All language subtitles for Air.Disasters.S20E10.Design.Flaws.1080p.SMIT.WEB-DL.AAC2.0.H.264-maldini_track3_[eng]

Damn it!

NARRATOR: When a passenger
jet owned by Formula One

superstar Niki Lauda
plummets to earth,

he's driven to find out why.

Where was the tail found?

I was upset that
the airplane did

something where a human
being couldn't react anymore.

NARRATOR: When two DC-10s are
brought down within two years--

What's happened?

The captain flew out!

NARRATOR: --investigators
unlock shocking discoveries.

Design flaw
issues can typically

affect the entire fleet.

Are there other ones
that are about to happen?

Get out of it!

I can't.

Come on.

NARRATOR: And a doomed
commuter flight in Georgia

comes down to a faulty design.

My god.

NARRATOR: Are three
horrific accidents

all because of deadly defects?

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.

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?

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

NARRATOR: Ascending
to cruise altitude--

OK.

NARRATOR: --Captain Thomas Welch
and first officer Josef Thurner

are flying a Boeing 767
from Bangkok, Thailand

to Vienna, Austria.

The flight is
operated by Lauda Air,

the brainchild of famed Austrian
Formula One driver Niki Lauda.

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.

NARRATOR: His new
airline is a small fleet

of just four planes.

One of the main reasons
was to give the passengers

a different way of flying--

better service, better food.

The airplane had to
look in a certain way.

NARRATOR: Lauda Air Flight 4
is less than 15 minutes into

its 11-hour flight to Vienna.

[non-english speech]

NARRATOR: In the cockpit,
Captain Welch and First Officer

Thurner monitor their
instruments while the autopilot

controls the climb.

[groaning]

NARRATOR: Suddenly, the plane
begins dropping from the sky.

[screaming]

Yeah, wait a minute.

NARRATOR: In an instant,
the pilots lose all control.

Damn it!

NARRATOR: And their jet
plunges toward the ground.

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

[screaming]

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.

They said one of our
airplanes crashed.

NARRATOR: When
rescuers arrive, it's

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

quick action from the National
Transportation Safety Board,

or NTSB.

Bob Macintosh is assigned
to the investigation.

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.

NARRATOR: He finds the badly
burned wreckage strewn over

a square mile of rough terrain.

The major components,
the cockpit, the tail,

the main body,
they were scattered

far enough apart
that certainly they

had not come down together.

NARRATOR: Macintosh
immediately knows the plane

broke up during the flight.

What he needs to uncover is why.

Where was the tail found?

NARRATOR: Airline owner Niki
Lauda arrives in Thailand

to help find the answer.

NIKI LAUDA
(VOICEOVER): The scene

for me, this was the
most horrific pictures

I've ever seen.

NARRATOR: Surrounded by
the scattered remains

of an airliner that
carries his name,

Lauda is overwhelmed by the
magnitude of the disaster.

NIKI LAUDA (VOICEOVER): When
you have 223 people killed,

the families all
want to know why.

And I think this is
the worst for people,

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

whatsoever.

NARRATOR: The black boxes may
have captured important clues

about the cause of
the in-flight breakup,

but they're badly burned.

They're sent to NTSB
technicians in Washington.

Then, days after the crash--

Well, would you look at that.

NARRATOR: --more
pieces of the plane

are found, including
one of the engines.

And investigators are
amazed by what they see.

The thrust reverser on
Flight 4's left engine

is fully deployed.

Once we actually saw that the
thrust reverser had deployed,

for me, it was shocking.

NARRATOR: Thrust reversers
are only used on landing.

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.

This discovery is the
biggest break so far,

and investigators need to know
was a deployed thrust reverser

responsible for the crash?

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

valuable tool.

The flight data
recorder is so badly

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

Thankfully, the cockpit
voice recorder has survived.

Ready to go?

NARRATOR: Investigators
and Niki Lauda

listen to the last moments
of Lauda Air Flight 4.

[radio chatter]

Check.

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

everything is normal.

Then the first sign of trouble.

[non-english] That keeps--

it's come on.

What's come on?

Some kind of warning.

NARRATOR: 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.

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.

OK.

Yeah.

NARRATOR: The crew doesn't
sound worried about the warning.

No, it's probably
moisture or something.

NARRATOR: For five
minutes, nothing happens.

Then--
- What?

Reversers deployed!

Wait a minute.

NARRATOR: --just 22 seconds
later, there's the sound

of the plane ripping to pieces.

The whole thing was so
difficult. You never forget.

NARRATOR: More motivated
than ever, Lauda sets out

to prove a controversial
theory explaining

why his plane went down.

NIKI LAUDA (VOICEOVER): I was
after Boeing day and night

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.

headquarters in
Seattle to investigate

the 767's thrust reversers.

NIKI LAUDA: It's my name,
my airplane, my crash.

And Boeing understood my
problem, or their problem.

And we kept on working together.

NARRATOR: Before the 767
first went into service,

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

of continued safe
flight and landing,

regardless of the position
of the thrust reverser.

The widespread feeling
of what would happen

if a reverser deployed in
flight was that the airplane

would be controllable.

NARRATOR: 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.

NARRATOR: 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.

OK.

Let's try it in a simulator.

NARRATOR: Niki Lauda works
with Boeing investigators

to recreate the flight.

OK, let's begin.

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

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

reverser certification.

Could the altitude difference
explain what happened?

It was incredible because the
airplane just turned around,

and you couldn't do anything.

NARRATOR: Lauda now knows
definitively why his plane

dropped so unexpectedly.

When the thrust
reverser deployed,

the smooth flow of
air over the wing

was disrupted, dramatically
decreasing lift.

Reversers deployed!

NARRATOR: This
forced the jet into

a sudden, terrifying nosedive.

It was just too late.

The aircraft was
going to assume a very

nose-low, high-speed attitude.

NARRATOR: Incredibly, Niki
Lauda has proven Boeing's

certification test was lacking.

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

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 operators of
Boeing 767s around the world

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.

NARRATOR: Now the 767's
thrust reversers undergo

intensive study at Boeing.

Engineers can imagine
only one scenario

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

The theory calls
for the activation

of the two valves in the
reverser's failsafe system

at the same time.

The first valve is the
isolation valve that controls

the flow of hydraulic fluid.

When the second valve, called
the directional control valve,

is activated, fluid
freely moves the reverser

from stowed to deployed.

You have to have both
the isolation valve open

and the directional control
valve at the engine open

to the deployed position.

NARRATOR: After
extensive testing,

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

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

NARRATOR: Investigators
consult with Boeing engineers

on the 767's infrastructure.

They discover that the
wires for both valves

were bundled in
the same harness.

A fault across several
wires in the same harness

could have triggered
simultaneous short circuits.

Two shorts in two valves
at exactly the same time?

What are the odds?

I was upset that
the airplane did

something where a human
being couldn't react anymore.

NARRATOR: Investigators
have revealed

a serious design flaw that
allows thrust reversers

to deploy accidentally.

Pressure builds for a
full review of the thrust

reverser system on all 767s.

There were other airlines
that joined in the questioning

of how could this
possibly affect

the flight path to the point
where you lose control?

NARRATOR: As a result of
the Lauda investigation,

a series of mechanical
locks now ensure

that even if both valves
get energized in flight,

the reversers can't deploy.

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.

And a lot of airplane
crashes been in the past

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.

NARRATOR: Even
when new airliners

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

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

go wrong even under the
best set of circumstances.

NARRATOR: 17 years earlier,
is a deadly design flaw--

The captain blew out.

NARRATOR: --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.

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

With all the new
passengers boarding,

Captain Nejat Berkoz and
co-pilot Oral Ulusman

are running a little
behind schedule.

So is baggage Handler
Mohammed Mahmoudi,

who's loading the luggage
of all 335 passengers.

NARRATOR: Not
expecting anymore bags,

Mahmoudi locks the
rear cargo door.

The DC-10 is set to go.

Just after 12:30
in the afternoon,

Flight 981 lifts off into
the skies above Paris.

TOWER: Tango Hotel Yankee.

91, you are cleared
to flight level 230.

91, roger.

NARRATOR: As it flies
away from the airport,

the DC-10 continues
to gain altitude.

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

[screaming]

The huge jet shudders
and begins to drop.

What happened?

The captain flew out.

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

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!

NARRATOR: In the cabin,
two rows of seats

have simply disappeared.

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!

[non-english]

[crash]

NARRATOR: Turkish Airways Flight
981 disintegrates at impact.

None of the 346 people
onboard survive.

It becomes the worst
plane crash to date.

Paul Eddy is a
journalist who covered

the story as it unfolded.

It was just a scene of
absolute, utter devastation.

I still have
nightmares about this.

NARRATOR: Investigators for the
French Accident Investigation

Bureau are on the scene.

My first job was to evaluate
the scope of the wreckage

and begin the first
investigation on the spot.

NARRATOR: Despite the
enormous force of the crash,

the black boxes survive.

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.

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

joins the investigation.

He was a very, very
professional man.

Chuck didn't sit
back in the office.

Chuck was always on the scene.

NARRATOR: 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

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

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

were able to land
their planes safely.

When the flight crew
finally saw the damage,

they were stunned by
a hole where the cargo

door would normally be.

The captain and I
walked back to the back,

and we just looked
up and saw this hole.

And it was just so weird.

NARRATOR: It didn't
take investigators long

to realize the very
design of the cargo door

carried a latent fatal flaw.

Take a picture of this here.

These don't look like--

NARRATOR: 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

push down on a
lever which drives

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

would rip the door
right off the plane.

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

pins were not in
the closed position.

Engage the lever.

NARRATOR: This meant baggage
handlers could believe the door

was locked when it wasn't.

In the Windsor incident,
there was an obvious flaw.

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.

NARRATOR: In 1972, Chuck
Miller's report strongly

recommended changes to
the DC-10's cargo door

locking mechanism be
made as soon as possible.

Two years later,
as Miller inspects

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

been ripped from its plane?

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

and that's when I
think his anger became

very, very strong indeed.

NARRATOR: Miller
takes an unusual step.

Although the official
investigation

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.

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

called Windsor, Ontario.

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

I'm Chuck Miller.

NARRATOR: --NTSB investigator
Chuck Miller shares his theory

with French investigators.

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

We asked for the report
on the Windsor accident.

And our American colleagues
were also volunteers

to give us a lot of details.

NARRATOR: The French
team is astonished

by what Miller shows them.

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.

NARRATOR: 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.

The latches that are supposed
to hold the cargo door closed

aren't locked.

What you're going
to now discover

is why wasn't that door fixed?

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

NARRATOR: In the wake of
the 1972 Windsor incident,

the NTSB had made very specific
recommendations to the Federal

Aviation Administration.

Engage the lever.

NARRATOR: Most importantly,
they suggested that a change be

made to the locking mechanism.

But in the two years
since that accident,

none of those
recommendations had

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.

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

and to come up with
recommendations as how

to prevent it happening again.

But it has absolutely no
authority to implement them.

NARRATOR: Investigators
learned that McDonnell

Douglas had made some minor
changes to the cargo doors.

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.

But sadly, those fixes just
created their own problems.

NARRATOR: 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

but not English, the
only language in which

the warning signs were written.

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

itself just two years later.

There is no question that if
an airworthiness directive had

been issued, as it should
have been after Windsor,

Paris would not have happened.

It was an entirely
avoidable accident.

NARRATOR: After the Paris crash,
foolproof changes were finally

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.

And unfortunately, we have
had to wait until we had

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

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

NARRATOR: Atlantic
Southeast Airlines Flight

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

you a pretty smooth ride.

NARRATOR: At the
controls of the Embraer

120 is Captain Mark Friedline.

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

First Officer Hank
Johnston has been

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.

Let's go 20
degrees to the right.

NARRATOR: Today's flight
is a short commuter route

from Atlanta, Georgia
to the city of Brunswick

on the Atlantic coast.

There are 20 passengers
on today's flight.

The runway's in sight.

NARRATOR: The crew is just five
minutes from touching down.

TOWER: ASA 2311.

Cleared direct to Jeff one.

Winco, report the airport sight.

Expect a visual.

We do have it in sight.

2311.

Slowing for approach speed.

The aircraft was normal.

There was nothing unexpected.

NARRATOR: Then the captain
notices an unusual sound.

It's weird.

Number one seems to
be spinning faster.

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

Bringing power down to the left.

NARRATOR: Captain
Friedline tries

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

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

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

What's going on?

Do you see anything?

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!

I can't.

Come on!

NARRATOR: The plane is
dropping out of the sky,

and the crew doesn't know why.

Come on.

No, no!

NARRATOR: --fights
desperately to save his plane.

It's no use.

[crash]

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.

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

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.

NARRATOR: At the same time,
witnesses tell investigators

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.

It was coming right
over these trees here,

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

to overcome.

What could make
it roll so far over?

NARRATOR: As investigators
scour the wreckage, searching

for clues, Ritter
knows they'll be

working without any
onboard flight recorders.

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

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

that matches that evidence.

NARRATOR: So far, the
investigators' best

clue is the steep left
roll before impact.

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,

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

NARRATOR: But when
investigators study the engines,

they find that they
were operating normally

at the moment of impact.

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,

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

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

NARRATOR: Deep inside
the propeller unit,

investigators uncover
an important clue.

Ah-hah.

There you are.

We have a witness mark.

Take a look.

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

slammed into its housing
during the crash.

It might be enough
to tell investigators

how the propellers
were operating

at the moment of impact.

You can literally match
up the scratch marks

between both pieces,
and you'll know

what the angle of
the propeller blade

was from that measurement.

It marked this one 22 degrees.

NARRATOR: The Embraer
120 is what's called

a constant speed propeller.

The blades spin at a
steady rate in flight.

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

in a car.

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

NARRATOR: The marks tell Ritter
the exact angle of the blades

when the plane slammed
into the ground.

We immediately
noticed a difference

between some of the
blade angle measurements

for the left engine
versus the right engine.

The blades were almost flat.

NARRATOR: At 3
degrees, the blades

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.

Will you look at this?

It's completely worn down.

NARRATOR: The teeth on
a key gear mechanism,

known as the quill, are
almost entirely worn away.

This is what it's
supposed to look like.

NARRATOR: With its
teeth worn away,

the quill can't effectively
lock onto the gear

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.

NARRATOR: Investigators
study the design

of the propeller mechanism.

They learn that shortly
before the accident,

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.

It turned into a giant file.

So the spines on
the transfer tube

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.

NARRATOR: Investigators
now understand why

the quill teeth were worn down.

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.

NARRATOR: --and test the
damaged propeller system

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,

we need to start
eliminating things.

NARRATOR: 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.

Thank you for doing this.

NARRATOR: Embraer's chief test
pilot, Gilberto Schittini will

fly an Embraer 120 that has
been modified to recreate

the failure of Flight 2311.

So we've modified the quill.

The teeth have been worn
down just like Flight 2311.

NARRATOR: A worn quill is placed
inside the propeller unit.

This was potentially
very high risk.

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.

It won't go past 22 degrees.

NARRATOR: A mechanical
lock has also

been added to stop the
propeller from going

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.

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?

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

This could be all
my responsibility.

Copy.

We are ready to
disengage the prop.

Propeller blade angle's
causing no problems.

No control issues.

NARRATOR: Then
the Brazilian test

flight takes a dramatic turn.

As the flight
continues, the blades

begin drifting toward
the deadly flat position.

Reducing speed.

Easy does it.

We started feeling a rolling
moment to the left and even

more movement to the left.

NARRATOR: 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.

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

Said, wow.

I mean, we now
know what happened.

It was obvious.

NARRATOR: Investigators finally
understand the full story

behind the crash of Flight 2311.

When the flight crew began
preparations for landing,

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

Slowing for approach speed.

NARRATOR: Preparing
for landing put

renewed pressure on the already
worn teeth in the quill.

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.

What's going on
with this thing?

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

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.

That's it.

Oh, God.

No!

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

NARRATOR: After the accident,
additional safeguards

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

affects not only Embraer
but several other turboprop

aircraft.

Airline manufacturers
are constantly

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.

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

let the chips fall
where they may.