Original subtitles

Roller coasters are supposed to be all about fun and thrill,

but when engineering goes wrong, it can be fatal.

2016 in Australia.

A raft capsizes on the Thunder River Rapids,

a tragic accident foreshadowed by perilously low water levels

on the ride that would claim the lives

of four people.

[Rob] The water levels at the loading and unloading bay

have dropped beneath where they're supposed to be.

1972.

Five die, and another 13 are injured

when a roller coaster leaves a horrific legacy.

The rope suddenly snaps.

[Brendan] And then, this whole set of carriages

start to roll backwards.

[Rob] Some passengers are projected from their seats.

[Dr. Shini] Others are crushed in the wreckage

as the train folds upon itself, one car at a time.

2023.

a replacement part, intended to increased safety, fails,

leading to a series of mechanical issues

that claim the life of one rider and leaves others

with serious injuries in Stockholm, Sweden.

The front car slumps towards the track.

The deceleration forces three riders out of their cars.

What did we learn from these tragic incidents?

What are the reasons behind such fatal engineering?

[enigmatic theme music playing]

We're in Stockholm, Sweden.

♪♪

On this summer Sunday, thrill-seekers are flocking

to the Gröna Lund amusement park.

Gröna Lund has a place in every Swedish person's heart.

It's in the heart of Stockholm. It's a very small family park.

Alongside its smaller attractions and music venues,

Gröna Lund also features a handful of roller coasters.

One of these is the Jetline Ride.

[beeping]

[roller coaster rumbling]

Built in 1988, the Jetline features

an 800-meter-long track that rises 30 meters

to its highest point.

The track is divided into four sections --

each section is separated from the other

by a braking point to ensure the permanent safety

of the attraction.

[Dr. Shini] Sensors along the track would monitor

the speeds of trains at various sections.

Fast speeds recorded at a section would trigger

the application of the braking system

at the following brake point to lower speeds.

Each train is made up of seven cars,

each able to carry two riders, for a total of 14.

This ride is the one you're going to go on

if you go to the park.

It's the one that everybody's talking about,

and the ultimate thrill on the park.

11:35 AM.

Eleven riders board the Jetline train

ready for the biggest thrill of their day.

[Brendan] So at the start of the ride,

everything's going as normal.

We're going up the lift hill.

Clack, clack, clack, clack, clack.

[roller coaster rumbling]

[Brendan] Then we're down into the first drop.

[roller coaster rumbling]

[riders screaming]

We're picking up speed.

[roller coaster rumbling]

[riders screaming]

[tense music starts]

As the train passes brake point one,

it begins to descend into a long right hand turn.

[roller coaster rumbling]

Little did they know that fatal engineering is about to play

its part in making this the final day

Jetline would ever run.

[roller coaster rumbling]

11:36 AM.

As it rolls down a curve, something breaks

under the leading car, turning this family attraction

into a runaway train.

[Dr. Shini] Front car slumps towards the track,

striking raised joints that scar the underside of the train

and cause it to decelerate

in a violent and uncontrolled manner.

Actually screeching along the rails,

and people can feel this inside.

Suddenly, there's this horrible grating sound.

[Rob] Because of the violent deceleration,

passengers' bodies are forced and pressed into the lap bar.

Mechanical parts fall onto the crowd below.

The train continues to scrape along

the Jetline's central axis.

[Brendan] The whole thing reverberates along the entire

length of the train.

As these collisions continue, the deceleration

that the riders and the train experiences

forces three riders out of their cars.

[Dr. Shini] The train now significantly decelerates

fails to make it up the next rise, instead,

reversing back down the track before coming to a rest.

[riders screaming]

Most of the occupants of this terrifying ride

suffer internal injuries.

Of the three riders ejected from the Jetline,

two are seriously injured, the third dies of his injuries.

The park is immediately closed.

Health and safety inspectors will come and look at the ride.

They will look at why there was this engineering failure.

It's at a forensic level.

All the components of the ride are inspected and reinspected,

right down to x-raying welding joints.

And they find the culprit.

Here's the cause of failure, a part that's vital

to the smooth operation of the roller coaster...

the control arm.

The control arms connects the main body, the chassis

of the roller coaster car, to the track below.

All cars have one fixed connection,

except for the lead car, where the connection

is a little different.

The control arm is flexible.

[Dr. Shini] This flexible connection enables the first car

more vertical motion in relation to the track,

helping it to absorb more of the gravitational forces

placed upon it as it enters each dip or change

in the ride's elevation.

It's this first set of control arms

that we have to worry about, because that's where

the fatal engineering failure is gonna happen.

To understand why the control arm on the first car broke,

we need to go back three months.

Following a maintenance inspection, the decision

was taken to replace several control arms

that were showing signs of wear.

In March 2023, new arms were installed.

but they did not fully meet safety regulations.

In the original control arms, a transverse plate of metal

welded inside the hollow, square tube of the control arm

helps to strengthen the metal and also distribute tension

throughout the structure.

In the replacement parts, this additional metal plate

hadn't been included.

Meaning, it was no longer as strong

as the original design and fabrication.

In the three months following its replacement, this part was

put through its paces at every turn of the Jetline.

Microscopic cracks have formed

and the control arm is in critical condition.

On the morning of June 25, 2023,

it reached its breaking point.

[roller coaster rumbling]

[Rob] Earlier in the day, regular riders of the Jetline

had reported vibrations and loud noises that they felt

as the train went around the track.

[Brendan] There's a bit of grating going on

and a slight juddering sensation.

It's not like it should be behaving.

It's actually screeching along the rails,

and people can feel this inside.

Little did they know this would be the last day

those trains ever ran.

11:35 AM.

As the train makes its way up the first slopes,

the muffled noises turn into ear-splitting screeches.

What the people in the train don't realize is that

there's something wrong with the control arm underneath.

11:36 AM.

As the train approaches the attraction's third incline,

the right-hand control arm connecting the train

to the track breaks on the lead car,

throwing it completely off balance.

The car drops suddenly onto the metal bars of the track.

[Dr. Shini] As the car screeches along the track,

it comes up against another element

of the ride's construction,

the joints of the track.

[Rob] In an ideal world, you'd build a roller coaster

without any joints.

But practicality in manufacturing

and assembling a ride such as Jetline

dictate that you need to build a ride in sections,

and then you join those sections together.

And you'd seek to join them as seamlessly as possible,

but wherever there's a joint, you need something

to hold that together.

The Jetline was built by connecting sections

of rail every four to six meters.

These sections have three main parts --

two rails on each side

on which the wheels of each train roll,

and a central section that supports the track structure.

It's in this central section that we find another element

that will prove disastrous to the train and its riders.

[Rob] On the Jetline, there are bolts that connect the track

and hold them securely together.

Under normal operating conditions,

trains pass over these track connectors,

which are slightly raised above the profile of the center rail.

When the train's control arm breaks, the leading car

collapses violently onto the track,

and these raised fittings.

[Brendan] This first carriage is now dragging

along this top steel tube,

banging into all these collections

of nuts and bolts along the way.

Each impact creates a violent shock wave

across the entire structure.

[Rob] And riders are slammed into the lap bars that should be

the safety component, keeping them within the car

and keeping them safe.

These lap bars have been designed for ordinary operation.

And the worst thing that can happen in ordinary operation is

a safety e-stop, where we bring the carriage

to a stop quite quickly.

Tragically, this is exactly what happens.

The accident investigation revealed that

the arm's failure, combined with repeated impacts

between the lead car and track joints,

generated forces that exceeded design limits.

This rapid and unexpected deceleration results

in the violent braking of the Jetline train.

That repeated bashing bends these safety bars out of shape,

leaving them practically wide open.

It's not there to stop this kind of impact.

It's like being hit in the chest by a sledgehammer

as they hit the safety bar.

These repeated blows deform and weaken the safety bars.

Worse still, some of them unlock.

The weight is enough to bend the lap bars out of place,

providing enough room for a passenger to fall

from the car.

We actually find in one of the cars that the bar's bent

so far forward, it's almost completely open.

On the Jetline's third descent, a passenger in car number five

is ejected from his seat, plunging some 15 meters

before hitting the ground.

With multiple fractures

and extensive internal injuries,

tragically, he dies.

As the train continues to collide with track joints,

two more passengers, both in the rear carriage,

are thrown from the ride.

[riders screaming]

[melancholic music playing]

One of the riders falls to the ground,

suffering fractures to his face, arms, and legs.

The other rider, ejected from his seat, lands on a beam

and climbs onto the track.

Miraculously, he suffers only minor injuries.

The remaining riders are not spared.

All suffer internal injuries due to the compression

of the safety bars.

[Dr. Shini] Considering the vital nature

of safety bars and restraints on roller coasters,

the failure of Jetline's lap bars

to keep all passengers firmly locked in place

while on the roller coaster was monumental.

Forever associated with this accident,

the Jetline closes permanently.

The Gröna Lund tragedy highlighted the vital need

for theme parks to pay closer attention to all renovation

and maintenance work carried out on their rides.

Had these retaining arms been manufactured

to the original specifications, the accident would most likely

never have happened.

All it took was a simple flaw to set off a chain reaction

that led the riders into a sequence of events

that no safety procedure had foreseen.

There is an inherent amount of risk deciding to go

on a roller coaster, and in many ways,

that's part of the fun.

Nobody should ever die because of that decision.

[Rob] The Jetline was a relatively modern ride with a number

of automated, mechanical, safety measures installed.

But in 1972, any safety measures were much more rudimentary,

and a ride at a theme park in London

still holds the record today

for the worst roller coaster disaster in history.

♪♪

Opened in 1951 as part of the festival of Britain,

the Battersea Park Funfair operated for over 20 years.

Designed to lift British spirits after the chaos

of World War II.

With about 40 rides, it is one of the oldest

and largest theme parks in the country.

And one of the great star attractions was the Big Dipper.

The Big Dipper, modeled after the American roller coasters

that were making waves across the Atlantic.

It was the largest roller coaster in England.

It's a classic coaster with eight slopes

and a 1.2-kilometer track.

One of the longest for its time.

[Brendan] It's a wooden coaster, so you get a sort of giant,

kind of, structure -- swirling structure

with lots of wooden beams,

and on there, there's this rail track.

And sat on top of the rail track, you've got three cars

and each car can take five rows of two people,

so that's 30 people in total on this car.

[archived reporter] No chance of going off the rails here.

It's a 55-mile-an-hour journey with your heart in your mouth

and your stomach trying to catch up.

A brakeman is positioned between the first

and second car.

Their task is to control a braking lever to ensure

the train doesn't exceed speed limits and guarantees

a controlled stop at the end of the ride.

[Brendan] The brakeman is like the driver of a train,

but he's only got one thing he can do,

which is apply a little bit of brake pressure on there.

So if anything happens, that's the only mechanism he has

to do anything about the ride.

The Big Dipper is a safe attraction

and a benchmark of its kind, symbolizing the rebirth

of an England looking toward the future.

[Brendan] The ride had got an excellent track record.

It had been running for just over 20 years.

[Dr. Shini] As a long-standing source of fun and enjoyment,

tourists flocking into the Battersea Funfair

could not have suspected that the park's greatest attraction

would one day turn into

one of the most deadly accidents

in the history of roller coasters.

Tuesday, May 30, 1972.

It's 2:00 PM.

School has just ended and children

from all over London are flocking to Battersea

to enjoy some time away from their studies.

Their destination, the Big Dipper,

where the queue stretches endlessly.

Passengers climb aboard

one of the Big Dipper's three carriages.

What they don't know is that it will end in tragedy.

[suspenseful chord strike]

[Brendan] So we've got 30 people packed on this ride.

On this particular fateful day, they're getting towards the top

of the ride, where it's just going to tip over,

and you expect it to go hurtling the other way.

But for the first time since the Big Dipper's creation,

the scene takes a completely different turn.

As the train is pulled up the first incline,

ready to begin its ride,

it suddenly starts rolling backwards down the track.

They think it's the guy's operating the ride,

having a bit of a laugh with them,

mixing up a bit making it exciting.

The brakeman is gonna put the brake on,

and then they continue again and so this sense in the car,

everybody's super excited, this is something new.

But it's only after a few seconds

as they continue to pick up speed going backwards

that people realize this is not supposed to be happening.

In reality the cable pulling the train upward

has just snapped. The train has nothing left

to hold it back.

These three carriages and 30 riders

are at the mercy of gravity.

They gain speed over a 27-meter drop.

Out of control and rolling in reverse,

the train hurtles past the station

before hitting a sharp corner and separating from the track.

[Rob] Some passengers are thrown from their seats.

Others are squashed in the collision that follows.

It was an absolute engineering catastrophe.

Five children lose their lives in the accident,

and 13 others are seriously injured.

Most of the people at the back all got crushed up now,

because they came down backwards.

Why did the Big Dipper fail?

What lessons have we learned from this tragedy?

To understand the reasons behind the Big Dipper accident,

we need to look at its construction.

Like all roller coasters, this ride features a number

of ramps and inclines.

Modern roller coasters often use strong chains

to lift passenger trains to the top of the first ramp.

But on the Big Dipper, the system is more rudimentary.

[Brendan] It's incredible to think about the technology

they're using here.

They're using a rope to haul a carriage

containing 30 passengers all the way up to the top.

The rope is connected to a winch,

and when that winch is wound, it drags the cars

to the top of the climb to a point where gravity

will do the rest as the train makes its way around the ride.

[Dr. Shini] If this rope snaps or fails,

the train would be unable to continue the ascent

and be at risk of rolling backwards down the incline.

Unfortunately, this would be only the first issue

the Big Dipper would face on Tuesday, May 30, 1972.

[tape rewinding]

It's 2:00 PM.

After waiting in line for a long time, 30 people,

some adults, and a majority of children take their seats

in the three carriages around the brake operator.

The ride begins as expected, with the passengers experiencing

the rattle of the carriage and the groan of the wood

as they climb up the first of the ride's eight peaks.

2:01 PM.

As the first carriage reaches the top,

disaster strikes in front of the train.

The rope suddenly snaps.

[tense music playing]

And then, this whole set of carriages

start to roll backwards.

Down below, the people in the queue suspect nothing,

thinking the brakeman is playing a trick.

But as the train begins to speed backward

down the track,

they quickly realize things are not normal.

But there's still hope.

An anti-rollback braking system had been fitted

to the Big Dipper in case of such an emergency.

This anti-rollback braking system,

also known as "Dogs," is essentially

the train's lifeline in case the rope snaps.

Located beneath the cars and along the initial lift hill,

it locks the train in place and prevents it

from rolling backward.

On the Big Dipper, this was pioneering technology.

They're little hooks on the ride,

or spigots, and there's this sprung bar which rolls

over them on the bottom of the carriage,

and every time it goes over one, it locks in.

It's precisely in these braking systems

that the next disaster strikes.

Originally, eight brakes were supposed to be installed

on the Big Dipper.

Unfortunately, when the rope breaks, only half of them are

in place to hold the weight of the train.

But that's not the worst part.

Not only were there only four in existence

when there should have been eight,

only one of them was shown to be operational.

Just one dog brake, trying to stop a train that heavy?

Disaster is inevitable.

The only operational brake fails,

unable to stop the weight of the fully loaded train.

With nothing left to stop it, the train picks up speed

and rolls backward down the track

toward the loading platform and the crowded queue.

[Brendan] But all is not lost, because we have a brakeman

who's sitting between the first and the second carriage.

He can apply his brakes.

Some riders may think it's just a joke at first,

but the brakeman is the first to realize the horror

of the situation.

He is the last line of defense against the catastrophe

unfolding before his eyes.

On a normal day, the brakeman will be braking,

going into the corners.

But on this particular day, when the ride starts

heading backwards, he has a job on his hands.

This is completely new territory to him.

The Big Dipper's brakeman pulls his lever in a desperate

attempt to stop the three packed cars.

But despite his best efforts, it's not enough to prevent

the worst case scenario.

It gathers too much momentum as it reverses downhill,

speeding into a sharp corner.

[Rob] The train rolls back through the station

and enters what should be the last corner of the ride,

and it enters it at far too high a speed.

2:02 PM.

The last car speeds into the curve.

It derails and slams into the safety barriers.

Some passengers are projected from their seats,

over the hoardings, and down onto the ground,

[Dr. Shini] Others are crushed in the wreckage

as the train folds upon itself, one car at a time.

The second car crashes into the first,

shattering the wooden structure, and causing

fatal injuries to the young riders.

The wooden supports of the track

are stained with blood.

People from the queue rush forward to help the victims.

[Brendan] You'd think that this was the end

of the catastrophes that could happen,

but there's one final thing that's gonna come

and bite everybody there, one final incident.

The lead car remains on the ramp,

about 15 meters above the ground.

Although the other cars have taken the brunt of the damage,

this car is relatively intact, and its riders are safe.

At least, that's what they think.

[tense music playing]

Tuesday, May 30th, 1972, The Big Dipper.

Although the other cars have taken the brunt of the damage,

the lead car remains on the ramp, and its riders are safe.

At least, that's what they think.

It's not just the rope and the brakes that were faulty

on the Big Dipper.

The truth is that the ride is in terrible condition.

Two years earlier, in 1970, part of the Big Dipper's track

and wooden fences had been destroyed in a fire.

As replacements, the roller coaster's management

decided to use salvaged materials from an older ride.

But these wooden parts were over 50 years old

and in poor condition.

And the hoardings are an important part of the track

because they would provide almost a safety barrier

containing the ride within itself.

As passengers climbed from the wreckage on the train

of May 30th, 1972, they wrongly presumed

the safety rails and hoardings which surround the track

are strong enough to hold their weight.

These decayed supports would lead to yet another death

in this terrifying accident.

One girl, only 14 years old, climbs free of the wreckage

and attempts to help her friends.

She leans on the railings at the side of the walkway...

[Rob] However, the hoardings weren't strong enough

to hold her weight.

...but it gives way.

She falls 15 meters before hitting the ground.

Her death was the final tragedy of the Big Dipper disaster.

In total, three people die at the scene.

Two others were rushed to the hospital,

but later succumbed to their injuries.

All were between 8 and 15 years of age.

Thirteen more were injured,

some left permanently disabled.

This accident shattered the lives of the survivors,

many of whom would live with post traumatic stress

and survivor's guilt for decades.

The disaster exposed a series of oversights

that proved fatal,

shaking public confidence in amusement park safety

for years to come.

Beyond the 66 structural defects,

the owners and operators of the Big Dipper

were accused of hiring untrained staff,

mainly teenagers who lacked the knowledge

to operate the ride safely.

It was also alleged that the Battersea Funfair staff

were intoxicated while on duty.

Despite this, the chief engineer

and the manager of the Big Dipper

were acquitted in 1973.

On the Big Dipper, the failure of a mechanical hauling rope

led to a runaway train

and the tragic loss of five lives.

But just because a ride doesn't have a track

doesn't mean it's guaranteed to be safe.

Even flowing water can lead to tragedy

in the wrong circumstances, as we see

on the Thunder River Rapids Ride in Queensland, Australia.

If you're a roller coaster enthusiast and you're there

with your friends and family,

you're going to visit Dreamworld.

Covering an area of one square kilometer,

the equivalent of 143 soccer fields,

it is Australia's largest theme park.

[commercial narrator] Dream at Dreamworld.

♪ Dreamworld is happening now ♪

It's the biggest entertainment complex

in the Southern Hemisphere and the best value anywhere.

There are thrilling...

Opened in 1981, Dreamworld features 47 attractions,

including an IMAX cinema, a zoo, and six roller coasters.

One of the most iconic and popular attractions

is the Thunder River Rapids.

[water rushing]

[Rob] It's a four-minute ride that courses its way

through various rapids and white water,

culminating in a conveyor belt that lifts the raft

back to the top of the ride, where passengers can unload

before new passengers reload and head off on their journey.

Originally classified as a medium-thrill attraction,

the Thunder River Rapids is opem to all riders over 1.2 meters tall.

With speeds reaching up to 45 kilometers per hour,

each raft spins on itself as it travels

along the 450-meter, man-made river.

And it's relatively slow compared to other rides,

but slow doesn't necessarily mean safe.

And this is when fatal engineering features

can start to show themselves.

[tape rewinding]

2:00 PM.

Four adults and two children -- a 10-year-old boy,

and a 12-year-old girl take their seats

in raft number five.

After completing the ride without any issues,

the riders are calmly ascending the conveyor belt,

still seated in their raft.

They have 42 seconds of carefree ascent,

unaware that their time is running out

due to an unexpected obstacle.

[Dr. Shini] Up ahead at the top of the conveyor belt,

an empty raft, raft six, has become stuck

between the conveyor belt and the tracks

which support the rafts

as they pass through the unloading and loading bays.

There is no alternative path for raft five.

It's destined for impact.

The two rafts collide. Raft five is lifted vertically.

The six riders are left dangling

above the moving conveyor belt,

but not for long.

Two adult passengers fall from their seats,

with the Velcro restraint belts doing nowhere near the job

that they're supposed to.

They fall down through the conveyor belt

and into the water below.

[Dr. Shini] Two additional passengers,

pummeled by the conveyor belt, are pulled from their seats

and into the mechanisms which drive the conveyor belt.

And the two poor children, who are now suspended

in the only two seats which survive,

they see all this unfolding below them.

All four adults perish in the accident.

The park immediately closes its doors.

How did a family-friendly ride turn into such a tragedy?

Could this disaster have been prevented?

The Thunder River Rapids.

Two rafts collide.

Two adult passengers fall from their seats.

Two additional passengers are pulled from their seats

and into the mechanisms which drive the conveyor belts.

All four adults perish in the accident.

A well known problem with the ride

concerned two large water pumps that would draw water

back to the top of the ride, providing the current

to send rafts on their way as the journey began,

and providing some of the white water and fast moving water

as the rafts made their way through the channels.

Sitting three meters from the base of the river,

with a diameter of 1.6 meters,

they pump up to 4,000 liters per second.

They're positioned at the highest point

of the attraction, just before the landing zone.

The issue is that if one pump stops working correctly

or switches off entirely, the water level

throughout the ride would lower dramatically

and incredibly quickly.

[Brendan] And what you thought was a safe environment to leave in,

by the time you get back,

it's a pretty dangerous environment

that you're gonna arrive back into.

However, this was beginning to happen quite regularly.

The water could sometimes drop 15 centimeters

in just 20 seconds, and 40 centimeters

in less than a minute.

On the day before the incident, continually experienced

an electrical fault, which meant that the pump kept tripping.

It kept cutting out.

On the day of the accident, the system broke down twice.

An electrician was supposed to arrive

on Thursday, October 27, but that would be too late.

Two days earlier, the Thunder River Rapids earned

its place among the deadliest attractions ever built.

You probably think that one of the previous engineering failures,

the loss of water coming out of the pump,

would trigger the whole ride to be shut down,

but this ride is different.

There is a significant flaw in this system.

There's no formal marking to determine

the correct operating levels of water within the ride,

and no automated system to monitor water levels

or to raise alarms

when they drop below a safe operating level.

Instead, operators depended on waterline marks

on the walls of the loading and unloading zone,

left behind by years of continuous use.

Despite recommendations to install

automatic monitoring systems to detect a drop in water level

below a critical threshold, the attraction continues

to operate based solely on unreliable visual checks.

2:00 PM. Tuesday, October 25, 2016.

Families gather in the boarding area,

awaiting the arrival of a raft to enjoy the thrills

of Thunder River Rapids.

As raft five pulls in, six individuals climb inside.

Two young children are accompanied

by two women and two men.

As they begin their tour of Thunder River Rapids,

they have no idea that in less than five minutes,

four of them will be fighting for their lives.

Two hours earlier, one of the pumps suffers

a power cut yet again.

The water levels at the loading and unloading bay

have dropped beneath where they're supposed to be.

It's 2:02 PM.

As raft five and its six riders begin their journey through

the Thunder River Rapids, the water level has dropped

by 30 centimeters at the ride's starting point.

Now, a new element comes into play.

A mechanism beneath the rafts, meant to ensure rider safety,

is supposed to allow them to disembark

even when water levels are low.

Underneath it, there's a metal structure

which stops them sinking too far.

They're supposed to be super safe,

but they are going to be the cause

of a fatal engineering catastrophe.

These steel frames, 1.25 meters wide,

are supported by cross beams spaced 1.27 meters apart.

They are supposed to remain underwater.

Due to the failure of one of the pumps

and the drop in water level,

these frames are almost fully out of the water.

[dramatic orchestral swell]

On the Thunder River Rapids, the top of the water pumps

are lower than the steel railings.

It's 2:03 PM.

It takes just one minute for the steel frames,

which are usually submerged under 30 centimeters of water,

to now sit 10 centimeters above the water.

Raft six, positioned in front of raft five,

reaches the unloading platform for its riders to disembark.

But instead of floating and continuing its course

once empty, it does something unusual.

Raft number six is stuck at the top of the conveyor belt.

[Dr. Shini] It's stranded upon the metal,

creating an incredibly dangerous obstacle.

At the same time, raft number five

has completed its journey around the rapids

and is now making its way up the conveyor belt,

getting closer and closer to raft number six.

A collision is inevitable, but there are more issues

with the conveyor belt itself that turns the collision

into a catastrophe.

[tense music playing]

The Thunder River Rapids' conveyor is essentially

a mechanical belt driven by a chain,

much like many others.

It consists of wooden flats fixed at regular intervals.

On this day in October 2016, the conveyor was no longer

in the same condition as when it was originally designed.

During the early years

of the Thunder River Rapids Ride,

the motors driving the conveyor belt

kept tripping out because the conveyor belt was too heavy.

To address weight issues, the initial solution was

to remove one out of every three wooden slats.

But when the conveyor was still deemed to be too heavy,

park engineers decided to remove a total of two

out of every three slats.

[Rob] And it worked.

With one every three slats, there was still enough friction

to grip the rafts and pull them to the top,

and the motors stopped tripping out.

It was a successful engineering modification.

Or so they thought.

2:04 PM.

The final minute before the Thunder River Rapids Ride

and Dreamworld plunge into horror.

Due to the drop in water level caused by the failure

of one of the two pumps. raft six is immobilized

on the steel frames of the passenger unloading dock

for 53 seconds.

[Dr. Shini] Raft five approaches from behind

with its six passengers still on board.

They can probably see that raft number six is stuck at the top,

but they don't think it's gonna be an issue.

Maybe they'll bump it.

But a very different scenario is about to unfold.

It's 2:05 PM.

In fact, they do get there, and they bump it several times.

But what they don't realize is that these big holes

in the conveyor belt, every time it bumps,

it's trying to get wedged into these big abysses,

and finally, the last one, it does get caught.

The gap in the wooden slats on the conveyor belt

is large enough for the flotation collar

of raft five to slip into,

with the wood now acting like teeth which bite down

and drag raft five into a vertical position.

A ride operator fails to hit the main emergency stop.

A fatal mistake.

Another operator triggers an alternate stop.

It takes eight seconds to stop the conveyor.

By this point, it's too late.

The damage has already been done.

Now completely vertical, raft five is at the mercy

of the conveyor mechanism.

At this moment, yet another failure seals the fate

of the riders.

[gears grinding]

[tense music playing]

[Dr. Shini] Two passengers fall from the raft,

their Velcro straps unable to hold their weight

as they fall between the wooden slats of the conveyor belt

and into the waters below.

[Rob] The conveyor belt now pummeling

those left inside the raft.

Two more adult passengers lose their life

as they get trapped in the conveyor belt mechanisms.

[Dr. Shini] Two seats which had been pulled lowest

by the conveyor belt are totally destroyed.

They couldn't escape in time.

And imagine this whole thing is evolving over several seconds.

It's not like an instantaneous disaster.

This is a slow disaster, which is unfolding horrifically

in front of these two children.

[indistinct, muffled shouting]

The four adults are pronounced dead at the scene.

The investigation reveals critical safety failures

in the ride's operation

and insufficient staff training.

I'm a mother, I have a family, I take my family to Dreamworld.

This could have been my family, and I am completely sympathetic

to what they must be going through,

and I hope that I will be able to help them,

and if I can't help them personally,

we will make sure that we have everything in place

to assist them through this tragedy.

Dreamworld was fined $3.6 million Australian dollars

and paid over $7 million Australian dollars

to victims' families.

♪ Wash away your worries and your woes ♪

[Brendan] The Thunder River Rapid

was actually made by the park themselves.

So they would have come up with a whole scheme

for maintenance and how this thing

should be looked after, repaired, inspected every day.

And they're the ones who are writing the rule book

for this thing.

But when you start to cut corners,

you're entering into a world of unknown

fatal engineering causes.

The Thunder River Rapids Ride will never reopen.

A memorial is erected in place of the ride.

honoring the four lives lost.

[Rob] These three examples of fatal engineering

shine a bright light on the consequences

of what can happen when best practice isn't followed.

And it's only by learning from these things that rides now

can be reopened, and everything is as safe as we hope.

But there's always one more disaster just around the corner.

Roller coasters are supposed to be all about fun and thrill,

but when engineering goes wrong, it can be fatal.