircraft following. To minimize the effect of wake turbulence, air traffic controllers (ATC) allow enough time for the vortex to dissipate by the appropriate separation between those departing. In the case of landing aircraft, controllers apply a safe distance between aircraft on approach.

Solid terrain, such as tall buildings and trees near the approach and takeoff, causes the wind to change direction and speed, creating vortices near the ground. These vortex systems impact turbulence on airplanes during takeoff and approach. This is called mechanical turbulence.

Mountain ranges cause the wind to flow perpendicular to it as it oscillates like a wave and can result in turbulence up to the lower stratosphere (the second layer of the atmosphere). Such waves pose a great turbulence danger to an airplane approaching the mountain from the leeward side.

Thunderstorms are associated with up-and-down movements of air currents, which cause turbulence when the aircraft enters them. The turbulence associated with thunderstorms exists even outside the storm, up to 50 miles in its vicinity. Pilots frequently alter headings to avoid areas of bad weather.

Disturbed wind flows around mountain ranges can cause turbulence for overflying aircraft. Photo: Michael Rodeback/Airways

How Pilots Deal With Turbulence

Turbulence can be unnerving to passengers. But it's generally not dangerous and is often more of an inconvenience than a safety issue. However, if turbulence happens or is expected in any phase of flight, pilots are trained to deal with it in the following ways:

Use of seat belts

To avoid passengers being shaken back and forth or hitting their heads, seat belts should be left loosely fastened at all times. It's very important to keep your seat belt on even when the seat belt sign is off. Flight SQ321 is a stark reminder of this, as passengers described being "launched into the ceiling" due to the turbulence.

It is important to note that pilots will always turn on the seat belt sign when turbulence is expected. Further, if turbulence worsens, the captain will inform the cabin crew through the public address system (PA) to take their seats and put on seat belts.

Analyzing SIGMET chart

Pilots analyze SIGMET (Significant Meteorological Information) reports to plan and prepare for necessary actions if they encounter hazards like turbulence. SIGMET features Jet streams, thunderstorms, heavy clouds, turbulence reports and Icing

Passengers could keep their seatbelts loosely fastened whilst seated incase of unexpected turbulence. Photo: Christopher Doyle from HorleyCC BY-SA 2.0

Use of Weather Radar

Weather Radar can detect precipitation and thunderstorm clouds, as they are associated with turbulence. Pilots use weather radar and coordinate with ATC to avert away from thunderstorms and heavy clouds.

PIREP

PIREP is a pilot's report of the preceding aircraft to the following pilots flying in a particular airspace. When pilots encounter turbulence, they report its intensity, location, time, altitude, and aircraft type so that the following pilots can adjust their height or track to avoid it. A pilot may ask clearance from the ATC to climb or descend to avoid reported or experienced turbulence.

Radio Transmission Frequency (RTF) and TCAS Display Monitoring

Traffic Alert and Collision Avoidance System (TCAS) display and designated RTF monitoring help awareness and enable pilots to be more proactive in asking ATC for assistance in avoiding wake vortex turbulence from other aircraft.'

"Liftoff before and land beyond" technique

When an ATC gives a wake turbulence caution to a pilot taking off or landing behind a larger aircraft, a pilot will be alert and avoid wake turbulence.

On the final approach and behind larger aircraft, the pilot of a smaller general aviation aircraft stays at or above the larger aircraft's final approach path and aims to land beyond its touch-down point, provided the remaining landing distance is adequate to bring it to a stop.

Airliners must plan their landing within the touch-down zone markings at the beginning of the runway. Controllers will vary the distance between landing aircraft based on the preceding and the following aircraft size.

All aircraft are now fitted with sophisticated weather radar. Photo: Joao Pedro Santoro/Airways

Mountain Wave Avoidance

When pilots expect to encounter turbulence when flying in mountainous areas, they usually plan to fly at least 50% higher than the height of the mountain peak above the surrounding base of terrain to provide an adequate margin of safety and recovery if strong turbulence is encountered.

Also, pilots approach mountain ranges at a 45-degree angle to make an immediate escape turn if severe turbulence is encountered and avoid the leeward side of the mountain ranges where strong downdraft may prevail.

Penetrating the turbulence

It is not possible for pilots to avoid flying in turbulence, such as in areas around the equator, areas with tall buildings that disturb the wind, or on a hot afternoon with a marked inversion. 

Pilots are professionally trained to bring you safe and comfortable flights. In such instances, pilots establish and maintain engine power settings to obtain and fly turbulence penetration speed and maintain level flight until the airplane gets out of turbulence.

Aircraft are designed to cope with even the roughest turbulence. Photo: Mateo Skinner/Airways

Averting Passenger Fears

Paul Williams, a professor of atmospheric science at the University of Reading in England, tells Newsweek that fatalities due to turbulence on commercial flights are very rare.

Williams notes that the last fatality caused by clear-air turbulence occurred on a United Airlines (UA) flight from Tokyo to Honolulu in December 1997. "Other forms of turbulence have caused more recent fatalities, but as far as I am aware there has not been a turbulence fatality on a commercial flight since 2009," Williams added.

The academic warned that turbulence incidents could double or even triple in the next decade if climate change was not curbed, saying there was "strong evidence" and adding that "severe clear-air turbulence in the North Atlantic has increased by 55% since 1979."

In a more technical explanation, a recent report by Forbes states that "warmer air caused by carbon dioxide emissions increases wind shear in the jet streams, strengthening clear-air turbulence globally."

Singapore Airlines 9V-SWF Boeing 777. Photo: Alberto Cucini/Airways

Fasten Your Seat Belts

Regardless of the causes, in the event of turbulence, experts advise passengers to remain calm and fasten their seat belts as soon as the seat belt sign turns on or when they are instructed to do so by the cabin crew.

Again, fastening seat belts minimizes the chances of injury during severe turbulence. According to an study from the U.S. National Transportation Safety Board (NTSB), most of the passengers and flight attendants "injured during turbulence were not wearing a seat belt at the time."

At the time of writing, the NTSB has informed via X that it was "sending an accredited representative and 4 technical advisors to support the Singapore TSIB's investigation of the turbulence accident on Singapore Airlines Flight 321, under ICAO's Annex 13."

One last thing—have faith in your pilots, because they are highly skilled, expertly trained and equipped with modern technology to fly through or avoid turbulence.

Airways extends its deepest condolences to the families and loved ones of the passengers affected by the tragic incident on Flight SQ321.

Pre-order the 2025 Airways Calendar