All language subtitles for Fatal.Engineering.S01E04.1080p.WEB.h264-FREQUENCY_track3_[eng]

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.