Original subtitles

JIM: What is going on out there?

FLIGHT ATTENDANT: Everyone, we're about to evacuate the plane.

Remain calm.

JIM: I certainly knew that with both engines on fire

it was not going to go well.

NARRATOR: 157 passengers rush to escape a burning 737.

JIM: Let's go, let's go!

KIT: It goes up like a bomb.

♪

NARRATOR: In the struggle to explain the inferno,

the smallest parts come under intense scrutiny.

The analysis leads to a stunning discovery.

INVESTIGATOR: Where is it?

BOB: It was a true breakthrough in the investigation.

NARRATOR: The failure that doomed China Airlines Flight 120

is putting more lives in danger every day.

KIT: We really didn't anticipate that there's a risk,

and it bit us.

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!

♪

NARRATOR: China Airlines Flight 120

is on final approach for landing.

CPT. YU: Ladies and gentlemen,

we're about to begin our descent into Okinawa Naha Airport.

Please give the flight attendants your full cooperation

as they prepare the cabin for landing.

NARRATOR: The Captain is 47-year-old Yu Chien-kou.

CPT. YU: What's the weather for approach?

FO. TSENG: The ceiling is 8,000 feet. Winds at eight knots.

NARRATOR: The first officer is 26-year-old Tseng Ta-wei.

The pilots have more than 8,500 hours of flight experience

between them.

CPT. YU: Almost straight down the pipe.

FO. TSENG: Yes, sir.

NARRATOR: Landing this Boeing 737 should be routine.

KIT: The 737 is probably the most popular airframe

in the world.

It's flown by most pilots as they start out in their careers.

It's a short-range domestic airplane.

NARRATOR: Flight 120 is a one-hour journey from Taiwan

to the southern Japanese island of Okinawa.

This morning, there are 157 passengers

and eight crew on board.

FLIGHT ATTENDANT: Please put your tray up and fasten your seat belt.

NARRATOR: Jim Caruso is a medical examiner for the U.S. Navy

stationed in Okinawa.

He and his family are on the last leg of a long trip home

from vacation.

JIM: The family was returning from Brisbane, Australia.

We had done probably 12 or 14 days on vacation,

and we were hoping to be home for lunch.

NARRATOR: Living in Okinawa has given the Carusos the chance

to travel through much of Asia.

JIM: We got to see China, Hong Kong, mainland Japan, Korea.

We had made use of our location to experience the area.

NARRATOR: Naha Airport sits at the southern end of Okinawa,

on the shore of the East China Sea.

The pilots reconfigure their plane as they slow down

and descend for landing.

CPT. YU: Let's go to flaps 25.

FO. TSENG: Flaps 25.

NARRATOR: They deploy flaps from the wing's back edge,

along with slats from the front edge.

These devices keep the plane airborne at lower speeds.

KIT: When we start configuring,

we begin to put out flaps and slats,

which extend the area of the wing and give us more lift

and allow us to fly slower.

And of course the landing and touchdown itself

are the most complicated part of the flight.

NARRATOR: They're now less than a minute from the runway.

JIM: Okay, honey. You have to remain in your seat.

JIM: I think everybody was looking forward to landing,

getting off the airplane and getting back to regular life.

AUTOMATION: 30, 20, 10.

NARRATOR: It's a textbook landing.

CPT. YU: Flaps up.

JIM: Once you're landed you figure you're home free.

The taxi is usually routine.

CPT. YU: After landing checklist.

FO. TSENG: Speed brakes.

NARRATOR: All that's left for the China Airlines pilots

is to park the plane.

FO. TSENG: Engine start levers.

CPT. YU: Engine start levers cut off.

NARRATOR: With the engines off they can finally relax.

KIT: The pilot is relieved. He's no longer at risk.

A catastrophic event after you're parked

is almost non-existent.

FO. TSENG: Seat belts. CPT. YU: Seat belts off.

JIM: The biggest challenge ahead of you from there

is making sure you can get through customs.

No one ever expects anything to go wrong,

especially once the engines are turned off.

NARRATOR: But one passenger has noticed

that something's not right.

JIM: My wife was next to a woman

who made some sort of exclamation

towards the engine on the right side.

FEMALE PASSENGER:

JIM: What's going on?

JIM: There was some smoke coming from that engine.

That was the first sign that something

was out of the ordinary.

JIM: What is going on out there?

FO. TSENG: Anti-ice, off. Start switches, off.

NARRATOR: The pilots are finishing the shut-down checklist.

FO. TSENG: Transponder T-CAS.

CPT. YU: Hey! What is this?

FO. TSENG: What's happening?

NARRATOR: Just when they thought they were safely parked...

GROUND CREW: Cockpit, ground. Number 2 engine fire.

NARRATOR: ...a radio call alerts them to an urgent danger.

Their plane is on fire.

FO. TSENG: Attention, crew on station.

Attention, crew on station.

KIT: Bringing the flight attendants to their station

tells them that what might likely be next

would be an emergency evacuation.

RADIO CALL: Dynasty 1-2-0, we are calling a fire truck.

Remain on standby.

FO. TSENG: We have a wheel fire. Please.

FEMALE PASSENGER:

FO. TSENG: Cabin crew, prepare for evacuation.

Prepare for evacuation.

♪

FLIGHT ATTENDANT: Everyone, remain calm.

We're about to evacuate the plane. Remain calm.

NARRATOR: Fear begins to spread throughout the cabin.

Outside, the fire is getting worse.

JIM: The engine on my side also started smoking and flaming.

So now we had both wings on fire.

And at that point people began to panic.

FLIGHT ATTENDANT: Remain calm! No pushing!

JIM: I had no idea at that point how things would play out,

but I certainly knew that with both engines on fire

it was not going to go well.

FO. TSENG: Parking brakes. Speed brakes. It's that lever.

NARRATOR: The pilots know they need to get their passengers

off the plane before flames reach the fuel tanks.

But they can't open the cabin doors yet.

CPT. YU: Engine fire warning switches. Override.

NARRATOR: They must follow an evacuation checklist.

KIT: We want the pilots to grab the list, simply read it

and do it.

No wondering what step is next,

because the sequence of the steps are very important.

CPT. YU: Pull and rotate.

FLIGHT ATTENDANT: Everybody remain calm.

Do not bring your luggage or personal belongings.

NARRATOR: Seconds feel like hours as the crisis escalates.

Finally, the pilots are ready to open the doors.

FO. TSENG: Evacuation required now. Required.

♪

FLIGHT ATTENDANT: No pushing. No pushing.

Please keep moving forward.

NARRATOR: But it will take time for all 157 passengers

to make it to the exit.

George Ishizaki is watching the unfolding disaster

from inside the airport terminal.

GEORGE: I just happened to have my camcorder with me.

I thought, oh my god, what is happening?

FLIGHT ATTENDANT: Keep moving forward.

NARRATOR: With the fire growing more intense,

time is running out.

JIM: Let's go. Let's go!

JIM: We were quite a ways back from any exit

since the over-wing exits were useless.

So my focus was really to get the kids moving forward

and off the aircraft.

FEMALE PASSENGER (In distress):

JIM: Go ahead. Go ahead, all right?

NARRATOR: Jim Caruso stays behind to help other passengers

get off the burning plane.

JIM: I don't actually remember hesitating.

It may have been a little difficult to make that decision

since the kids were already moving forward.

FLIGHT ATTENDANT: Hey, no pushing. Keep moving forward.

GEORGE: The smoke actually started building, and

that's when everything started happening really quickly.

♪

NARRATOR: Jim Caruso is separated from his family.

The heat and smoke are getting worse. He hears a cry for help.

FEMALE PASSENGER:

JIM: The woman behind me pointed towards the overhead bin.

I was concerned if she was pointing towards

flames coming in.

FEMALE PASSENGER:

JIM: I looked up and I saw a pair of crutches.

GEORGE: Everybody was just sliding down the slides,

and once they got on the ground they were just scrambling.

♪

JIM: Once the smoke and fire started building,

the cabin became rather warm.

I do recall some of the windows actually cracking

from the heat.

NARRATOR: Finally, they make it to the exit.

♪

The plane has been burning for close to three minutes.

It could explode at any moment.

FLIGHT ATTENDANT:

Captain, all passengers are evacuated.

You're the last ones.

CPT. YU: We gotta get out of here.

KIT: Typically, the captain will stay until everybody's off

and he will verify that the airplane is empty.

NARRATOR: The pilots have put their passengers' safety first.

But now it may be too late for them.

CPT. YU: We're going to have to climb out through the window.

You first.

FO. TSENG: Yes, sir.

NARRATOR: All 737 cockpits are equipped

with an emergency escape rope.

It's designed to help pilots exit through the side window,

but it's no easy maneuver.

KIT: Exiting the airplane is more difficult than it sounds.

It's a relatively small window. Going down the rope has a risk.

♪

Then...

WITNESS: Oh. Whoa. Whoa. Oh my god!

GEORGE: You felt a huge kaboom.

I've never felt anything like that.

JIM: We actually could feel the ground shake.

♪

NARRATOR: Passengers run to safety as a fiery explosion

engulfs the plane they just escaped.

The fate of the pilots is still unknown.

JIM: As I looked back after the first explosion,

I recall crew members fast-roping, as it were,

out of the cockpit.

NARRATOR: The force of the blast overpowers the first officer.

GEORGE: He dropped down from the height of the cockpit window

onto the ground.

NARRATOR: Incredibly, he's able to get up and run away

from the flames. The captain quickly follows.

GEORGE: It was good that he did that,

because the fire just gutted the airplane.

NARRATOR: More explosions rock the airplane.

GEORGE: The fuselage I guess melted.

The back half just kind of fell to the ground.

NARRATOR: Finally, fire trucks arrive on the scene.

JIM: Everybody had exited the aircraft at that point

and was gathering in the terminal.

It was a huge relief to have the kids

and my wife and I together.

We certainly were looking back at the aircraft again

in amazement.

NARRATOR: Fire on an airplane can quickly become lethal.

Incredibly, on Flight 120, all 165 people on board

have escaped unharmed.

KIT: I've never heard of any evacuation

where somebody wasn't hurt.

To get this many people off in such a dire circumstance

in a very short period of time with no injuries is miraculous.

♪

NARRATOR: If there's a next time,

passengers may not be so lucky.

Pressure to figure out what happened falls

on an international team of air crash investigators.

THOMAS: Okay. Let's get to work.

NARRATOR: They need to explain how an airliner

that had landed safely and turned off its engines

suddenly burst into flames.

THOMAS: We have no clue. We do not know what happened.

We tried to find out from the wreckage that remained

still on the apron.

BOB: Normally fires occur in-flight,

perhaps during taxi sometimes,

but rarely in a parking spot after the engines are shut down.

THOMAS: The challenge for the investigator is,

most of the evidence will be destroyed by the fire.

NARRATOR: Across the globe, there are more than 5,000

Boeing 737s in service.

KIT: There's a 737 taking off and landing

every three or four seconds in the world.

NARRATOR: If the plane has a design flaw

that somehow leads to uncontainable fire,

countless passengers could be at risk.

BOB: We were aware that the 737

is probably the most popular airliner out there.

So there's a reason once an accident occurs

to try to figure out what happened pretty darn quickly.

♪

NARRATOR: The search for Flight 120's black boxes

begins immediately.

BOB: One of our main goals initially

is to try to find the cockpit voice recorder

and the flight data recorder.

These are important, because a lot of times

they tell us what happened.

NARRATOR: But investigators know that

after such an intense fire

there's a chance the black box data will be lost.

They need other leads.

BOB: Good investigators don't rely totally

on flight data recorders, for instance,

or cockpit voice recorders because they can be destroyed.

So we rely on witnesses to tell us their impression

of what happened.

CPT. YU: Well, we taxied off the runway

down the apron to our assigned parking spot.

FO. TSENG: Once we parked, we shut off the engines,

and some time after that we heard the aircraft was on fire.

BOB: We needed to know what type of fire it was,

what the ignition source would be,

what the fuel source would be.

Those were the areas of our main questioning right off the bat.

FO. TSENG: I radioed the controller letting him know

we had a wheel fire.

NARRATOR: Investigators know that if a wheel caught fire

on Flight 120, there's more than one possible cause.

A deflated tire can result in burning rubber.

Overheated brake pads could potentially

ignite hydraulic fluid.

BOB: In a wheel well of an aircraft

there are a lot of hydraulic lines

going to the landing gear assemblies and things like that.

Hydraulic fluid is very flammable.

If a hydraulic leak occurred and it happened to drip

onto a hot brake for instance, well, there you go.

CPT. YU: Hey. What is this? FO. TSENG: We have a wheel fire.

NARRATOR: If the pilots are right about where the fire started,

investigators should be able to find proof.

They examine the plane's right side wheel well

and landing gear assembly.

They find scorched wreckage, but not enough to convince them

that this is where the fire began.

BOB: Once we were able to closely examine

that part of the aircraft,

we were very confident that a wheel well fire per se

did not occur.

The seat of the fire seemed to be forward

and a little bit to the right of the wheel well area.

NARRATOR: It seems the pilots were mistaken

about the origins of the fire.

Where it started remains a mystery.

♪

Solving that mystery may have just become easier.

Investigators have recovered the plane's black boxes.

THOMAS: The Japanese team retrieved both the Boeing's CVR and FDR.

THOMAS: Let's get working on the FDR immediately.

BOB: On modern 737s, the flight data recorder

has thousands of parameters,

data bits that come in to the recording device itself.

It will take time to download and verify all the data.

♪

NARRATOR: Meanwhile, the charred fire scene

continues to challenge investigators.

THOMAS: With all this heat damage it's nearly impossible

to tell where the fire started.

NARRATOR: They're almost certain the fire began

on the right side of the plane as witnesses reported.

But where exactly?

THOMAS: Wait a sec.

NARRATOR: Scorched wires provide a promising new lead.

THOMAS: What do you think?

NARRATOR: Did an electrical fire destroy Flight 120?

Serious electrical failures are rare, but not unheard of.

In 1998, the cockpit of Swissair Flight 111

began filling with smoke shortly after takeoff.

The pilots tried to make an emergency landing

in Halifax, Canada. They never made it.

Their plane disappeared into the Atlantic Ocean,

killing all 229 people onboard.

Investigators found that an electrical fault

in the entertainment system

almost certainly sparked the fire that doomed the plane.

THOMAS: Okay. Let's see what we got.

NARRATOR: If the fire aboard Flight 120

was caused by faulty wiring,

investigators may now be able to confirm it.

They've successfully downloaded the black box data.

Any electrical problem or failure in any onboard system

should show up in the data.

BOB: Almost every system on the aircraft is recorded.

Its status is recorded.

So we looked very quickly through

these hundreds of electrical possibilities

and we found nothing.

THOMAS: It wasn't electrical.

NARRATOR: The analysis comes up empty.

THOMAS: At that time we can rule out some wheel fire

or some other electrical fire.

NARRATOR: Investigators turn their attention

to the aircraft's right engine.

In flight, the CFM-56 power plant

generates internal temperatures of more

than 2,000 degrees Fahrenheit, hotter than molten lava.

Could an engine fire have sparked the inferno?

BOB: We of course looked at the engines very carefully,

the right engine especially

because there was a lot of fire damage around it.

It was fairly easy to look inside the engine

and examine the core, so to speak,

where all the damage in an engine failure usually occurs.

And we found nothing wrong.

The engine was damaged externally but not internally,

so we eliminated it as a possible factor.

THOMAS: What could have burned this entire plane down?

NARRATOR: The investigation has hit a wall.

The cause of the catastrophic fire remains unknown,

while every day thousands of 737s continue to fly.

There's growing pressure on investigators

to find the answer.

BOB: We knew we had a little bit of detective work ahead of us

and we pressed on.

♪

NARRATOR: The video capturing the fiery destruction

of China Airlines Flight 120

could provide investigators with clues

to what started the fire.

THOMAS: Whoa.

NARRATOR: The footage reveals just how quickly

the flames spread through the passenger jet.

But for investigators, the most important clue is missing.

The recording hasn't captured the critical moment

the fire started.

THOMAS: From the video, we can only understand

there was fire and the location of the fire

and it seems that something was feeding to the fire.

But we cannot understand why.

♪

NARRATOR: Investigators widen the search for leads.

THOMAS: What did you see?

NARRATOR: The effort pays off when an airport ground worker

provides a critical detail.

BOB: A ramp worker on the right side of the aircraft

said very distinctly that he saw a liquid

running down the leading edge of the right wing

before the fire broke out.

THOMAS: Thanks.

NARRATOR: Fluid leaking from this part of the wing

of the aircraft can be only one thing: jet fuel.

The 737 holds 4,390 gallons of fuel,

much of it in tanks located

inside the plane's two massive wings.

BOB: When we figured out that an actual fuel leak had occurred,

it was a breakthrough,

a true breakthrough in the investigation.

We now needed to know why it originated.

THOMAS: We know the fuel was leaking, but from where?

A fuel line?

NARRATOR: Finding solid evidence amongst the burnt remains

of the plane's fuel system won't be easy.

The Boeing 737-800 has high-pressure pumps

inside the wing.

They deliver 200 gallons of fuel per hour to the engine.

All that fuel flows through flexible pipes.

Could one of those fuel pipes be the culprit?

BOB: Fuel lines are probably in a sense

the most vulnerable part of a fuel system.

They take bends and sometimes they're exposed

where they could get knocked or punctured by something.

So we tried to trace the entire fuel system of the aircraft.

♪

NARRATOR: It's another dead end.

THOMAS: It wasn't the fuel lines.

NARRATOR: None of the fuel lines are ruptured.

BOB: We had a lot of fire damage,

but the fuel lines that we examined seemed to be intact

and functional.

NARRATOR: Investigators still can't explain the fire.

They know enough fuel leaked from the plane

to feed the flames, but they don't know where it came from.

BOB: Once we eliminated fuel lines per se

as a possible problem,

pretty much the only thing out in that area

that could have gone wrong is a leak in the fuel tank itself.

NARRATOR: The plane's fuel tanks are made from aluminum alloy

and designed to withstand the rigors of flight for years.

They should never crack or leak.

Examining them

presents one of the most difficult challenges yet.

They hope a tool called a borescope will do the trick.

It's a small camera that can peer into tight spaces.

It gives them a unique view

inside the plane's right wing fuel tank.

What it reveals changes the entire course

of this investigation.

THOMAS: Whoa. Would you look at that?

BOB: All of a sudden, clear as a bell,

we saw this bolt sticking out of the fuel tank itself.

Where the bolt came from is a complete mystery,

but it has ruptured the tank

right where the ground worker spotted leaking fuel.

BOB: It's hard to describe how significant this was.

I mean, this was the core of the investigation.

Now we knew what happened.

The rest of the investigation was trying to figure out

why this occurred.

NARRATOR: A punctured fuel tank was the cause

of one of the most infamous air crashes in history.

A supersonic Air France Concorde burst into flames on takeoff

after running over a piece of metal debris on the runway.

Did a similar scenario lead to the total destruction

of China Airlines Flight 120?

THOMAS: Okay. We need to cut into this wing.

NARRATOR: Investigators need to get a closer look

at the mysterious bolt that made a hole in the fuel tank.

BOB: The investigator in charge said, yep,

now's the time to start cutting into that thing.

♪

♪

THOMAS: Okay. Got it.

NARRATOR: Now they need to figure out where the bolt came from.

They study schematics of the 737 wing structure.

BOB: We went back to the drawings

and went back to things like maintenance records

to try to figure out exactly what it was.

NARRATOR: They soon get their answer.

THOMAS: A downstop assembly.

NARRATOR: The downstop assembly is part of the slat mechanism

on the wing's leading edge.

CPT. YU: Let's go to flaps 25. FO. TSENG: Flaps 25.

NARRATOR: Pilots extend flaps and slats

during every takeoff and landing.

The downstop is fixed to the end of a track

that slides back and forth.

The device prevents the slats from moving too far forward.

RODNEY: The downstop is there, quite frankly, to stop it

when it reaches its maximum deployment length.

If it didn't exist then there would be no way

to retain the slat on the aircraft.

NARRATOR: Investigators have identified the piece

that penetrated the fuel tank.

They know it's not from another plane,

like the runway debris that caused the Concorde disaster.

But they have other important questions that need answers.

BOB: We had the assembly. We knew it punctured the tank.

Our next step was trying to figure out

how this could possibly have occurred.

♪

NARRATOR: Investigators pore over Boeing service documents

to learn more about downstop assemblies on the 737.

They make a surprising discovery.

THOMAS: This has happened before,

and it's happened more than once.

♪

BOB: There'd been two previous instances

of this device coming apart and causing minor fuel leaks,

but this was the first instance of an actual destructive fire.

In both previous instances, parts from a downstop assembly

punctured a fuel tank, just like on Flight 120.

THOMAS: Clearly, they knew it was a problem.

NARRATOR: Boeing was so concerned about the problem,

it issued a special work order

to secure the downstop assembly on all 737s worldwide.

RODNEY: The solution that Boeing had recommended

was to remove the nut from this particular device

and install some thread hardening material,

and then you reinstall the nut. It hardens in place.

THOMAS: What if the work order on this plane

was never completed?

NARRATOR: Investigators review the Boeing work orders.

If the plane that burned in Okinawa was never fixed,

that could explain the accident.

THOMAS: We tried to figure out

when is the last time anybody touched that assembly.

NARRATOR: But according to the records,

the proper work was carried out very recently.

BOB: We discovered that this particular component,

this downstop, had been manipulated

only a couple of weeks or so prior to the accident.

THOMAS: It just doesn't make sense.

NARRATOR: Investigators can see that the nut

on the downstop assembly is still attached.

It seems that the work order to replace it was completed

just as the records show. So what went wrong?

THOMAS: That's the weird part that we wanted to figure out

at that time.

NARRATOR: They examine the downstop assembly from Flight 120.

♪

They check all the component parts.

Finally, they spot something.

BOB: We decided to count parts, and lo and behold,

a washer was missing.

♪

THOMAS: Where is it?

NARRATOR: There's supposed to be a washer right behind the nut.

Could a single missing washer

have played a role in the accident? It seems unlikely.

But investigators can't rule it out.

They need to find the washer.

BOB: We thought initially that the washer

may have somehow gotten inside the fuel tank,

but that's not the case at all.

We examined very carefully the rest of the wing,

and we found that particular washer

in the leading edge assembly of the wing,

just laying in there loose.

THOMAS: The washer is a fit.

NARRATOR: Recovering the missing washer

raises a puzzling question.

BOB: The nut was on there and it was torqued down correctly,

but there was no washer on it at all.

How did the washer become detached from the bolt

but not the nut?

THOMAS: If the nut is still on the bolt,

why is there something between that fell off?

NARRATOR: It seems like an impossibility,

and yet somehow it happened.

The Flight 120 fire investigation heads to Taiwan

and the headquarters of China Airlines.

Investigators hope to shed some light

on the mystery of the detached washer.

THOMAS: I appreciate you making the time.

THOMAS: We went to China Airlines

to ask them to demonstrate how they do the maintenance work.

THOMAS: Do you think you can show me how you completed

this repair on the downstop assembly?

BOB: Sometimes maintenance records don't tell you

the true story.

They can tell you that according to somebody,

a maintenance procedure had been done correctly.

But to get a better story,

you have to actually watch the procedure being done.

NARRATOR: A mechanic demonstrates

how he performed the downstop repair.

MECHANIC: You won't be able to see much of what I'm doing.

RODNEY: Performing maintenance on this particular downstop

is a little tricky.

The mechanic is going to be in a very restricted visual area.

So he's going to have to work with his hands.

He's going to have to feel the apparatus.

MECHANIC: After applying glue, you put the bolt into place.

♪

THOMAS: You can imagine that you're under the wing

and you cannot see it.

MECHANIC: Sorry. I just dropped it. Don't worry.

It's easy to pick up again.

NARRATOR: It's an eye-opening demonstration.

MECHANIC: And that's how it's done.

THOMAS: Thank you. You have been very helpful.

THOMAS: It's not very easy for them to confirm

they finished their job and everything is in order there.

♪

NARRATOR: Records show that the work order repair

was the only time mechanics ever serviced the downstop

in the history of the accident airplane.

There's only one possible explanation

for how the washer found in Okinawa came loose.

It fell off during the maintenance procedure in Taiwan.

BOB: It could have just slipped off the gentleman's fingers

when he was trying to install it.

It could have stuck to the nut and then fallen off

just before he touched them together.

A lot of things could have happened.

The bottom line is, the washer was not there.

♪

NARRATOR: But understanding what happened to the washer

still leaves investigators scratching their heads.

The downstop assembly had a well-tightened nut

that was also glued to the bolt. How could it fall out?

And how did this piece start a raging fuel fire

that destroyed a $70 million airplane

and threatened the lives of 165 people?

♪

THOMAS: All right. Now let's test it without a washer.

NARRATOR: Investigators experiment with the suspicious part

from Flight 120 to see how it performs without the washer.

RODNEY: The design of this assembly requires each component

to play a specific role.

So any piece that is not reinstalled is critical.

NARRATOR: They make a stunning discovery.

The small washer is the only thing

preventing the unit from falling out of its mount.

THOMAS: Without the washer, it fails.

BOB: We discovered that the nut and the bolt

were smaller than the rest of the assembly

and that the washer was a required item.

This is an example of an intact downstop assembly.

You take the nut off. You take the washer off.

Put the nut back on, like they did in Taiwan,

and now you'll note that the assembly itself

is pretty ineffective.

It falls apart without much problem at all.

♪

NARRATOR: Finally, it's clear why the downstop assembly

was able to fall out of the slat track.

How it punctured the fuel tank

is the final piece of the puzzle.

But investigators believe

the design of the slat mechanism itself may hold the answer.

RODNEY: When the engineers are designing these aircraft,

they take into consideration maximum space utilization

and for the design of a leading edge slat

they came up with something called a can.

NARRATOR: The slat can is the area inside the wing

that houses the moving track.

RODNEY: The can is a void that extends into the fuel tank

and allows for the device that operates the slat

to move in and out.

NARRATOR: The space inside the slat can is tight,

just big enough for the sliding track.

Any foreign metal object taking up any space inside the can

would be an accident waiting to happen.

BOB: It would render the entire assembly

not only non-functional but dangerous.

♪

THOMAS: China Airlines flight 1-2-0,

157 passengers, eight crew.

NARRATOR: Investigators believe they finally understand

the sequence of events that led to a devastating fuel fire

in Okinawa.

♪

It all begins weeks before the accident

with a botched repair to a critical component.

A single missing washer turns a downstop into a hidden danger,

a bolt that can work its way loose over time.

BOB: Washers really aren't supposed to hold things together,

but this washer did because of its design.

NARRATOR: Six weeks after the failed repair...

CPT. YU: Please give the flight attendants your full cooperation

as they prepare the cabin for landing.

NARRATOR: A routine descent into Okinawa requires the pilots

to deploy the flaps and slats as usual.

Inside one of the track cans on the right wing,

the loosened downstop is just barely holding on.

Touchdown is enough of a jolt

to finally knock the downstop out of its track.

CPT. YU: Flaps up.

NARRATOR: The unsuspecting crew soon retracts

the flaps and slats.

The plane's powerful hydraulics move the slat track

back into the can. But now the downstop bolt is in the way.

The track pushes it to the back of the can and then

straight through, puncturing the right wing fuel tank.

THOMAS: A hole in the fuel tank caused the leak.

The plane would have been carrying

thousands of liters of fuel.

NARRATOR: As the plane taxis, the engine exhaust

is powerful enough to disperse the leaking fuel.

It can't come in contact with the hot tail pipe or brakes.

CPT. YU: After-landing checklist.

NARRATOR: But once the pilots park and shut down the engines,

the situation instantly becomes much more dangerous.

FO. TSENG: Engine start levers.

CPT. YU: Engine start levers cut off.

NARRATOR: The leaking fuel starts dripping

directly onto the scorching-hot tail pipe.

PASSENGERS:

JIM: What is going on out there?

NARRATOR: The leaking jet fuel ignites on contact.

KIT: The speed of the development of the fire is incredible.

Obviously fuel burns very well, and it goes up like a bomb.

FLIGHT ATTENDANT: Everybody remain calm.

NARRATOR: The cabin crew's professional conduct

gets 157 passengers off the plane

in just one minute and 42 seconds.

♪

The Naha Airport fire leads investigators

to a striking realization.

THOMAS: The repair that was ordered actually caused the fire.

BOB: It was kind of ironic.

The Taiwanese maintenance procedure was to prevent

an accident, and in essence the procedure had a lot to do

with why this particular accident happened.

KIT: It's a great study in unintended consequences.

We really were trying to fix a problem.

We really didn't anticipate that

every time we handle a maintenance piece like this

there is a risk. And it bit us.

NARRATOR: In the wake of the Naha Airport inferno,

aviation authorities around the world

order the inspection of the entire fleet of 737s.

In the US alone, 21 planes are found to have the same defect,

all of them at risk of a catastrophic fuel leak and fire.

Boeing takes immediate action.

It redesigns the downstop mechanism

and ensures that the improved part is installed

on each and every plane.

KIT: Boeing made the changes necessary

to ensure that the accident didn't occur again.

It took a little time, as it normally does,

to get to the final resolution of it,

but they did what they needed to do.

♪