SINGAPORE, May 31 — On the back of this week’s early official findings on the severe turbulence that hit Singapore Airlines (SIA) flight SQ321 on May 21, TODAY spoke to aviation experts to better understand what they mean.

On Wednesday (May 29), the Transport Safety Investigation Bureau, an arm of the Ministry of Transport, gave a chronology of events of the flight based on early findings using the flight data recorder and cockpit voice recorder.

The severe turbulence that hit the London-Singapore flight left one passenger dead after a suspected heart attack and dozens of others injured including crew. The flight was diverted to Bangkok in Thailand for emergency support.

In a press release, the Transport Safety Investigation Bureau outlined how SQ321 likely encountered rough weather, referred to in the statement as "developing convective activity”, when it was flying over Myanmar mid-afternoon Singapore time on May 21.

The aircraft experienced an "uncommanded” rise in elevation of up to 110m, causing the autopilot function to pitch the aircraft downwards to correct it back to the selected altitude of 37,000ft (about 11,300m).

The pilots also noticed an increase in airspeed that they arrested by extending the speed brakes.

The changes in elevation and airspeed were likely caused by an updraft, the preliminary findings showed, and that autopilot was engaged during this period.

At 3.49.40pm Singapore time, the aircraft experienced a rapid change in gravitational force or G-force as "recorded vertical acceleration” fell from +1.35G to -1.5G within 0.6 of a second.

The aircraft also fell about 54m in elevation.

This worked out to a 2.85G change, which likely caused the people onboard not wearing seat belts to become airborne.

About one second later, the vertical acceleration changed from -1.5G to +1.5G within four seconds. This likely resulted in the people who were airborne to fall back down.

During the rapid changes in G-force, the pilots disengaged autopilot and manually stabilised the aircraft before re-engaging autopilot 21 seconds later.

What is G-force and what does a change in G-force feel like? What does vertical acceleration mean?

When do pilots engage in autopilot, and when do they take over the plane in instances of severe turbulence?

The aviation experts interviewed by TODAY helped to answer these questions as well as why rapid G-force changes can have such severe consequences.

G-force is the force of gravity or acceleration on an object. It is measured in units of G, where 1G is the normal force of gravity at the Earth’s surface.

For example, 2G means an object feels twice the normal gravitational force. A person experiencing 2G will feel as though their body weight is double their normal weight.

Mr Abbas Ismail, the course chair of aviation management at Temasek Polytechnic, said: "This force is experienced by pilots during rapid acceleration, deceleration or sharp turns, affecting their bodies and aircraft.”

G-force can be measured both horizontally and vertically.

Horizontal G-force "measures the force parallel to the ground, experienced during turns, acceleration and deceleration”, Mr Abbas explained.

In the case of SQ321’s findings, it is vertical G-force, which means the force perpendicular to the ground that is experienced during vertical manoeuvres such as takeoff or landing, for example.

Mr Faris Iskandar, co-founder of flight school Aeroviation, said that a G-force in vertical acceleration means that there is a "sudden drop in height”.

"Should there be a sudden change in the acceleration, G-forces will be felt,” he added.

He also outlined the difference between positive and negative G-forces.

If a car driver suddenly accelerates quickly, the passenger will be thrown backwards in the seat and experiences positive G-force.

In the case of an amusement park’s drop tower, the point where it drops causes a sudden acceleration downwards, and people will experience negative G-force or a "floating sensation”.

The passengers aboard flight SQ321 experienced a change of 2.85G. Mr Abbas said that experiencing 3G means feeling a force three times stronger than the normal pull of gravity on Earth.

"For a person, this sensation would feel like their body weight has tripled. For instance, if someone weighs 70 kg, under 3G, they would feel as if they weigh 210kg.”

Mr Faris said that when an aircraft makes a turn, a passenger will feel an average of 1G to 1.5G, depending on the angle of the turn. "On a rollercoaster, a passenger can feel up to a maximum of 4G on extreme rollercoasters.”

During these changes in G-force, passengers might find it hard to lift their arms or heads and feel strong pressure against their bodies.

"These forces are intense but generally manageable for short periods,” Mr Abbas said.

"However, prolonged exposure to high G-forces can lead to discomfort and potential health risks, such as difficulty breathing or blood pooling away from the brain, potentially leading to loss of consciousness,” he added.

How does autopilot on a commercial aircraft work?

Autopilot is designed to assist the aircraft in staying within its assigned parameters, as set by the pilot, thereby ensuring a smooth and controlled flight.

Mr Faris said that autopilot is commonly engaged during long flights worldwide because it ensures that the plane remains within its assigned parameters and relieves the pilot’s workload.

"Usually, the pilot inputs parameters such as speed, direction and altitude, and the plane adjusts itself to bring itself back to those parameters if disturbed.

"Throughout the years, the autopilot has been used most of the time as airplane systems are advanced enough for the aircraft to follow the assigned route and maintain its flight.”

In terms of the autopilot systems, Mr Abbas said that they can range from basic flight controls to highly sophisticated systems capable of handling almost all aspects of flight, from takeoff to landing.

He added that autopilot is usually engaged in the various stages of a flight, depending on the specific flight operations and conditions.

For instance, during the climb phase, the autopilot might be engaged to maintain a steady climb and adhere to the planned route. During descent, autopilot might be engaged to allow for a more controlled and smooth descent.

Some advanced autopilot systems can perform automatic landings during landing, especially in poor visibility conditions.

However, Mr Abbas said that pilots typically disengage the autopilot just before landing to manually control the final approach and touchdown.

"While autopilot enhances safety and efficiency, pilots remain actively involved, monitoring systems and ready to take manual control if necessary.”

In what situations do pilots override the autopilot function and when do they re-engage it?

Some modern autopilot systems are designed to handle moderate turbulence effectively, Mr Abbas said.

"These systems can make precise adjustments to maintain a stable flight path, which might be challenging for manual control in rapidly changing conditions.”

However, in "clear air turbulence”, the decision to intervene manually or rely on autopilot depends on the specific circumstances and the capabilities of the autopilot system.

Clear air turbulence (CAT) is turbulence that occurs in clear skies without any visible warning such as clouds or storms.

"In severe CAT, pilots may choose to disengage the autopilot if they feel manual control would better handle the sudden and unpredictable movements,” Mr Abbas added.

"Experienced pilots can sometimes react more intuitively to turbulence.”

In severe turbulence, autopilot would not be able to hold parameters due to drastic changes in the aircraft’s attitude and would automatically disengage, Mr Faris said.

Pilots in the cockpit would usually be given an audio and visual warning that autopilot is disengaged.

"The pilot will then take control of the aircraft to stabilise the aircraft and ensure that the aircraft is still in stable flight.”

The pilot would then most probably follow a checklist and decide with support personnel what to do next.

Mr Faris said that the pilot might either choose to climb or descend to fly in smoother air and avoid CAT.

Mr Abbas said: "In general, during severe CAT, pilots closely monitor the situation and decide whether to intervene based on the severity of the turbulence, aircraft behaviour, and autopilot performance.

"Safety is the primary concern, and pilots are trained to use their judgement to either rely on the autopilot or take manual control as needed.” — TODAY