Original subtitles

[Explosion]

First Officer: What the hell?

[Explosion]

Narrator: A massive explosion erupts

inside the cabin of a commercial jet.

[Screaming]

Captain: Mayday, mayday, mayday!

Narrator: Terrified passengers fight for their lives.

Flight Attendant: You have to stay calm!

Man: The chaos ensued when the fire was spreading.

Man: Now the question became what created this explosion?

Man: We just weren't sure what we were looking at.

Narrator: Investigators scour the scorched debris for clues.

Investigator: Let's check the rows

near the blast area.

Man: There's a lot of pressure on a post-blast investigator

to solve the case quickly.

Investigator: I found something.

Narrator: But then they discover something

that has no business being anywhere near an aircraft.

Man: We immediately thought it must be a bomb.

Flight Attendant: Ladies and gentlemen,

we are starting our approach.

Pilot: We lost both engines!

Flight Attendant: Put the mask over your nose.

Emergency descent.

Pilot: Mayday, mayday!

Flight Attendant: Brace for impact!

Controller: I think I lost one.

Man:...investigation starting into this tragedy...

Man: He's gonna crash!

Captain: Hualien tower, good afternoon.

Uni Air 873.

Visual approach, 15 miles out.

Narrator: Uni Air Flight 873

is nearing Hualien Airport in Taiwan.

Controller: Uni Air 873, continue approach.

Report at five miles out.

Narrator: The Taiwanese crew

works for regional airline Uni Air.

The first officer is flying under the watchful eye

of the more experienced captain.

They've been in the air for only 20 minutes...

traveling south,

hugging the coastline from Taipei to Hualien.

Al Dickinson: It was what you'd call a puddle jumper.

It was a short flight.

You don't expect anything to happen.

Narrator: Hualien is a popular destination

for Taipei residents.

On a warm August day,

passengers are looking forward to some vacation time.

The plane is a twin-engine MD-90,

a recent addition to Boeing's fleet.

Cindy Keegan: The MD-90 was fairly new at the time.

It was only about four years old since its manufacture.

And it was generally a very solid aircraft.

First Officer: Flaps 11.

Captain: Flaps 11.

Narrator: The crew prepares to land right on schedule,

On the small single runway

used by both civilian and military flights.

But as they close in,

the first officer struggles with the landing.

Captain: Too high.

First Officer: Too high.

Keegan: It was a little bit of a bumpy landing.

It wasn't anything highly unusual.

Captain: You were sinking too fast.

I have control.

The captain takes over the controls.

First Officer: You have control.

Al Dickinson: The pilot in command

had over 5,000 hours of flight time.

The co-pilot, he was the newer of the two.

And that's good to put people together with,

one with a lot of experience and one with a little.

It trains people.

Narrator: The captain engages the thrust reversers

and hits the brakes.

Keegan: Nobody would have felt anything highly unusual

during that landing.

Narrator: The plane touches down,

with plenty of room on the runway to come to a stop.

Dickinson: You start rolling, and it all irons out.

So you're on the ground, and you start to relax.

But as the plane slows...

[Explosion]

[Screaming]

[Alarm Bell Ringing]

First Officer: What the hell?

[Screaming]

Narrator: A fire erupts immediately

and spreads quickly.

[Screaming]

Passengers instantly panic.

[Screaming]

Keegan: When you introduce fire to an enclosed vessel,

it becomes a very dangerous situation.

[Screaming]

There's nowhere for that fire to exasperate itself,

and there's oxygen to propagate the fire.

Narrator: Moments after the fire starts,

the power goes out.

Captain: Mayday, mayday, mayday!

Narrator: Black smoke spews from the still-rolling airplane.

[Screaming]

Keegan: They would have been faced with an urgency

to get out of that airplane as quickly as they could.

[Coughing]

Narrator: Panic is as dangerous as the fire.

[Screaming]

Flight Attendant: You have to stay calm!

Narrator: The terrified passengers

aren't waiting for the plane to stop.

They rush for the exits.

Dickinson: The chaos ensued because

the aircraft was still rolling when the fire was spreading.

You can't expect any passenger to just sit in a seat

and wait for an evacuation.

Narrator: Seconds after the blast,

the plane comes to a complete stop.

The captain issues the order.

Captain: Evacuate, evacuate, evacuate!

Narrator: Fire is consuming the passenger cabin

with terrifying speed.

Keegan: You have trouble seeing.

You have trouble breathing.

You have the threat of burn injuries.

Captain: Go check on the cabin!

Go!

Narrator: The plane is losing its battle with the fire.

Smoke pours out from a huge gash in the fuselage.

[Speaking Chinese]

Wu Tsai-Hsun, Translated: I was the correspondent

for China Television in Hualien

at the time.

When I first heard about this accident,

my reaction was this could be a very serious one.

One to two hundred passengers' lives could be in danger.

[Screaming]

Narrator: The 90 passengers and 6 crew members

only have seconds to escape.

[Screaming]

Dickinson: Fires are dangerous in aircraft

because you can't go anywhere.

And they erupt surprisingly fast.

So you don't have much time to get away from it.

Flight Attendant: Next. Go now!

Come on!

Next!

Next! Go now!

Narrator: The captain is the last crew member

to leave the aircraft.

[Coughing]

Captain: Anybody there?

Anybody?

Dickinson: Like some ship captains do,

he stayed with his aircraft

and made every effort to make sure everybody was off.

[Sirens]

Wu: When I first arrived at the scene,

the fire was so big.

Firefighters were trying very hard to put out the fire.

I wondered how many people were on the plane,

how many casualties.

[Speaking Chinese]

Narrator: It takes more than an hour

to contain the massive fire.

By the time it's out,

only 59 of the 90 passengers are accounted for.

[Sirens]

When firefighters finally extinguish the massive inferno,

they board the ruined plane,

looking for the missing passengers.

Dickinson: A lot of people sometimes get knocked over.

People step on them.

There's all sorts of chaos going on inside that cabin

because of the fire.

We thought for sure somebody would be stuck in there somehow.

The firemen didn't know how many people were still in there.

Narrator: To everyone's relief,

there are no bodies aboard the burned wreckage

of Uni Air Flight 873.

The missing passengers are located.

28 people are hospitalized...

many suffering from serious burns.

They include a pregnant woman who loses her unborn child.

One passenger will later die in the hospital.

But it could have been much, much worse.

Dickinson: If that aircraft

had still been up at cruise altitude,

nobody's probably gonna survive.

Narrator: The passenger jet is heavily damaged.

It will be up to a small team of investigators to figure out

what caused so much destruction and threatened so many lives.

Kay Yong: I was in Hong Kong.

I got a call,

and they said there was a major accident

happened in Hualien.

I immediately rushed back to Taiwan.

Narrator: It's the first major investigation for this team.

Taiwan's aviation safety council, or ASC,

is little more than a year old.

Dr. Kay Yong is the managing director of the safety council.

Yong: The aviation safety council was formed

because Taiwan's safety record was pretty poor.

Narrator: Investigators review the debris

collected from the runway.

Investigator: It doesn't look like they hit anything.

Everything looks like it came from our plane.

Narrator: What could have caused a relatively new airplane

to catch fire on a runway after a safe landing?

Dickinson: We've seen lots of inflight fires.

We've seen crashing into things on landing.

I had never seen one where they had some kind of fire erupt

after the aircraft got on the ground.

Very unusual.

Narrator: More than 100 MD-90s are in service.

Large airlines like Delta use the passenger jets

For short to medium-haul flights.

If something is wrong with the aircraft,

investigators need to know in a hurry.

Dickinson: Questions arise

because a perfectly good aircraft

getting back on the ground

should have taxied up to the gate

and offloaded everybody with no problem.

Narrator: Investigators start questioning witnesses.

Investigator: Tell me when you first noticed

something wrong.

Flight Attendant: It all happened so fast.

[Screaming]

Narrator: A flight attendant reports

that she was just about to make the landing announcement

when she heard an explosion.

Flight Attendant: It was very loud.

Definitely from inside the plane.

Yong: The crew indicating

as they still taxied on the runway,

They heard a boom.

[Boom]

[Screaming]

Narrator: Investigators discover

they're dealing not just with a fire,

but also an explosion aboard the plane.

[Screaming]

Flight Attendant: My ears buzzed,

and I felt a blast of hot air.

Narrator: The flight attendant recalls

the explosion came from above,

near the front left side of the cabin.

Investigator: Did anything stand out before the blast?

Anything out of the ordinary?

Narrator: Everyone who was in the cabin

says much the same thing.

There were no warning signs,

no unusual smells or sounds

until the blast.

Yong: Everyone heard a boom.

[Screaming]

And immediately the smoke started filling up the cabin.

[Screaming]

Investigator: They all confirm an explosion.

So what could have caused it?

Yong: Right away you thought, it must be a bomb.

[Reporter, Translated]: Because the fire started

in an overhead compartment,

investigators did not rule out the possibility

that explosives were on board.

Narrator: The pressure to find answers is building.

There's speculation that a bomb might be to blame.

Yong: The media's attention make the investigation harder.

That's always the case.

Narrator: Police launch a criminal investigation,

separate from that by Dr. Yong's aviation team.

Yong: The criminal investigation

is looking for who is responsible.

The safety investigation

is focused on finding out the probable cause

And how to try to avoid such similar accident

would happen again.

Narrator: Within four days,

investigators from the United States

National Transportation Safety Board

and Federal Aviation Administration

arrive to assist in the investigation.

Dickinson: The aircraft was relatively new.

So we were concerned that something in the design

might have set this fire off.

Investigator: It's a real mess.

Burnt right down to the windows.

Narrator: The damage is minimal

from the main entrance door forward.

But the plane has sustained heavy fire damage

from aft of the door to the middle of the tail.

Investigator: There's no damage to the wings

or the landing gear.

Ed Kittel: And the worst of it's here.

Narrator: Explosives expert Ed Kittel

Is part of the investigation team.

Kittel: We looked at the area of the major damage

where the flap of the aircraft skin had been blown back.

It could have been a bombing,

and we just weren't sure what we were looking at.

Investigator: Come on. I have something to show you.

Everything we found on the runway is here.

Narrator: The Taiwanese have started moving debris

from the runway into the hangar.

Any one of these pieces could be crucial evidence.

Kittel: There's a lot of pressure

on a post-blast investigator

to solve the case quickly.

There's also a lot of predetermined

"I know this was a bomb," you know, "it had to be a bomb."

What you have to do

is you have to dissociate yourself from all of that

and you have to follow the evidence.

Narrator: That approach soon takes Kittel

on a reconnaissance mission inside the burnt-out aircraft.

Everything is soaked

by the water used to douse the flames,

damaged in the evacuation,

and buried in dust.

Kittel: My senses are overwhelmed

with the smell of a burned-out fuselage...

burn marks everywhere.

There's soot, there's debris.

Narrator: The scorched wreckage is overwhelming,

even to a veteran like Kittel.

Kittel: I was absolutely shocked by the amount of damage.

You're thinking people were injured here,

and I have to try to figure out what happened.

Narrator: Kittel must pinpoint

where a bomb could have detonated.

To a layman's eye, the debris all looks the same.

But Kittel knows exactly what to look for.

Kittel: You look all the way back to the rear,

look forward towards the cockpit

and then stop and focus on, ok, where did this start?

Where is the center of all of this?

You always try to go to the point of detonation.

Narrator: His trained eye

quickly identifies the likely detonation point...

the burnt-out hole above row eight

where the overhead bin once was.

Kittel: When a high explosive goes off,

it gives off jets

of really, really high temperature hot gases,

and they literally will burn through a piece of metal...

tiny, tiny little holes

that will almost look like the surface of the moon.

Finding those pieces

is essentially what we would call the silver bullet.

Narrator: But instead of the telltale holes,

he finds the skin ripped open along the rivet lines

that join sections of fuselage together.

Kittel: We had a whole section of fuselage

that tore along the rivet lines.

The effects were more of a pushing and heaving

Than a shattering

and a really high pressure, high explosive event.

[Explosion]

[Screaming]

Narrator: If not a bomb,

what could have caused an explosion powerful enough

to destroy a passenger jet?

Kittel: We have to try to figure out what caused it.

Investigator: Ok. Let's see if the tape can tell us anything.

Narrator: Taiwanese and American investigators

are struggling to determine

what triggered the massive explosion

aboard Uni Air Flight 873.

Could it have been a bomb in an overhead bin

in the passenger cabin?

Or something else?

They turn to the black boxes for clues.

Captain On Tape: Hualien Tower, good afternoon.

Narrator: It's a short flight,

just 20 minutes in the air.

And as investigators listen,

they soon discover it's uneventful.

Yong: The flight crew's conversation with the tower,

ATC tower,

everything was quite normal.

Narrator: Except for a somewhat bumpy landing.

Captain: Too high.

You were sinking too fast.

I have control.

Narrator: After touchdown,

the captain engages the thrust reversers

and hits the brakes.

Investigator: The plane gets on the ground without incident.

So far so good.

Tony James: You could hear the thrust reversers.

You could hear the speed brakes come up.

You could hear everything from the cockpit as being normal.

But something happened during that landing sequence.

[Boom]

First Officer: What the hell?

Investigator: It came right out of the blue.

Narrator: There was absolutely no hint of trouble

before the explosion.

Captain: Mayday, mayday, mayday!

Dickinson: We could pretty much rule out the flight crew

having anything to do with this.

The first they knew something was different

was when they heard that sound.

Investigator: Can we hear that explosion again?

[Whirring]

[Boom]

First Officer: What the hell?

Yong: The boom was fairly dull,

the time interval slightly prolonged.

Narrator: The sound itself

could be the biggest clue on the audio tape.

It wasn't the very rapid high frequency spike of a bomb.

Investigator: Send it in for analysis, please.

Narrator: The sound of the explosion

on board flight 873

Is compared to sounds of various types of explosions.

Yong: So we compared all the acoustic data from the NTSB

from various different source.

[Boom]

We determined it was a gaseous explosion.

Narrator: It's a new theory

that fits with what Ed Kittel saw in the wrecked cabin.

Kittel: The effects were more

of a slower, rolling blast or shockwave

than the hard-hitting punch of a high explosive.

And one thing that came to mind

was the possibility of a fuel-air explosion.

Narrator: Fuel explosions on airplanes are rare,

but deadly.

The worst was in July 1996,

when a TWA 747 exploded soon after takeoff from New York.

All 230 people on board died,

making it one of the deadliest aviation accidents

in U.S. history.

But the flight data recorder shows no irregularities

With the fuel systems on Uni Air Flight 873.

Investigator: Nothing here.

Narrator: Investigators can't find

any other mechanical issues either.

The MD-90 is off the hook.

Investigator: Whatever it was exploded here.

Narrator: Investigators now focus

on the overhead bin above seat 8B

that Ed Kittel pinpointed as the point of detonation.

They wonder if someone brought explosive materials on board.

♪

Investigator: The flight attendants said

they noticed nothing unusual going into that bin.

Narrator: So what happened in this bin

during the short 20-minute flight to Hualien?

Investigator Tony James decides to examine the burnt-out plane

for clues that may have been missed.

Investigator: Let's check the rows...

near the blast area first.

James: The carpet was sopping wet.

Everything was just goo.

There was shards of metal, plastic everywhere,

so you needed to be extremely careful not to get injured.

We found backpacks. We found suitcases.

We found all kinds of things that were burned,

but we didn't know what they meant.

Narrator: James focuses on the area

around the overhead bin at row 8.

James: We had no idea what we were looking for.

As a trained investigator,

you do not go in with any kind of preconceived ideas.

You just look for anything

that's just a little bit different

from something else.

Narrator: On the far side, underneath seat 7C,

he spots something unusual.

James: It looked like a bottle or a jug of some sort.

It looked like it had been ripped apart.

Not burned, but ripped.

It turned out it was the top third of a bottle.

The rest of it was gone.

Investigator: I found something.

Narrator: It's not the look of the fragment

that grabs his attention,

but the smell.

James: You could smell gasoline instantly,

not kerosene, or the fuel that is used in a jet engine.

Investigator: It smells like gasoline.

James: What is this doing on this airplane?

Gasoline is prohibited from any kind of airplane that I know of,

whether it's domestic or foreign.

The aviation fuel that runs in the jets

versus the gasoline that you'd run in your automobile

have two totally different smells.

I knew that there was something different about this accident.

Narrator: The bottle fragment was found in row 7,

just one row forward of the exploded bin.

Could this be the source of the gas explosion?

Investigators order up a chemical analysis

of the bottle fragment.

Kittel: I don't understand why there'd be gasoline here,

but that's really piqued my interest

that that could be the fuel that somehow ignited,

causing that explosion to rip open the side of the airplane.

I'm really excited that we may be on to something.

Narrator: The fragment is soon identified

as coming from a bottle

of a common brand of laundry bleach.

But lab results show that it wasn't bleach in the bottle.

Investigator: No question about it,

it's gasoline.

Yong: It was indeed gasoline.

92 gasoline,

which you can find very commonly from any gas station.

Narrator: It's a remarkable discovery.

An innocent-looking 750-milliliter bottle

is now the prime suspect in the accident investigation.

Yong: Everybody sort of was saying,

well, bingo, we found the golden nugget.

Narrator: Investigators believe

Someone swapped the bleach for gasoline.

Investigator: The gasoline

could have leaked out during the flight.

Investigator: Maybe.

Narrator: The team gets their first big break.

But it raises new questions.

Why would anyone bring gasoline onto an airplane?

Yong: The next question was,

how is gasoline aboard the aircraft?

Narrator: Could an explosion and fire aboard Uni Air Flight 873

really have been caused by a small bottle of gasoline?

Investigator: This could take a while.

Narrator: The discovery of a bottle fragment

with traces of gasoline on it has investigators perplexed.

Kittel: I want to look at the screening tapes

from Taipei Airport

and see if there's anything unusual,

see if we can tell who carried this bottle onto the airplane.

Narrator: It's a painstaking process,

with hours of footage to review.

Investigator: Nothing yet.

Kittel: We're looking for things that are beyond the normal.

We're looking for suspicious behavior,

we're looking for items that are singled out by the screeners,

we're looking for something that's not right.

Investigator: Look at this.

Narrator: A security guard has pulled over a passenger

to inspect his bag.

The contents prove to be unusual.

Kittel: There were four items that were cause for concern.

There was a bottle of camping fuel.

There was another large can of insecticide,

which typically has propellants that are explosive.

Narrator: the inspector confiscates

the camping fuel and insecticide.

Kittel: And then two bottles

that we can't quite tell what they are.

Narrator: He deliberates over the other two bottles.

Kittel: The screeners really drilled down

on reading the labels of these two bottles.

Investigator: Let's see if we can get any closer.

Narrator: Could one of these be the offender?

Investigator: It sure looks like a match.

Narrator: It turns out the two bottles are identical

and are the same brand of bleach

as the fragment found on the plane.

Investigator: The guard just put them back.

James: The security person

then looked at the contents and read the contents,

did not remove the cap, did not smell it,

but gave it back and put it back down.

Narrator: Investigators now question the security inspector

to find out more.

Investigator: Why'd you let the bleach bottles through?

Why didn't you open them to check?

Inspector: The bottle said bleach.

He said it was bleach.

Yong: At that time there was a procedure

that whenever there is a bottle,

the inspector should open the bottle and smell it

To make sure it's not a hazardous material.

Passenger: We're opening the family cabin

and need it to clean.

Inspector: Oh, OK.

Yong: He did not actually open the cap and smell it

and just let it go.

Investigator: Can you tell me anything about the passenger

carrying the bleach bottles?

Inspector: Sure. I recognized him right away.

Narrator: Investigators discover

that the man who carried the bottle through security

was a Taiwanese celebrity named Ku Chin-Shui.

Yong: The suspect was a very famous decathlon champion.

Almost everybody knew him.

[Speaking Chinese]

Wu: Ku Chin-Shui was from Hualien

but was famous for decathlon throughout Taiwan.

[Speaking Chinese]

Narrator: Ku won multiple medals for Taiwan

at international track meets in the 1980s and '90s.

After retiring from sport, he became a schoolteacher.

But the nation still remembers him

as a star athlete and a national hero.

When Ku Chin-Shui is brought in for questioning,

he explains that after passing security

he gave his bag to a relative who carried it on board.

He pleads his innocence to the media.

Ku Chin-Shui, Translated: Because my family

was on the plane.

Why would I do such a stupid thing

as to put gasoline on board?

Wu: People were so surprised

to find out that he was a suspect in this case.

Yong: That's sort of a complicated issue,

because, number one, he was a very well-known personnel.

And number two,

it immediately became a criminal investigation.

Narrator: It's the job of the police

to investigate whether Ku has criminal responsibility.

Meanwhile, investigators have to determine

If these two gasoline-filled bottles

were the cause of the explosion.

Yong: We did not look into the motive of the suspect.

What we did care is how to improve safety.

So our focus is what happened,

how did it happen?

Investigator: Two bottles of gasoline

end up in an overhead bin.

But what caused them to explode?

Narrator: The team has made significant progress,

but identifying two small bottles of gasoline

is a long way from explaining an explosion.

Gasoline in liquid form does not burn.

But gas vapor leaking from the bottles

could produce highly explosive fumes.

Investigators have a key question to answer.

Kittel: Now that we've established

that we had gasoline in an overhead bin,

I've got to find the ignition source.

So where could that spark have possibly come from?

Narrator: To understand how two small bottles of gasoline

could destroy a commercial jet,

kill one passenger,

and injure dozens more,

investigators need to determine how the gasoline ignited.

[Screaming]

Dickinson: Here we were with a compartment

that we knew had two bleach bottles

that had gasoline in them.

We're pretty certain fumes leaked out of them,

and you get to a point when that becomes very flammable,

and it doesn't take much to set that off.

Narrator: Investigators get to work

looking for whatever might have ignited the leaking gasoline.

They focus on the wire bundles

that run close to the damaged bin.

Investigator: An exposed wire near the bin

could have caused a spark.

James: We start with every possible items

that are around there.

And one of them happens to be the overhead wiring,

which runs the P.A. system, runs the lights,

and it could generate some kind of a spark

if the insulation was gone off the wire.

Investigator: Sure, but none come close

to the bin's interior.

James: After a thorough investigation,

we found nothing to conclude that this is what happened.

Nothing had exposed wires.

The insulation was all intact.

Narrator: Even if a wire was faulty,

it wouldn't have contacted the fuel.

Investigator: Ok, then,

what about the emergency oxygen generators?

Narrator: An oxygen generator,

which provides a passenger with oxygen in an emergency,

is another possible culprit.

They've been collected from the wreckage site.

Investigator: We need to check these retaining pins.

Narrator: They're part of the safety demonstration

At the start of every flight.

If the cabin depressurizes,

the mask drops down

and you pull on it to release oxygen.

The action of pulling the mask

releases a retaining pin in the generator.

That causes chemicals to react with each other

and produce oxygen.

These generators have been implicated

in a previous plane crash.

In 1996,

a ValuJet flight crashed into the Florida Everglades

killing all 110 people on board.

A fire in the DC-9's cargo area

was linked to improper storage of oxygen generators.

The ensuing investigation showed the units were highly flammable

under certain conditions.

James: They were stored in a box,

and they were not properly secured with a pin,

and it generated the heat and created a major explosion.

Narrator: A missing pin on one of the Uni Air generators

may have led to the explosion.

James: When you pull the pin to use them,

they generate a tremendous amount of heat.

So this is one of the areas we wanted to check.

Investigator: All the pins are intact.

Narrator: But investigators find

that none of the generators have been activated.

It's another dead-end.

As Ed Kittel considers what else

could have caused the spark on board the flight...

Lab technicians go through the exhaustive process

of identifying all the debris from the aircraft and runway.

They make a curious discovery.

Yong: It was a motorcycle battery,

and that motorcycle battery

was also actually something should not be on the aircraft.

Narrator: This burnt, beaten-up 12-volt battery

takes the investigation down an entirely new path.

Kittel: This motorcycle battery

Was found a couple of rows away from the bottle,

and the top of the battery has been damaged,

and it's exposing the, uh, the poles,

the positive and the negative terminals.

They're open,

and the area around them is damaged.

So all of a sudden

we have another possible ignition source.

Narrator: The poles have wire fragments attached to them,

but not the kind used to hook the battery up to a motorcycle.

Kittel: It's another question.

What was this battery used for?

If it had wires attached to it,

it probably wasn't being used on a motorcycle.

Maybe it was being used to power a light

or an inverter or something,

but this is highly unusual.

Narrator: If the live wires were jostled during the flight,

the two ends could make contact, causing a spark.

A hard landing could have disturbed the battery enough

To cross the wires.

Captain: You were sinking too fast.

I have control.

Narrator: The bumpy landing

seemed innocent enough at the time,

but it might have set the whole accident scenario

in motion.

[Boom]

[Screaming]

Kittel: Two 750-milliliter bottles of gasoline

and a 12-volt motorcycle battery in an overhead bin

is just a recipe for disaster.

Narrator: Scientists at Taiwan's Chung Shan Institute

test whether a 12-volt battery with loose wires

was the ignition source

of the explosion on board Uni Air Flight 873.

They test one identical

to the one found in the plane's debris.

Kittel: The Taiwanese forensic experts

were able to create the spark with the same type of battery.

Investigator: It looks like it was the battery.

Narrator: Investigators finally believe

they have all the pieces of the puzzle...

Vapor from the leaking bottles of gasoline

and a battery that provides the spark.

James: The gasoline may have leaked,

created a vapor inside of that overhead bin,

and as the aircraft came to a sudden stop,

then the battery slid and created an arc.

It would have created the explosion

to blow the door off of the overhead bin.

Narrator: Now the final step is to prove it.

Yong: We did quite a few simulations

to prove that such scenario could happen.

Narrator: They place a leaky bottle of gasoline

inside a similarly sized bin,

and then use a 12-volt battery to create a spark.

Investigator: Wow. The lid blows right out

and there's a flash fire.

Narrator: It's the final piece of evidence investigators need

To wrap up the case.

Investigator: That would do it.

Kittel: At the conclusion of this test series,

the Taiwanese investigators were able to prove

that that battery and a gasoline-air mixture

could cause an explosion.

[Explosion]

Narrator: It was the perfect storm of unlikely events.

Leaking gas filled the bin with fumes.

♪

The plane hit the ground hard,

which normally wouldn't be a problem.

Kittel: They put on the brakes, they put on the reversers,

and everything goes flying forward in the overhead bin.

Then about halfway down the runway

they let off of the reverse thrusters and the brakes,

and everything goes sliding back.

Somewhere in that sequence, the battery short-circuited.

It caused a spark, which caused the explosion.

[Boom]

[Screaming]

[Screaming]

Kittel: Just a tragic collection of missteps

leading to this disaster.

Narrator: After a long, difficult investigation,

Taiwan's Aviation Safety Council

releases its accident investigation report.

The probable cause...

a short in a battery

that ignites a gasoline vapor and creates an explosion.

Some nagging questions remain.

Why would anyone bring a banned substance onto an airplane?

No clear answer was ever found.

Keegan: Although we had completed all our investigation

and we had all our notes compiled,

there was this criminal investigation

that went on for a lot longer

after we had left the site in Taiwan.

Narrator: Ku Chin-Shui's court case dragged on for years.

After six separate trials,

He was ultimately found not guilty.

The court didn't think there was strong enough evidence

to justify a criminal conviction.

Yong: We can only conclude

that there was a possibility such event could happen.

No one can know exactly what happened.

Narrator: The aviation report

calls for an overhaul of the system

that screens for hazardous materials in Taiwan.

Training for security personnel was improved

in the hopes of preventing another disaster.

Kittel: So the lessons learned

are to really inspect the contents of bags

and make sure that they don't cause a tragic accident.

I'm willing to bet that nobody in screening in Taiwan

ever misses another motorcycle battery.