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Aircraft had 54 metre altitude drop in 4.6 seconds

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People not wearing seatbelts forced into air, probe says

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Aircraft encountered rapid change in altitude, air speed

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28 passengers still in Thai hospitals - airline

(Adds changes in numbers in hospital, seat belt requirement

Greater Bay Airlines is introducing in paragraphs 10, 15-16)

SINGAPORE, May 29 (Reuters) - Preliminary findings

released on Wednesday of an investigation into a Singapore

Airlines flight hit by severe turbulence last week

showed that a rapid change in gravitational force and a 54-metre

altitude drop caused injuries.

A 73-year-old passenger died of a suspected heart attack and

dozens were injured after flight SQ321, flying from London to

Singapore, encountered what the airline described as sudden,

extreme turbulence while flying over Myanmar.

The May 21 flight on a Boeing 777-300ER plane

carrying 211 passengers and 18 crew diverted to Bangkok for an

emergency landing after the plane was buffeted by turbulence

that flung passengers and crew around the cabin, slamming some

into the ceiling.

"The aircraft experienced a rapid change in G (gravitational

force) ... This likely resulted in the occupants who were not

belted up to become airborne," the Singapore Transport Ministry

said in a statement.

"The vertical acceleration changed from negative 1.5G to

positive 1.5G within 4 seconds. This likely resulted in the

occupants who were airborne to fall back down," it said, citing

information extracted from the flight data and cockpit voice

recorders.

"The rapid changes in G over the 4.6 seconds duration

resulted in an altitude drop of 178 ft (54 m), from 37,362 ft to

37,184 ft. This sequence of events likely caused the injuries to

the crew and passengers," it added.

CHAOTIC SCENES

Shaken passengers described scenes of chaos in the minutes

after the incident, with the turbulence throwing people upwards

then into the aisle, many left with bleeding and head wounds.

Photographs of the cabin showed gashes in the overhead cabin

panels, oxygen masks and panels hanging from the ceiling and

luggage strewn around. A passenger said some people's heads had

slammed into lights above the seats and broken the panels.

Singapore Airlines said it acknowledged the report and was

cooperating fully with the investigation.

The airline late on Tuesday had said 42 people who were on

board the flight were still in Bangkok, including 26 passengers

receiving medical treatment in hospital.

Among those initially hospitalised were patients with spinal

cord injuries and some with brain and skull injuries, according

to Thai medical officials.

The preliminary report said that upon the flight

encountering slight vibrations there was an uncommanded increase

in altitude, resulting in the autopilot pitching the aircraft

downwards. The pilots experienced an increase in airspeed and

responded by applying speed brakes.

"While managing the airspeed ... it was heard that a pilot

called out that the fasten seat belt sign had been switched on,"

it said.

Hong Kong's Greater Bay Airlines said on Wednesday it would

require passengers to fasten seatbelt at all times during the

flight even when the seat belt sign is off, starting from

Thursday.

The company said it's not a mandatory requirement but a

precautionary measure for passenger safety.

The investigation team comprised Singaporean investigators,

and U.S. representatives from Boeing ( BA ), the National

Transportation Safety Board (NTSB) and the Federal Aviation

Administration (FAA). The Singapore transport ministry said the

probe was ongoing.