CHICAGO, May 21 (Reuters) - Buckle up. That is the

message from flight attendants and pilots following the severe

turbulence encountered by a Singapore Airlines flight

on Tuesday that resulted in the death of one passenger and

injured dozens of others.

The London-to-Singapore flight hit heavy turbulence over the

Indian Ocean and descended 6,000 feet (around 1,800 meters) in

about three minutes, before an emergency landing in Bangkok.

Weather forecasting service AccuWeather said satellite and

lightning data showed "explosive thunderstorms" developing close

to the flight path. Developing thunderstorms can leave pilots

with little time to react, it said.

Airlines are required by law to switch on the seatbelt sign

during takeoff and landing, but carriers have their own

procedures to deal with mid-air turbulence.

A witness on the Singapore Airlines flight said numerous

people who were not in their seatbelt were thrown around the

cabin when the plane dipped, many hitting their heads.

Singapore Airlines CEO Goh Choon Phong said the plane

encountered sudden, extreme turbulence.

Sara Nelson, international president of the Association of

Flight Attendants-CWA representing over 50,000 at 20 airlines,

said initial reports seemed to indicate clear-air turbulence,

considered to be the most dangerous type of turbulence.

Clear-air turbulence cannot be seen and is virtually

undetectable with current technology, making it all the more

important for passengers to wear seatbelts whenever seated, she

said.

"It is a matter of life and death," Nelson said.

The Air Line Pilots Association (ALPA), which represents

more than 77,000 pilots at 41 U.S. and Canadian airlines, echoed

that message, saying the safest way for passengers to protect

themselves is by making sure their seatbelts are always

fastened.

Turbulence-related airline accidents are the most common

type of accident, a 2021 study by the U.S. National

Transportation Safety Board said.

Most recently, in March, a Boeing 787 plane operated

by LATAM Airlines dropped abruptly mid-flight, causing

injuries to more than 50 people.

Aerospace safety expert Anthony Brickhouse said passengers

need to minimize their movement on flights and always stay

buckled in, regardless of the seatbelt light.

American Airlines ( AAL ) requires its pilots to turn the

seatbelt sign on and instruct passengers and flight attendants

to sit immediately when turbulence is severe.

Flight attendants then have to remain seated until notified

by the flight's captain or the seatbelt sign is turned off.

Other airlines have similar protocols.

Some pilots and attendances say that leaving a seatbelt sign

on throughout the flight would backfire - as passengers would

start to ignore it.

"The seatbelt sign means something, and if you leave it on

all the time, it means nothing," said Dennis Tajer, a

spokesperson for the Allied Pilots Association, American

Airlines ( AAL ) pilot union. "Everyone will just say it's a sign that

doesn't mean anything."