Original subtitles

Damn it!

When a passenger jet owned by Formula One

When a passenger jet owned by Formula One

superstar Niki Lauda plummets to earth,

he's driven to find out why.

he's driven to find out why.

Where was the tail found?

I was upset that the airplane did

I was upset that the airplane did

something where a human being couldn't react anymore.

When two DC-10s are brought down within two years...

When two DC-10s are brought down within two years...

What's happened?

The captain flew out!

...investigators unlock shocking discoveries.

...investigators unlock shocking discoveries.

Design flaw issues can typically

affect the entire fleet.

affect the entire fleet.

Are there other ones that are about to happen?

Get out of it!

I can't.

I can't.

Come on.

And a doomed commuter flight in Georgia

comes down to a faulty design.

comes down to a faulty design.

My god.

Are three horrific accidents

all because of deadly defects?

all because of deadly defects?

A design flaw that leads to a loss of an aircraft,

A design flaw that leads to a loss of an aircraft,

it draws the attention of the entire world.

Ladies and gentlemen, we are starting our approach.

We lost both engines.

We lost both engines.

Keep the mask over your.

Emergency.

Mayday, mayday.

Brace for impact!

He's gonna crash!

You need a little bit of outer trim to the left, huh?

What was that?

What was that?

You need a little bit of outer trim to the left.

Ascending to cruise altitude...

OK.

OK.

...Captain Thomas Welch and first officer Josef Thurner

are flying a Boeing 767 from Bangkok, Thailand

are flying a Boeing 767 from Bangkok, Thailand

to Vienna, Austria.

The flight is operated by Lauda Air,

The flight is operated by Lauda Air,

the brainchild of famed Austrian Formula One driver Niki Lauda.

the brainchild of famed Austrian Formula One driver Niki Lauda.

For me it was a logical step after retiring

For me it was a logical step after retiring

from racing to start in this kind of business.

And that's what I did with Lauda Air.

And that's what I did with Lauda Air.

His new airline is a small fleet

of just four planes.

of just four planes.

One of the main reasons was to give the passengers

a different way of flying...

better service, better food.

better service, better food.

The airplane had to look in a certain way.

Lauda Air Flight 4 is less than 15 minutes into

Lauda Air Flight 4 is less than 15 minutes into

its 11-hour flight to Vienna.

In the cockpit, Captain Welch and First Officer

Thurner monitor their instruments while the autopilot

Thurner monitor their instruments while the autopilot

controls the climb.

Suddenly, the plane begins dropping from the sky.

Yeah, wait a minute.

In an instant, the pilots lose all control.

In an instant, the pilots lose all control.

Damn it!

And their jet plunges toward the ground.

In a matter of seconds, the jet falls more than 20,000 feet.

The plane slams into a remote jungle, 110 miles

The plane slams into a remote jungle, 110 miles

Northwest of Bangkok.

I was at home, and I had a phone call from the news.

I was at home, and I had a phone call from the news.

They said one of our airplanes crashed.

They said one of our airplanes crashed.

When rescuers arrive, it's

immediately obvious that there's no one to be rescued.

immediately obvious that there's no one to be rescued.

All 213 passengers and 10 crew members are dead.

It's the first time a Boeing 767 has ever crashed.

The loss of a US-made aircraft prompts

The loss of a US-made aircraft prompts

quick action from the National Transportation Safety Board,

or NTSB.

Bob Macintosh is assigned to the investigation.

A first crash of an aircraft that

A first crash of an aircraft that

was in service intercontinental for almost a decade

with a perfect service record...

it was extremely important for us to learn what had happened.

it was extremely important for us to learn what had happened.

He finds the badly burned wreckage strewn over

He finds the badly burned wreckage strewn over

a square mile of rough terrain.

The major components, the cockpit, the tail,

The major components, the cockpit, the tail,

the main body, they were scattered

far enough apart that certainly they

far enough apart that certainly they

had not come down together.

Macintosh immediately knows the plane

broke up during the flight.

What he needs to uncover is why.

Where was the tail found?

Airline owner Niki Lauda arrives in Thailand

Airline owner Niki Lauda arrives in Thailand

to help find the answer.

NIKI LAUDA The scene

NIKI LAUDA The scene

for me, this was the most horrific pictures

I've ever seen.

I've ever seen.

Surrounded by the scattered remains

of an airliner that carries his name,

Lauda is overwhelmed by the magnitude of the disaster.

Lauda is overwhelmed by the magnitude of the disaster.

When you have 223 people killed,

the families all want to know why.

And I think this is the worst for people,

And I think this is the worst for people,

if they do not know why they lost their husband, children

whatsoever.

The black boxes may have captured important clues

The black boxes may have captured important clues

about the cause of the in-flight breakup,

but they're badly burned.

but they're badly burned.

They're sent to NTSB technicians in Washington.

They're sent to NTSB technicians in Washington.

Then, days after the crash...

Well, would you look at that.

Well, would you look at that.

...more pieces of the plane

are found, including one of the engines.

And investigators are amazed by what they see.

And investigators are amazed by what they see.

The thrust reverser on Flight 4's left engine

The thrust reverser on Flight 4's left engine

is fully deployed.

Once we actually saw that the thrust reverser had deployed,

Once we actually saw that the thrust reverser had deployed,

for me, it was shocking.

Thrust reversers are only used on landing.

Thrust reversers are only used on landing.

Once activated, they direct air flow forward,

Once activated, they direct air flow forward,

helping the 767 come to a stop.

They should never be deployed in the air.

And even if it does happen, a deployed reverser

shouldn't cause a crash.

shouldn't cause a crash.

This discovery is the biggest break so far,

This discovery is the biggest break so far,

and investigators need to know was a deployed thrust reverser

responsible for the crash?

responsible for the crash?

But the team will have to solve the mystery without their most

valuable tool.

valuable tool.

The flight data recorder is so badly

burned that technicians at the NTSB can't recover any data.

burned that technicians at the NTSB can't recover any data.

Thankfully, the cockpit voice recorder has survived.

Thankfully, the cockpit voice recorder has survived.

Ready to go?

Investigators and Niki Lauda

Investigators and Niki Lauda

listen to the last moments of Lauda Air Flight 4.

Check.

5 and 1/2 minutes into the 15-minute long flight,

5 and 1/2 minutes into the 15-minute long flight,

everything is normal.

Then the first sign of trouble.

Then the first sign of trouble.

That keeps...

it's come on.

What's come on?

What's come on?

Some kind of warning.

They discover Lauda pilots

They discover Lauda pilots

discuss a mysterious cockpit warning

moments before the crash.

It's a warning that relates to a hydraulic valve in the

left engine's thrust reverser.

left engine's thrust reverser.

The hydraulic isolation valve controls

the flow of hydraulic fluid to the reversers.

Opening the valve allows the reverser

to be stowed or deployed.

When it's closed, the reversers won't budge.

When it's closed, the reversers won't budge.

OK.

Yeah.

The crew doesn't sound worried about the warning.

The crew doesn't sound worried about the warning.

No, it's probably moisture or something.

For five minutes, nothing happens.

- Then... - What?

- Then... - What?

Reversers deployed!

Wait a minute.

...just 22 seconds later, there's the sound

...just 22 seconds later, there's the sound

of the plane ripping to pieces.

The whole thing was so difficult. You never forget.

The whole thing was so difficult. You never forget.

More motivated than ever, Lauda sets out

More motivated than ever, Lauda sets out

to prove a controversial theory explaining

why his plane went down.

why his plane went down.

I was after Boeing day and night

so that the people understand that we're working on it,

so that the people understand that we're working on it,

that we hopefully find the cause and make sure it

will never ever happen again.

Because this is the answers these families needed.

Niki Lauda travels to Boeing

headquarters in Seattle to investigate

the 767's thrust reversers.

the 767's thrust reversers.

It's my name, my airplane, my crash.

It's my name, my airplane, my crash.

And Boeing understood my problem, or their problem.

And we kept on working together.

And we kept on working together.

Before the 767 first went into service,

Boeing ran certification tests to prove that pilots could

Boeing ran certification tests to prove that pilots could

keep flying safely in the event of a mid-air reverser

deployment.

The tests proved that the 767 was capable

The tests proved that the 767 was capable

of continued safe flight and landing,

regardless of the position of the thrust reverser.

regardless of the position of the thrust reverser.

The widespread feeling of what would happen

The widespread feeling of what would happen

if a reverser deployed in flight was that the airplane

if a reverser deployed in flight was that the airplane

would be controllable.

But despite Boeing's certification tests,

But despite Boeing's certification tests,

Lauda believes the reverser is to blame.

I think we need to use a simulator.

I know what it says, but that's not what happened in Thailand.

I know what it says, but that's not what happened in Thailand.

And he's determined to figure out why a failure that's

And he's determined to figure out why a failure that's

not supposed to be dangerous killed 223 people flying

on his airline.

It's my airplane.

It's my crash.

It's my crash.

OK.

Let's try it in a simulator.

Niki Lauda works with Boeing investigators

Niki Lauda works with Boeing investigators

to recreate the flight.

OK, let's begin.

OK, let's begin.

He sets the flight's altitude to 24,000 feet, more

than double the 10,000 feet that Boeing used for their thrust

than double the 10,000 feet that Boeing used for their thrust

reverser certification.

Could the altitude difference explain what happened?

Could the altitude difference explain what happened?

It was incredible because the airplane just turned around,

It was incredible because the airplane just turned around,

and you couldn't do anything.

Lauda now knows definitively why his plane

dropped so unexpectedly.

dropped so unexpectedly.

When the thrust reverser deployed,

the smooth flow of air over the wing

was disrupted, dramatically decreasing lift.

was disrupted, dramatically decreasing lift.

Reversers deployed!

Reversers deployed!

This forced the jet into

a sudden, terrifying nosedive.

It was just too late.

It was just too late.

The aircraft was going to assume a very

nose-low, high-speed attitude.

nose-low, high-speed attitude.

Incredibly, Niki Lauda has proven Boeing's

certification test was lacking.

certification test was lacking.

He's shown that above 20,000 feet, where planes fly faster,

He's shown that above 20,000 feet, where planes fly faster,

the accidental deployment of a thrust reversers can be fatal.

the accidental deployment of a thrust reversers can be fatal.

It was evident to the Boeing Company

and to the FAA certification authorities

and to the FAA certification authorities

and to the operators of Boeing 767s around the world

that this was going to be a major, major issue.

that this was going to be a major, major issue.

The reverser deployed, and those guys couldn't recover.

Let's figure out why that happened.

Now the 767's thrust reversers undergo

Now the 767's thrust reversers undergo

intensive study at Boeing.

Engineers can imagine only one scenario

Engineers can imagine only one scenario

that might make a reverse or deploy unexpectedly in mid-air.

The theory calls for the activation

The theory calls for the activation

of the two valves in the reverser's failsafe system

at the same time.

at the same time.

The first valve is the isolation valve that controls

the flow of hydraulic fluid.

the flow of hydraulic fluid.

When the second valve, called the directional control valve,

is activated, fluid freely moves the reverser

is activated, fluid freely moves the reverser

from stowed to deployed.

You have to have both the isolation valve open

You have to have both the isolation valve open

and the directional control valve at the engine open

to the deployed position.

to the deployed position.

After extensive testing,

engineers finally make a major breakthrough.

engineers finally make a major breakthrough.

They're able to trigger the double failure needed to

accidentally deploy a reverser.

We could get that system to activate

We could get that system to activate

by introducing a short, a direct short in a system.

by introducing a short, a direct short in a system.

Investigators consult with Boeing engineers

on the 767's infrastructure.

on the 767's infrastructure.

They discover that the wires for both valves

were bundled in the same harness.

were bundled in the same harness.

A fault across several wires in the same harness

could have triggered simultaneous short circuits.

could have triggered simultaneous short circuits.

Two shorts in two valves at exactly the same time?

Two shorts in two valves at exactly the same time?

What are the odds?

I was upset that the airplane did

I was upset that the airplane did

something where a human being couldn't react anymore.

something where a human being couldn't react anymore.

Investigators have revealed

a serious design flaw that allows thrust reversers

a serious design flaw that allows thrust reversers

to deploy accidentally.

Pressure builds for a full review of the thrust

Pressure builds for a full review of the thrust

reverser system on all 767s.

reverser system on all 767s.

There were other airlines that joined in the questioning

of how could this possibly affect

of how could this possibly affect

the flight path to the point where you lose control?

As a result of the Lauda investigation,

As a result of the Lauda investigation,

a series of mechanical locks now ensure

that even if both valves get energized in flight,

that even if both valves get energized in flight,

the reversers can't deploy.

For Niki Lauda, the outcome of this investigation

For Niki Lauda, the outcome of this investigation

is total vindication.

For me, the worst thing in life is gray areas.

I hate gray areas.

I hate gray areas.

And a lot of airplane crashes been in the past

where you really do not know exactly what happened.

where you really do not know exactly what happened.

And in this crash here, thank God,

it was clear what was the cause.

And it was fixed for all airplanes worldwide.

And it was fixed for all airplanes worldwide.

Even when new airliners

go through exhaustive testing before they're put in service,

go through exhaustive testing before they're put in service,

a design flaw can occasionally slip through the cracks.

a design flaw can occasionally slip through the cracks.

I think due to the complexity of these aircraft,

there are any number of things can

there are any number of things can

go wrong even under the best set of circumstances.

17 years earlier, is a deadly design flaw...

The captain blew out.

The captain blew out.

...responsible for the worst air crash to date?

Turkish Airlines Flight 981 is boarding at Orly airport

in Paris.

Normally, the last leg of this trip from Turkey to England

isn't very crowded.

isn't very crowded.

But today, the DC-10 is filling up fast.

With all the new passengers boarding,

Captain Nejat Berkoz and co-pilot Oral Ulusman

Captain Nejat Berkoz and co-pilot Oral Ulusman

are running a little behind schedule.

So is baggage Handler Mohammed Mahmoudi,

So is baggage Handler Mohammed Mahmoudi,

who's loading the luggage of all 335 passengers.

Not expecting anymore bags,

Not expecting anymore bags,

Mahmoudi locks the rear cargo door.

The DC-10 is set to go.

The DC-10 is set to go.

Just after 12:30 in the afternoon,

Flight 981 lifts off into the skies above Paris.

Tango Hotel Yankee.

91, you are cleared to flight level 230.

91, roger.

91, roger.

As it flies away from the airport,

the DC-10 continues to gain altitude.

the DC-10 continues to gain altitude.

They're still climbing at 10,000 feet when disaster strikes.

They're still climbing at 10,000 feet when disaster strikes.

The huge jet shudders and begins to drop.

The huge jet shudders and begins to drop.

What happened?

The captain flew out.

The captain flew out.

As the nose pitches down, the plane picks up speed,

and the crew struggles to save their plummeting jet.

and the crew struggles to save their plummeting jet.

Bring it up!

Up, up, up!

I can't bring it up.

She doesn't respond!

She doesn't respond!

In the cabin, two rows of seats

In the cabin, two rows of seats

have simply disappeared.

Anything not bolted down flies out of the plane

Anything not bolted down flies out of the plane

through a gaping hole in the cabin floor.

It looks like we're going to hit the ground!

Turkish Airways Flight 981 disintegrates at impact.

Turkish Airways Flight 981 disintegrates at impact.

None of the 346 people onboard survive.

None of the 346 people onboard survive.

It becomes the worst plane crash to date.

Paul Eddy is a journalist who covered

Paul Eddy is a journalist who covered

the story as it unfolded.

It was just a scene of absolute, utter devastation.

It was just a scene of absolute, utter devastation.

I still have nightmares about this.

I still have nightmares about this.

Investigators for the French Accident Investigation

Bureau are on the scene.

Bureau are on the scene.

My first job was to evaluate the scope of the wreckage

My first job was to evaluate the scope of the wreckage

and begin the first investigation on the spot.

and begin the first investigation on the spot.

Despite the enormous force of the crash,

the black boxes survive.

the black boxes survive.

Their contents could provide valuable clues.

Their contents could provide valuable clues.

The inquiry takes a bizarre turn when

investigators are called to a field

nine miles from the main crash site.

nine miles from the main crash site.

They find the rear cargo door and two rows of seats that

They find the rear cargo door and two rows of seats that

somehow fell from the DC-10.

Since the accident involves an American plane,

Chuck Miller from the National Transportation Safety Board

Chuck Miller from the National Transportation Safety Board

joins the investigation.

He was a very, very professional man.

He was a very, very professional man.

Chuck didn't sit back in the office.

Chuck was always on the scene.

Chuck was always on the scene.

For him, the scene is eerily familiar.

For him, the scene is eerily familiar.

For the second time in two years,

he's dealing with a DC-10.

In June 1972, a plane's cargo door and some cargo

In June 1972, a plane's cargo door and some cargo

were found in a field 18 miles from Windsor, Ontario, Canada.

were found in a field 18 miles from Windsor, Ontario, Canada.

They had been ripped from a brand new DC-10

They had been ripped from a brand new DC-10

as it flew to Buffalo.

But the Windsor incident ended differently,

as the pilots of American Airlines Flight 96

as the pilots of American Airlines Flight 96

were able to land their planes safely.

When the flight crew finally saw the damage,

When the flight crew finally saw the damage,

they were stunned by a hole where the cargo

door would normally be.

door would normally be.

The captain and I walked back to the back,

and we just looked up and saw this hole.

and we just looked up and saw this hole.

And it was just so weird.

It didn't take investigators long

It didn't take investigators long

to realize the very design of the cargo door

carried a latent fatal flaw.

carried a latent fatal flaw.

Take a picture of this here.

These don't look like...

When it closes, hooks in the DC-10's cargo door grab

When it closes, hooks in the DC-10's cargo door grab

hold of a bar on the plane's door frame.

To make sure it's locked, baggage handlers

To make sure it's locked, baggage handlers

push down on a lever which drives

locking pins through the hooks that hold them in place.

locking pins through the hooks that hold them in place.

Without this heavy-duty mechanism,

the extreme air pressure thousands of feet in the air

the extreme air pressure thousands of feet in the air

would rip the door right off the plane.

A 1972 discovery was terrifying.

A 1972 discovery was terrifying.

The NTSB realized it was possible to close the lever

on the outside of the door, even if the hooks and locking

on the outside of the door, even if the hooks and locking

pins were not in the closed position.

Engage the lever.

Engage the lever.

This meant baggage handlers could believe the door

was locked when it wasn't.

In the Windsor incident, there was an obvious flaw.

In the Windsor incident, there was an obvious flaw.

And that's where the NTSB said, let's make sure we

And that's where the NTSB said, let's make sure we

change this system right now.

Every DC-10 operator needs to know this.

I want everything checked, all the bolts checked.

I want everything checked, all the bolts checked.

In 1972, Chuck Miller's report strongly

recommended changes to the DC-10's cargo door

recommended changes to the DC-10's cargo door

locking mechanism be made as soon as possible.

Two years later, as Miller inspects

Two years later, as Miller inspects

the Turkish Airlines crash site, he's immediately troubled.

the Turkish Airlines crash site, he's immediately troubled.

If Miller's recommendations were implemented after the Windsor

incident, why has another DC-10 cargo door

incident, why has another DC-10 cargo door

been ripped from its plane?

When he saw the door, saw that the fix hadn't been made,

When he saw the door, saw that the fix hadn't been made,

and that's when I think his anger became

and that's when I think his anger became

very, very strong indeed.

Miller takes an unusual step.

Miller takes an unusual step.

Although the official investigation

is just beginning, he gives journalist Paul Eddy

is just beginning, he gives journalist Paul Eddy

an important tip.

I said have you got any ideas what made the door come off?

And he said, yeah.

And he said, yeah.

If I were you, I'd go and look at a place

called Windsor, Ontario.

Suspecting the two similar

incidents within two years with a DC-10 are connected...

incidents within two years with a DC-10 are connected...

I'm Chuck Miller.

...NTSB investigator Chuck Miller shares his theory

...NTSB investigator Chuck Miller shares his theory

with French investigators.

These were taken on June 12, 1972, right after the incident.

These were taken on June 12, 1972, right after the incident.

These were taken on June 12, 1972, right after the incident.

We asked for the report on the Windsor accident.

We asked for the report on the Windsor accident.

And our American colleagues were also volunteers

And our American colleagues were also volunteers

to give us a lot of details.

The French team is astonished

The French team is astonished

by what Miller shows them.

Now we had an American Airlines flight from Detroit

Now we had an American Airlines flight from Detroit

to Buffalo have its cargo door blow off.

And he has been very frank.

And I'll explain what he was thinking

of the Windsor accident.

of the Windsor accident.

With the information from Chuck Miller,

With the information from Chuck Miller,

French investigators take a closer

look at the plane's cargo door.

There is no new problem.

It's just like the 1972 American Airlines case all over again.

It's just like the 1972 American Airlines case all over again.

The latches that are supposed to hold the cargo door closed

The latches that are supposed to hold the cargo door closed

aren't locked.

What you're going to now discover

What you're going to now discover

is why wasn't that door fixed?

So I don't think it's connected...

In the wake of the 1972 Windsor incident,

the NTSB had made very specific recommendations to the Federal

the NTSB had made very specific recommendations to the Federal

Aviation Administration.

Engage the lever.

Most importantly, they suggested that a change be

Most importantly, they suggested that a change be

made to the locking mechanism.

But in the two years since that accident,

But in the two years since that accident,

none of those recommendations had

been implemented by McDonnell Douglas, the company

been implemented by McDonnell Douglas, the company

who manufactures the DC-10.

In fact, the FAA never issued an airworthiness directive,

a legal requirement that would ensure fixes were made.

a legal requirement that would ensure fixes were made.

It's the job of the NTSB to discover what's happened,

It's the job of the NTSB to discover what's happened,

and to come up with recommendations as how

and to come up with recommendations as how

to prevent it happening again.

But it has absolutely no authority to implement them.

But it has absolutely no authority to implement them.

Investigators learned that McDonnell

Douglas had made some minor changes to the cargo doors.

Douglas had made some minor changes to the cargo doors.

A peephole was cut in the bottom of the door,

A peephole was cut in the bottom of the door,

so baggage handlers could see if the locking pins had engaged.

Several warning signs were also attached to the plane's door.

Several warning signs were also attached to the plane's door.

But sadly, those fixes just created their own problems.

But sadly, those fixes just created their own problems.

Many baggage handlers didn't know what the small hole

Many baggage handlers didn't know what the small hole

in the door was for.

The baggage handler in Paris read and spoke three languages

The baggage handler in Paris read and spoke three languages

but not English, the only language in which

the warning signs were written.

the warning signs were written.

But in the eyes of Chuck Miller, these were band-aid solutions.

But in the eyes of Chuck Miller, these were band-aid solutions.

The fundamental flawed design of the locks

remained the same, allowing history to repeat

remained the same, allowing history to repeat

itself just two years later.

There is no question that if an airworthiness directive had

There is no question that if an airworthiness directive had

been issued, as it should have been after Windsor,

Paris would not have happened.

Paris would not have happened.

It was an entirely avoidable accident.

After the Paris crash, foolproof changes were finally

made to the DC-10's cargo door.

made to the DC-10's cargo door.

Well, in aviation, it's called tombstone technology.

In other words, we always have the balance of money.

In other words, we always have the balance of money.

And unfortunately, we have had to wait until we had

enough people die in an accident to say, you know,

enough people die in an accident to say, you know,

we really are going to have to spend the money over here.

Atlantic Southeast Airlines Flight

2311 cruises at 15,000 feet.

2311 cruises at 15,000 feet.

This is Captain Friedline on the flight deck.

We've got a bit of weather ahead of us,

but we're going to go around it and give

but we're going to go around it and give

you a pretty smooth ride.

At the controls of the Embraer

At the controls of the Embraer

120 is Captain Mark Friedline.

He's an experienced pilot with almost 12,000 flight hours.

He's an experienced pilot with almost 12,000 flight hours.

First Officer Hank Johnston has been

flying with Atlantic Southeast Airlines

flying with Atlantic Southeast Airlines

for nearly three years.

This was a normal day in the life of the crew.

doubt they were expecting any difficulties with the flight.

doubt they were expecting any difficulties with the flight.

Let's go 20 degrees to the right.

Today's flight is a short commuter route

from Atlanta, Georgia to the city of Brunswick

from Atlanta, Georgia to the city of Brunswick

on the Atlantic coast.

There are 20 passengers on today's flight.

There are 20 passengers on today's flight.

The runway's in sight.

The runway's in sight.

The crew is just five minutes from touching down.

The crew is just five minutes from touching down.

ASA 2311.

Cleared direct to Jeff one.

Winco, report the airport sight.

Winco, report the airport sight.

Expect a visual.

We do have it in sight.

2311.

Slowing for approach speed.

Slowing for approach speed.

The aircraft was normal.

There was nothing unexpected.

There was nothing unexpected.

Then the captain notices an unusual sound.

Then the captain notices an unusual sound.

It's weird.

Number one seems to be spinning faster.

Number one seems to be spinning faster.

The left is... the left is pulling a bit more.

The left is... the left is pulling a bit more.

Bringing power down to the left.

Captain Friedline tries

to compensate for the plane's unexplained pull to the left.

to compensate for the plane's unexplained pull to the left.

Flight 2311 is less than 1,000 feet from the ground,

Flight 2311 is less than 1,000 feet from the ground,

and the plane is getting more and more difficult to control.

and the plane is getting more and more difficult to control.

What's going on?

Do you see anything?

There's nothing.

There's nothing.

The crew were apparently caught completely

by surprise by something.

What's going on with this thing?

I can't hold it.

Get out of it!

Get out of it!

I can't.

Come on!

The plane is dropping out of the sky,

and the crew doesn't know why.

As Flight 2311 plummets to earth, captain Friedline...

As Flight 2311 plummets to earth, captain Friedline...

Come on.

No, no!

...fights desperately to save his plane.

...fights desperately to save his plane.

It's no use.

All 20 passengers and three crew members

are killed at the moment of impact.

Wreckage is still smoldering when

NTSB investigator Jim Ritter arrives at the crash site.

NTSB investigator Jim Ritter arrives at the crash site.

OK, let's start here and work backwards to first impact.

I want a record of everything.

I want a record of everything.

You need to look at the crash site

to collect the physical evidence.

That's the most important aspect of any aircraft investigation.

That's the most important aspect of any aircraft investigation.

At the same time, witnesses tell investigators

they saw the plane roll hard to the left

they saw the plane roll hard to the left

before it hit the ground.

And it come right over top of the house,

and it got real loud.

and it got real loud.

It was coming right over these trees here,

and then it got extra loud.

and then it got extra loud.

We knew that it was some kind of very abrupt failure

that would have been difficult for the flight crew

that would have been difficult for the flight crew

to overcome.

What could make it roll so far over?

What could make it roll so far over?

As investigators scour the wreckage, searching

for clues, Ritter knows they'll be

for clues, Ritter knows they'll be

working without any onboard flight recorders.

working without any onboard flight recorders.

In 1991, commuter planes aren't required to carry them.

Without the black boxes, it's basically

Without the black boxes, it's basically

a process of elimination.

We analyze all of the physical evidence

and come up with the most compelling scenario

and come up with the most compelling scenario

that matches that evidence.

So far, the investigators' best

So far, the investigators' best

clue is the steep left roll before impact.

When the airplane rolled to the left,

When the airplane rolled to the left,

it could really only be due to two things.

Perhaps the pilot wanted to roll to the left,

Perhaps the pilot wanted to roll to the left,

or there was a malfunction that the pilots couldn't counteract.

But when investigators study the engines,

they find that they were operating normally

they find that they were operating normally

at the moment of impact.

Ritter turns his attention to the other main part of

Ritter turns his attention to the other main part of

the plane's propulsion system.

Let's take a look at these propellers.

When we started doing testing to the propeller system,

When we started doing testing to the propeller system,

we didn't knew where it was going to lead us,

but it was something we had to eliminate if nothing else.

but it was something we had to eliminate if nothing else.

Deep inside the propeller unit,

investigators uncover an important clue.

investigators uncover an important clue.

Ah-hah.

There you are.

There you are.

We have a witness mark.

Take a look.

There's a small mark where the base of the propeller

slammed into its housing during the crash.

slammed into its housing during the crash.

It might be enough to tell investigators

It might be enough to tell investigators

how the propellers were operating

at the moment of impact.

at the moment of impact.

You can literally match up the scratch marks

between both pieces, and you'll know

between both pieces, and you'll know

what the angle of the propeller blade

was from that measurement.

It marked this one 22 degrees.

It marked this one 22 degrees.

The Embraer 120 is what's called

a constant speed propeller.

The blades spin at a steady rate in flight.

The blades spin at a steady rate in flight.

When the pilots need more power, the blades

When the pilots need more power, the blades

twist, changing their angle to take

a bigger bite out of the air and provide more thrust.

Slow in for approach speed.

And in flight, it acts like the automatic transmission

And in flight, it acts like the automatic transmission

in a car.

It's as if it's changing gears to match the engine load.

It's as if it's changing gears to match the engine load.

The marks tell Ritter the exact angle of the blades

when the plane slammed into the ground.

when the plane slammed into the ground.

We immediately noticed a difference

between some of the blade angle measurements

between some of the blade angle measurements

for the left engine versus the right engine.

The blades were almost flat.

The blades were almost flat.

At 3 degrees, the blades

are so flat they would act like a wall,

are so flat they would act like a wall,

blocking the flow of air the plane needs to maintain lift.

It might have caused the pilots to lose control.

Investigators study the mechanism used

to control the left propeller.

to control the left propeller.

Will you look at this?

It's completely worn down.

It's completely worn down.

The teeth on a key gear mechanism,

known as the quill, are almost entirely worn away.

known as the quill, are almost entirely worn away.

This is what it's supposed to look like.

With its teeth worn away,

the quill can't effectively lock onto the gear

system that controls the angle of the propeller blades.

system that controls the angle of the propeller blades.

That really was a eureka moment for us,

because now we had a serious malfunction

that we could examine.

that we could examine.

Investigators study the design

Investigators study the design

of the propeller mechanism.

They learn that shortly before the accident,

the propeller manufacturer started using a harder, more

the propeller manufacturer started using a harder, more

abrasive coating on a key part known

as the transfer tube, which meshes with the quill teeth.

as the transfer tube, which meshes with the quill teeth.

It turned into a giant file.

It turned into a giant file.

So the spines on the transfer tube

were much harder and rougher than the quill teeth.

were much harder and rougher than the quill teeth.

And it was almost like sandpaper,

so the tube was actually wearing down the teeth on the quill.

so the tube was actually wearing down the teeth on the quill.

Investigators now understand why

Investigators now understand why

the quill teeth were worn down.

Without functional quill teeth, the propeller blades

Without functional quill teeth, the propeller blades

could slip into a dangerously flat position.

To prove their new theory, investigators

need to take a huge risk...

We've got to see what happens in the air.

We've got to see what happens in the air.

...and test the damaged propeller system

under full flight conditions.

under full flight conditions.

I said, well, the only way to really know

is let's do a flight test and find out.

Because we are at a point in the investigation,

Because we are at a point in the investigation,

we need to start eliminating things.

The investigation into the crash of Flight 2311

The investigation into the crash of Flight 2311

moves to Embraer headquarters in Brazil.

Investigator Tom Haueter meets with the representatives

from Embraer and the propeller manufacturer.

from Embraer and the propeller manufacturer.

Thank you for doing this.

Embraer's chief test pilot, Gilberto Schittini will

Embraer's chief test pilot, Gilberto Schittini will

fly an Embraer 120 that has been modified to recreate

the failure of Flight 2311.

the failure of Flight 2311.

So we've modified the quill.

The teeth have been worn down just like Flight 2311.

The teeth have been worn down just like Flight 2311.

A worn quill is placed inside the propeller unit.

A worn quill is placed inside the propeller unit.

This was potentially very high risk.

Because once we disconnected the transfer tube in flight,

Because once we disconnected the transfer tube in flight,

the pilots would have no way to control the propeller.

We put a pitch lock here.

We put a pitch lock here.

It won't go past 22 degrees.

A mechanical lock has also

been added to stop the propeller from going

been added to stop the propeller from going

flatter than 22 degrees, not 3 degrees as happened

flatter than 22 degrees, not 3 degrees as happened

on Flight 2311.

It would be too dangerous in a flight test

to have the propeller blade go all the way to flat pitch.

to have the propeller blade go all the way to flat pitch.

You'd lose control of the airplane.

That was almost guaranteed.

EMC 120, do you copy?

What if this airplane crashes?

What if we lose the airplane?

What if we lose the airplane?

I'm the one who's basically running this test.

This could be all my responsibility.

This could be all my responsibility.

Copy.

We are ready to disengage the prop.

Propeller blade angle's causing no problems.

Propeller blade angle's causing no problems.

No control issues.

Then the Brazilian test

flight takes a dramatic turn.

As the flight continues, the blades

begin drifting toward the deadly flat position.

begin drifting toward the deadly flat position.

Reducing speed.

Reducing speed.

Easy does it.

We started feeling a rolling moment to the left and even

more movement to the left.

more movement to the left.

The blades go as flat as this test will allow.

For Haueter, the risky test flight has paid off.

The investigator's theory about the crash

is back on solid ground.

is back on solid ground.

Seeing the data right then, it took a load off.

Said, wow.

I mean, we now know what happened.

It was obvious.

It was obvious.

Investigators finally understand the full story

Investigators finally understand the full story

behind the crash of Flight 2311.

When the flight crew began preparations for landing,

When the flight crew began preparations for landing,

the teeth on the quill were worn down but still operational.

Slowing for approach speed.

Slowing for approach speed.

Preparing for landing put

renewed pressure on the already worn teeth in the quill.

renewed pressure on the already worn teeth in the quill.

The teeth could no longer stay locked on the left propeller

The teeth could no longer stay locked on the left propeller

mechanism.

And once the teeth gave way, the propeller blades

were free to drift into a fatally flat position.

were free to drift into a fatally flat position.

What's going on with this thing?

I can't hold it.

I can't hold it.

The propeller's design couldn't hold the blades

at a safe angle, and the plane became

less and less controllable.

Human error is almost always underneath the causes

Human error is almost always underneath the causes

of an accident, even if it wasn't the pilot or a mechanic.

In this particular case, an engineering change was made,

which well-intentioned actually, did not work out.

which well-intentioned actually, did not work out.

That's it.

Oh, God.

No!

The worn quill teeth was a time bomb waiting to go off.

The worn quill teeth was a time bomb waiting to go off.

After the accident, additional safeguards

are added to prevent this type of failure, a change that

are added to prevent this type of failure, a change that

affects not only Embraer but several other turboprop

affects not only Embraer but several other turboprop

aircraft.

Airline manufacturers are constantly

Airline manufacturers are constantly

trying to stay ahead of potentially fatal design flaws.

trying to stay ahead of potentially fatal design flaws.

Air crash investigators are determined

to make sure they do.

What's important is getting to the truth.

What's important is getting to the truth.

So if the truth is something that may hurt the company,

So if the truth is something that may hurt the company,

let the chips fall where they may.