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 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.

Singapore Airlines did not say what type of turbulence was

involved, but aviation experts suspect it to be clear-air

turbulence, considered to be the most dangerous type of

turbulence.

Clear-air turbulence (CAT) is virtually undetectable with

current technology, meaning it can hit without warning - making

it all the more important for passengers on a plane to wear

seatbelts whenever seated, safety experts said.

Airlines are required by law to switch on the seatbelt sign

during takeoff and landing of flights, 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, with many hitting their heads.

Sara Nelson, international president of the Association of

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

said CAT instances are on the rise and cannot be seen, stressing

the importance of being buckled during flight.

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

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."