All language subtitles for Mayday.S06E02.Fatal.Flaw.1080p.AMZN.WEB-DL.DD+2.0.H.264-playWEB_track3_[eng]

In the ocean
near Los Angeles...

on a lonely mountain
in Japan...

off the East coast
of Canada...

a plane crash can reduce

an enormous jet plane
to mangled pieces.

The cause is buried
somewhere in the wreckage.

In the business, we refer to,

often, "finding
the golden nugget."

A modern jet plane
is made up of hundreds

of thousands of parts.

A failure in any one of them
can lead to disaster.

A missing screw can jeopardize
the safety of flight.

Constant checks
keep planes flying,

and passengers safe.

A single oversight can
end in tragedy.

Mayday!

Something exploded.

Oh, my God!

Oh, my God!

Help me,
help me hold it.
Help me hold it!

Mayday, mayday.

A hot summer night
in Phoenix, Arizona.

It's eleven o'clock,

but the maintenance workers
at Southwest Airlines

are just getting started.

Tonight, they're going
to open up

a state-of-the-art
Boeing 737-700.

Almost 40 inspectors
and mechanics are going

to spend the night making sure
the plane is fit to fly.

Without proper maintenance,
airplanes don't fly.

Pilots are usually the focus

for the operation
of the airplane.

But maintenance has
an equally high priority role

in the safe operation
of any aircraft.

To keep airplanes
in peak condition,

they get more health checks
than most passengers.

It's a very intricately
weaved web

between the operation
of the airplane

and the maintenance
of the airplane

and the management
of the airplane.

Passenger planes
are examined

every time they come
to a stop.

This is the A-check.

A brief walk-around inspection
turns up

the most obvious problems.

The more intensive work is
done at set intervals.

These are the B and C checks.

Tonight, workers are
performing a C-check.

From start to finish,
it can require

hundreds of man-hours of work.

It all has
to get finished tonight,

so the plane is
back flying in the morning.

It's a massive challenge

because modern jets are made

of hundreds of thousands
of individual pieces.

In 1903,

when the Wright Brothers took
their historic first flight

near Kitty Hawk,
North Carolina,

their plane had
some 1,500 parts.

A 737 has more than 360,000.

You have to ensure that
every one of those components

is doing its respective job.

It doesn't matter
how big the part is.

A missing screw can jeopardize
the safety of flight.

It's a lesson
the aviation industry has

learned the hard way.

January 31st, 2000.

On board Alaska Airlines
flight 261,

the situation is desperate.

Operating a damaged plane,

the captain is trying to land
at Los Angeles airport.

But the aircraft is not
responding to controls.

The MD-83 is plunging
towards the Pacific Ocean.

Holy!

Other pilots
flying nearby report

the nightmare scene back
to L.A. Air Traffic Control.

Push the blue sign up!

Here we go!

Flight 261 crashed
off the coast of California

at over 400 kilometers
an hour.

All 88 passengers
and crew are killed.

Investigators from
the National Transportation
Safety Board

begin their work quickly.

The cockpit voice recordings
provide some
of the earliest clues.

We have a jammed stabilizer

and we're maintaining altitude
with difficulty.

We immediately suspected

some problem in the tail
of the airplane,

which is where
the controls are.

Something was
wrong back there.

Investigators examine

the MD-83's
horizontal stabilizer.

The stabilizer controls
the plane's pitch,

its ability to tilt
up and down.

As the stabilizer moves up,

the plane's nose tilts down.

As the stabilizer moves down,

the nose moves up.

In the MD-83,

a motorized jackscrew
on the tail moves

the stabilizer up and down.

When investigators recover
the tail from the crash site,

they make
a puzzling discovery.

The jackscrew wasn't mated

with the nut
that it screws into.

It was just by itself.

And the nut was found
in another piece of structure

a few feet away
from where the jackscrew was.

To have a screw
separate itself from a nut

with very thick threads
surprised us.

Without the jackscrew,

the stabilizer was
beyond control.

Without the stabilizer,
the plane was doomed.

The investigators
very quickly figured out

how the accident happened.

Now they want to know why.

The answer is tragically simple.

There was no lubrication
or visible grease

on the working area
of the screw.

That was surprising
and strange.

The Federal
Aviation Administration orders

an immediate check
on all MD-80s in the USA.

At Alaska Airlines,

the jackscrews on six
of its fleet

of 34 MD-80s fail inspection.

Investigators discover
even more alarming evidence

as they go through
the carrier's
maintenance records.

Mechanics at Alaska Airlines
report that they are

under tremendous pressure

to cut corners
to keep the planes flying.

We interviewed
all the mechanics

who had worked
on these airplanes.

We knew that they had
been falsifying records

or not doing the work
they had indicated.

To survive
an economic recession

in the 1990s,

Alaska Airlines slashed
their maintenance regime.

With air carriers,

especially those that may
be economically strapped,

they're going to stretch
inspection cycles

to the maximum.

The FARs, the Federal
Aviation Regulations,

set a minimum level of safety.

Now if you're going
to operate on a shoestring,

you're only going to meet
that minimum level of safety.

If I'm a good carrier,
or I want to be a good carrier

and I want to show
that we're going to operate

at the highest levels
of safety,

I'm going to typically
exceed the minimums.

It's going to cost more,
but I'm going to exceed it.

A lot of companies that say,

"Wait, the regulations
only say

"I only have to go to here.
That's what I'm going to do."

Jackscrews in
the company's fleet had been

inspected every 500
to 700 flight hours.

But in 1996, to cut costs,

Alaska Airlines began
checking the jackscrews

every 2,500 hours.

At the same time, they doubled

the average daily use
of their fleet.

If you had 600 hours
between inspection points

and greasing points,

we have no chance
of ever having

a metal-to-metal
contact situation.

But if you put that up
to 2,000 hours
or 2,500 hours,

now what you do is eat into

some of these
protective stages,

these barriers that we have
towards catastrophic failure.

Proper maintenance
becomes even more critical

when there is no backup
to a component.

On the MD-83,
there was no alternative

if the jackscrew failed.

So proper maintenance was
a matter of life and death.

But in the aviation industry,

it's also a matter
of dollars and cents.

There's a lot of pressure
in the airline industry,

when you look at it,
whether you're hauling boxes

or hauling people.

The fact of the matter is, is
that competition is stiff

and how do you get
the competitive advantage

against the next guy?

How am I gonna
get more for less?

And a lot of times,
it's labor.

The other times,
it's maintenance.

If I can stretch
the inspection

to 500 hours
instead of 400 hours,

that saves me a lot of money.

To stay afloat financially,

Alaska Airlines put
countless lives at risk.

But disaster can erupt
even when an airline doesn't

cut back
on its maintenance regime.

We're going
to hit the mountain!

Keep trying!

It's past midnight
in Phoenix, Arizona.

A maintenance crew works
through a 737-700.

They're performing
a so-called C-check,

one of the most
detailed inspections

any plane can go through.

We work overnight because
that's when nobody flies.

It's better for the airline
to keep the airplanes
on ground overnight

to fix them up.

Tonight, 339
individual inspections are

set to be made.

Each one of these is
tracked by computer.

Anything that comes up yellow
is an unscheduled procedure,

a problem that's
just been spotted.

Unscheduled maintenance are
those kinds of things

typically that people
experience with their car

where they're driving
down the highway

and, all of a sudden,
the air conditioner
doesn't work.

Well, the same
with an airplane.

Tonight,
the inspectors discover

a tire on one of
the main landing gears
is worn out.

They add it the list
of unscheduled
maintenance items.

It has to be replaced

before the plane goes
back into service.

Obviously, the stakes
are extremely high.

Every night, we come to work

and try to do
our best job possible,

make sure everything's
in working order

so people get
to where they need to go.

But sometimes,
despite all the maintenance,

the worst-case scenario
comes true.

A simple repair can
unexpectedly lead to disaster.

August 13th, 1985.
Mount Osutaka, Japan.

This is the wreckage
from the deadliest
single plane disaster

in aviation history.

JAL Flight 123 crashed
the night before,

killing 520 passengers
and crew.

Only four people survived.

Because the 747 jet was
built in the United States,

the National Transportation
Safety Board joins

the investigation.

When I arrived in Tokyo,

the atmosphere in Japan was
extremely stressful.

The news media
were everywhere.

There was a tremendous
amount of anger.

Soon after the crash,

experts get a helping hand
from an amateur photographer.

He managed to take a picture

of the 747
minutes before it crashed.

The picture reveals
that JAL Flight 123 was

flying without
its massive tail fin.

The tail fin houses
critical control surfaces

like the rudder,

as well as tubes that carry
the hydraulic fluids.

What force could be
strong enough

to tear off the tail fin?

Digging through
the 747's maintenance history,

investigators discover that,
seven years earlier,

the jet had landed
with its nose too high.

The tail hit the ground
and scraped along the runway.

The rear part of the plane
had to be repaired,

including the pressure bulkhead.

Japan Airlines called in
Boeing technicians

to help repair
the cracked bulkhead.

After this
unscheduled maintenance,

the 747 was given
a clean bill of health

and flew
for another seven years.

But this bulkhead becomes

a prime suspect
for the investigators.

We had an idea
that we wanted to find

the rear pressure bulkhead,

because we had
a flight attendant

who had been interviewed,
that described an explosion

in the back of the airplane
and she could see out.

So we wanted to focus
on the bulkhead.

During his investigation,

Schleede finds
a piece of the panel

that had been spliced
into the bulkhead

seven years before.

The mystery
of Flight 123 is solved.

The 747 went down
because of a faulty repair.

The repair had,
in fact, not been
done correctly.

There was only
one row of rivets

holding that joint together,

uh, where there should
have been two rows

of rivets holding
the joint together.

With only one row
of rivets straining to hold

the repaired panel in place,

this was a disaster
waiting to happen,

especially because this
was such a busy jet.

This particular airplane was

used in Japan
on a domestic operation.

So, it made
multiple takeoffs and landings

on domestic operations,

unlike most 747s
that make long-range hauls.

So, this was considered
a high cycle airplane.

Investigators calculate

that, with the repair job,
the bulkhead would survive

approximately 10,000 flights
or cycles.

But on the day of the crash,

the 747 had already racked up
over 12,000 cycles.

On 747 jets, the cabin is
pressurized, but not the tail.

During flight,
the pressurized cabin air
presses against

the repaired bulkhead.

After some 12,000 cycles,

this pressure stretched
the faulty repair

to breaking point.

The highly-pressurized air

blasted into
the hollow tail fin

and blew it off.

Flap up! Flap up!

Losing part
of the tail crippled

the plane's hydraulic systems.

The Boeing 747 had

four independent
hydraulic systems

to power its systems.

So it had
quadruple redundancy.

Unfortunately,
these four lines
came together

in the lower part of the spar,

and when it separated,
it sheared those four lines.

All four hydraulic systems
were depleted.

Both hands!

For some 30 minutes,

the crew tried to fly
their 747 using only thrust.

This is like trying to drive

a car using
only the accelerator,

no steering wheel, no brakes.

Despite their heroic efforts,

it was a losing battle.

All this death
and destruction boils down

to a missing row of rivets.

Why had
the growing metal fatigue

in the bulkhead
remained undetected

through seven years
of scheduled maintenance

and inspections?

The primary inspection method
for the bulkhead area

and the seams was
a visual inspection.

And at heavy maintenance
periods when they...

they would take
the insulation out,

uh, off the walls
and everything
and off the bulkhead,

they would do
a detailed visual inspection.

And during
subsequent maintenance checks,

the faulty repair was
never found.

Two decades
after JAL Flight 123,

airlines are constantly
looking for hidden flaws

that aren't visible
from the outside.

Back at the Southwest
maintenance hangar,

inspectors are
using a borescope,

a tiny flexible camera,
to inspect the engines.

Engines are the heart
of passenger planes.

If they stop working,
pilots don't have the option

of pulling over
to the side of the road.

Yeah, there we go.

In this area,
we're looking for cracks.

Looking at the, uh, blades,
the rotor blades,

and we're looking
for missing material
off of them.

You know, any hot spots

that have worn
through the metal cracks,

radial and axial cracks.

Any kind of crack
or trace of metal fatigue

in any of the fan blades
could spell disaster.

- Take-off, check.
Below the line.
- I'll clear your lights.

August 21st, 1995.

Atlantic Southeast Airlines
Flight 529,

an Embraer Brasilia,
is about to take off

with 29 people on board.

It's bound
for Gulfport, Mississippi.

It was, at the time,

the fastest, sleekest
turbo-prop around.

Before the plane
even reaches

its cruising altitude,

something seems
to explode outside.

Autopilot, engine control...

The sound of that
was tremendous.

It was as if someone
had taken a baseball bat

and hit
an aluminum garbage can
as hard as they could.

It was just a...
gigantic crashing sound.

The airplane immediately
lurched to the left.

No matter what
the flight crew tries to do,

the plane pulls violently
to the left.

Autopilot. Engine control.

Help me hold it.
Help me hold it!

Over there!

The captain
and co-pilot are pushed

to the brink
of their experience.

Help me. Help me!

Help me hold it.
Help me hold it!

Atlantic Southeast
Flight 529 crashes

near the small
farming community

of Carrollton, Georgia.

Emergency.
Yes,

we have a plane crashed
in our backyard.

A plane crash?

All 29 people
survive the violent landing.

But ten passengers
eventually die
from their injuries.

Called into action,
the NTSB creates teams

to examine various parts
of the plane.

Jim Hookey,
an aerospace engineer,

is in charge of the Propeller
Maintenance Group.

We came along a lot of pieces
of the wing,

came along the, um...

the propeller assembly

that was missing
one part of the blade.

The blade broke
in a very specific fashion,

leaving behind
all the telltale signs

of a fatigue fracture.

A fatigue fracture tends to be
a very flat fracture.

It also has
what we call beach marks

radiating out from the origin.

So you see these radiating
concentric rings

coming from the origin
of the crack.

Hookey had good reason

to focus
on the broken propeller blade.

Seventeen months
before ASA 529,

identical propeller
blades broke

on separate flights
over Canada and Brazil.

Fortunately, in both cases,

the aircraft managed
to land safely.

The manufacturer
of the propeller was
Hamilton Standard.

Hookey and his team
start combing

through Hamilton Standard's
maintenance records.

They're looking for anything
out of the ordinary.

It's
whatever's abnormal.

You really don't know
what you're looking for
until you find it.

But you just go through,

and there's a lot
of routine maintenances done,

regular inspections, A-checks,
B-checks, C-checks.

And there's
the non-routine maintenance
that's occurred.

If something is broken,
or a truck hits the airplane

or they have a bird strike
or something like that.

And it's those
that you look for.

The maintenance records reveal

that the broken propeller blade
had earlier problems.

We found out that
that propeller blade had

actually been removed
from service once already,

uh, for a crack indication.

And that became the first clue
about there may be a problem

with that propeller blade
and those inspections.

Deep inside
the hollow propeller,

investigators find
what they're looking for.

In the hollow interior,
or taper bore,

weights are inserted
to balance the prop.

They're kept in place
by a cork.

This simple cork
was the trigger

in a deadly chain of events.

About 95% of the cork
that's produced in the world

is used by
the medical industry.

And, for aesthetic purposes
and for sterilization,

they like to have
the light color.

So, the cork is
bleached with chlorine.

The NTSB discovers

that moisture
inside the propeller caused

the chlorine in the cork
to leach out

and corrode the propeller's
aluminum alloy.

They also notice
something else
on the broken blade.

On the inner surface,

extending about
four centimeters

from the fracture, there are
a series of sanding marks.

Going through
the blade's repair records,

Hookey notices
the initials "CSB".

Christopher Scott Bender.

This technician worked

at a Hamilton Standard
repair facility.

When Christopher Bender
watches news of the accident,

he learns that
the investigators

are examining
the Hamilton propeller.

As soon as I heard that,

my heart just sank.
I was like...

you know, I think I might
have even cried a little bit

'cause I was just, you know,
just emotionally overwhelmed

that, you know, something
I had put my hands on,

a procedure somebody
trusted me to do, failed.

Uh, and because of it,
somebody had died.

After discovering
that it was Bender

who last worked
on the deadly propeller blade,

the NTSB now has to find out
how the blade had

passed inspection
at Hamilton Standard.

Investigators ask Bender
to perform

his standard
maintenance technique

on the propeller.

He demonstrated how he would
go down into the barrel

of the taper bore

with a fiberoptic borescope
and look for cracks.

And therein lied one
of the primary problems.

The borescope
that he was using

had a bright white light

that would put
a lot of glare

back into
the inspector's eyes.

It really did not lend itself

to the inspection
that was required.

And investigators
also find a gap

in Bender's training.

He had never been shown
what a crack would look like.

He was just told
to find a crack,

and he would look for a crack.

When he was
examining the propeller blade,

Bender had been
unable to detect

any evidence of corrosion.

He then did what
he had been told to do,

polish the inside
of the blade.

He was given a directive
to use a repair

to blend out the inside
of the taper bore.

He blended it out,
he did an inspection.

And the blending
that he had done

had roughened the surface,

so it actually masked
the indication of the crack

in the subsequent inspection.

And the blade was
returned to service,

where the crack continued
to propagate

until it ultimately reached
critical length and separated.

The draft accident report
we present to you today

involves Atlantic
Southeast Airlines Flight 529.

According to the NTSB,

by polishing the blade,
Hamilton Standard had

unwittingly removed
all traces of the crack.

Even a later, more thorough
ultrasound examination

could not detect it.

The company that manufactured
Flight 529's propeller is

now renamed
Hamilton Sundstrand.

Its inspection
and repair process was made

more stringent, in some cases
exceeding FAA requirements.

Flight 529 was the last time

one of its propellers
failed in flight.

You know, I wish
this had never happened.

I wish I could go back
in time and fix it

and take care of it
and that it didn't happen.

Out of the thousands
of parts on board

an Embraer Brasilia,
a simple small cork was

the key to
a horrific accident.

ASA Flight 529 underlines

the critical need
for proper maintenance.

But sometimes maintenance can

create the potential
for disaster

when a new component is

installed into
an older airplane.

Moncton center, Swiss Air 111
heavy is declaring

Pan Pan Pan. We have
smoke in the cockpit.

It's early morning
in Phoenix, Arizona.

Southwest Airlines engineers
are continuing

their scheduled maintenance
of a 737.

Southwest is unique
among larger airlines.

It flies just one kind
of plane, the 737.

Tonight, engineers are
working on a 700 model,

one of the newest 737s.

But the company's
very first 300 model,

bought in the mid-1980s,
is still flying.

You can
still operate an old airplane,

as long as you have
inspection protocols.

When you look at some
of the cargo carriers,

they're operating airplanes
that are 30 and 40

and even 50 years old.

They're still
reliable airplanes.

They've been maintained,

they've been retrofitted
with modern day equipment.

Updating older planes is

a standard part
of maintenance.

But sometimes,
installing a new component

in an older plane can
lead to tragedy.

Inside this makeshift lab
are the shattered remains

of Swiss Air Flight 111.

On September 2nd, 1998,

the passenger jet crashed
off the coast

of Nova Scotia, Canada,

killing everyone on board.

Recovered from the seabed,
the debris is overwhelming.

There is almost 250 kilometers
of wiring alone.

In Swiss Air, we'd had
about two million pieces
of airplane.

And we pretty much almost
had to look at them all.

In the business, we refer

to often "finding
the golden nugget."

That's saying, "A-ha, there's
the cause of the accident."

Somewhere in this wreckage,

investigators hope to find
that golden nugget,

the one piece that will
reveal the reason

why Swiss Air 111 crashed
into the Atlantic ocean.

The cockpit
voice recorder gives

investigators their
first critical clues.

Do you smell something?

Yeah. What is that?

Go have a look.
I'll take the controls.

Roger. You have control.

The First Officer checks

the area around
the air conditioning vent.

Nothing seems wrong.

Don't see anything there.

And there's nothing
up there now.

Captain Zimmerman
is troubled

by the smell of smoke.

There it is again.

He starts
to divert the plane

to the nearest airport.

Find the closest place
to land, Stefan.

He radios
air traffic control

in Moncton, New Brunswick.

Moncton Center,
Swiss Air 111 heavy is

declaring Pan Pan Pan.
We have smoke in the cockpit.

"Pan Pan Pan" is
an international term used

to notify air traffic control
of an urgent situation.

...to fly to the threshold.

It's one step below
declaring Mayday.

Uh, I guess Boston, we need...

Swiss Air 111
is directed to Halifax

and starts its descent.

Can I vector you to set up
for runway zero-six
at Halifax?

The pilots appear
calm and in control.

Halifax is just
20 minutes away.

They want us
to turn to the south.

At that point,
everything was normal.

I gave the pilot
an initial descent

and he requested to level off

at an intermediate altitude

to get the cabin in order
for the landing,

which took to mean
that they needed

to pack away dinner trays
and things like that.

Cabin bus off.

Cabin bus off. Roger.

But the seemingly
controlled situation

on board
Swiss Air Flight 111 escalates

into a full scale emergency.

Autopilot, disconnect.

We are declaring
emergency now!

Swiss Air 111 at time 0-1-2-4!

All my screens are down!

I'm planning
on steering by instruments.

Maintaining 300!

Shortly after declaring an emergency,

the plane goes silent.

It was probably one
of the most helpless feelings

that any individual can have,

not being able to do anything

but just sit
and watch the target

and hope that it would turn
back toward the airport.

And of course, it didn't.

At 10:31 p.m. Atlantic time,

residents of Peggy's Cove hear
a devastating explosion.

From the cockpit
voice recorder,

investigators know
they are dealing with a fire

and not a plane
that was malfunctioning.

We found no anomalies

or no problems
in any of that flight data

that suggested there was
a problem with the aircraft.

Investigators work
their way through the hangar

of wreckage recovered
from the Atlantic Ocean.

Finally, they find
scorch marks,

which reveal that the source
of the fire was

in the back of the cockpit,

directly behind
the First Officer.

Following this trail
leads the team to
an unlikely suspect,

the entertainment system
in First Class.

The Swiss Air MD-11s
provided First Class

with one of the world's
most sophisticated
entertainment systems.

Passengers in first class

could choose
their own movies,

access the Internet,
and even gamble.

This entertainment system
was not part

of the original MD-11 design.

The system had
some major deficiencies.

It was getting very hot,

it drew a lot of power.

And, uh, thereby,
for example,

raising the cabin temperature

considerably, because it
was always running.

They did not install
a simple "off" switch,

nor did they install
appropriate cooling systems.

Any time you have
an electrical system

where you're putting
an after-market install

into an airplane,

you run the risk
of compromising

the integrity
of the aircraft itself

as it was originally designed.

When informed
about the flaw in the wiring,

Swiss Air immediately disabled
the entertainment system

on the rest of its fleet.

But investigators discover
that another piece

of the jet had helped
the fire spread

with alarming speed.

And in this instance,
we did discover a wire

that arced in that way.

And right next to it was
some very flammable material

called metalized polyethylene
terephthalate covering material

that covers
the insulation blankets.

This polyethylene insulate was

common on commercial airlines
around the world.

It had somehow passed

the industry's
flammability tests,

which require materials
to self-extinguish

after a reasonable period
of time.

This thermo-acoustical material

that was in this aircraft
was very flammable.

Even though it passed a test,

it does sustain
and it does propagate flame.

The fire spread
quickly from the cockpit

back into
the First Class galleys.

Some metals showed heat damage

from temperatures as high
as 1,100 degrees Fahrenheit.

Less than 12 minutes
after the crew declared

a Pan Pan,
the fire disabled

all electronics
in the cockpit.

In the aftermath,
Swissair removed

the flammable insulate
from its entire MD-11 fleet.

The rest of the industry was
required to follow suit.

In Phoenix, Arizona,
flight engineers continue

their C-check on the 737.

It includes testing
the plane's rudder.

The rudder is one of the jet's
most vital control surfaces.

It allows a plane
to turn left and right.

Okay, C-Bass,
rudder should turn.

All right, go ahead.

A problem here could
have terrifying consequences.

Uh, you're clear on the left.

In fact,
despite years
of proper maintenance,

a problem with
a tiny component

with the 737 rudder killed
more than 100 people.

Not even the most diligent
maintenance workers
could have spotted it.

March 3rd , 1991.

United Flight 585 begins

its final approach
into Colorado Springs.

Another ten-knot gain.

Thirty flaps.

Oh, God, flip!

- Fifteen flaps.
- Fifteen!

Oh, my God!

Oh, my God!

- Oh, God!
- Oh, my God!

All 20 passengers
and five crew are killed.

Investigators from
the National Transportation
Safety Board descend

on Colorado Springs.

My first sense that it was
going to take some time

to investigate
the accident was

the damage that we saw
on the parts.

An aerospace
engineer by training,

Greg Phillips is in charge

of investigating United 585's
flight control systems.

We focused in
after eliminating

other flight control surfaces

that we thought could
contribute to the roll.

We started looking
at the rudder.

But investigators
face a critical obstacle.

Most of the plane's parts
are too crushed

or burned for testing.

Luckily, one vital component
is still reasonably intact.

This is the power control unit
or PCU.

Used constantly during flight,

especially during landings,

the PCU performs like
a car's power steering.

When the pilot pushes
on a rudder pedal,

the PCU uses hydraulic fluid

to convert
the gentle movements

of a pilot's foot into
the pressure needed to move

the 737's enormous rudder.

The heart of the PCU is
the dual servo valve.

This valve is roughly
the size of a soda can.

It contains
two extremely thin slides

that glide past one another.

These slides direct
the flow of hydraulic fluid

which moves the rudder.

When a technician opens up
the power control unit,

it seems to be
in working order.

We didn't have
any absolute indication

or information that we could
point to that said

the rudder power control unit,
the servo valve or any part

of that flight control system
caused that accident.

It's a pass.

For only the fourth
time in its history,

the NTSB releases a report

that does not
reach a conclusion.

We had put
a lot of time and effort in

into the investigation

and we just weren't sure
what had happened.

Less than
two years later,

Greg Phillips and the NTSB
would be brought back

to the mysterious disaster

with the crash of another 737.

Hold on. Hold on.

- Hold on.
- Shoot!

What the hell is this?

Oh, God, no!

September 8th, 1994.

US Air Flight 427 has
crashed near Pittsburgh,

killing everyone on board.

When we first arrived
at the crash site,
first of all,

there was no aircraft there.

There were only bits
and pieces of the airplane.

It wasn't really recognizable
as an airplane.

Investigators begin
to see that this crash is

a mirror image
of United Flight 585.

On final approach,
United 585 rolled right,

while US Air 427 rolled left.

Both crews are
caught by surprise.

After a terrifying struggle,
both crash with no survivors.

Once again, investigators test
the dual servo valve.

But they come up empty-handed.

That unit passed
all its operational tests.

There wasn't any indication
that it had failed.

We were going up against
an aircraft that had
an incredible safety history.

It was really, everything
you could see for 30 years,

this had been
a great airplane.

We were trying to prove that
there was something wrong

with the straight A student.

The team reaches
another dead-end.

But almost two years later,

they get a third chance
to solve this deadly mystery.

In Phoenix, Arizona,

a long night of maintenance is
winding down.

Over 30 mechanics

and some
half-a-dozen inspectors

have combed through
a Southwest Airlines 737.

The team has made more
than 400 separate checks.

Parts have been replaced.

Southwest 737 N427WN is
almost ready to fly again.

Hopefully it'll push back
out of the hangar.

We'll do some leak checks,
make sure
everything's working,

nothing's leaking.

Then it'll go to the gate
for departure time.

But it's a truth
in maintenance

that engineers can only fix
what they know is broken.

For several years,
every 737 that flew had

a hidden danger that not even
the most careful technician
could have spotted.

In the early 1990s,

two 737s crashed
in mysterious accidents.

In both cases,
the jets spiraled
out of control.

In 1996, the same malfunction
strikes again.

It's June 9th.

Captain Brian Bishop prepares
to land in Richmond, Virginia.

Then, just like United 585
and US AIR 427,

his plane rolls
out of control.

I turned the yoke
the opposite direction

and stood
on the opposite rudder pedal.

The pedal didn't move for me.

We didn't know to what extent,

but we knew we had
a problem with the rudder.

For over 30 seconds,

Bishop struggles to control
his renegade plane.

And then, just as suddenly,

the 737 calms down
and goes back to horizontal.

We had
started the checklist.

And almost before I could
finish the sentence,

all of a sudden
there was just a 'wham!'

The 737 is
once again out of control.

Then, out of the blue,
Eastwind 517 is back on track.

Wasting no time,
Captain Bishop gets it
onto the tarmac.

Taxiing in is
when I realized
my legs were shaking.

We marched
to the scene.

The airplane
literally didn't move.

Stayed at its location
at the airport

until we got down there.

Suddenly they had a 737
that had had
a rudder incident,

that was intact,

and they had a pilot
who was alive
and who could talk about it.

I think they were much happier
to have the airplane than me.

Investigators zero in

on the 737's rudder controls.

The power control unit is
tested again and again.

But it performs perfectly.

Refusing to give up,
investigator Tom
Haueter decides

to try a different test.

One fellow mentioned a test
they had done in the military

of a thermal shock.

The power control unit
is soaked in dry ice

and blasted with nitrogen gas
at minus 40 degrees Celsius.

Then it's injected
with super-heated
hydraulic fluid.

It's then given a command
to start working.

As we were
standing there listening

to the actuator move left
and right, left and right,

it stopped, and it was not
commanded to stop.

It just jammed.

Stopped working completely.

The team
has discovered

that a small hydraulic valve
that controls the rudder

of the world's
most popular jetliner can jam

in the right circumstances.

And the valve can jam
without leaving behind
any traces.

When investigators
double-check their results,

they discover
another major flaw.

In careful analysis
of the data,

a couple
of the engineers recognized

that it not only
stopped working,

but actually left became right
and right became left.

There was actually a movement

of fluid in... into places
that it shouldn't have gone.

And the reversal is
like driving your car.

You turn to the right,
it goes left.

You're not gonna
figure out this failure mode

until you go off the road.

And in these cases,
that's what the pilots
were faced with,

something so unusual

that they didn't understand
what was happening.

What the hell is this?

That would explain why
the First Officer Chuck Emmett
would keep his foot

on the rudder pedal,

because he's thinking,

"Why isn't
the plane going right?",

and he's feeling
the plane go to the left.

In the aftermath
of these disasters,

pilots received
better training

on how to deal
with sudden rudder problems.

Boeing spent hundreds
of millions of dollars

redesigning and replacing
the rudder's dual servo valve

on thousands of 737s
around the world.

One thing we don't like
at the Safety Board is to have

an undetermined accident,

because then we can't make
a change to improve safety.

So out of US Air 427,

United 585,

we have a much
safer 737 fleet.

It's 7 a.m.

After an eight-hour shift,
the maintenance is finished

on this
Southwest Airlines 737.

According to their
maintenance reports,

the team has conducted
78 unscheduled procedures

and 339 scheduled inspections.

I think each one of us
out here has

a sense of pride
in themselves,

that we do the best job
we possibly can,
day in and day out.

Obviously,
the stakes are high.
Every life is important.

Despite the horror
of airplane disasters,

they are still extremely rare,

especially given how often
passenger planes take off
and land.

Sometimes we obscure the fact

that we fly millions
and millions

and millions of people
day in and day out,

without putting a scratch
on even the airplane,
let alone the people.

This is
the most amazing system.

This system depends
on the dedicated team

of professionals committed
to taking care

of these
21st century masterpieces,

planes so well built
that they could fly

almost as long
as we're willing
to take care of them.

We've learned now
how to inspect
and maintain these things

and even rebuild them
to where they should have
an indefinite life.

They're built tough

and they should be
able to last forever,

if they're
maintained properly.