All language subtitles for Air.Disasters.S18E23.Fatal.Flaw.1080p.SMIT.WEB-DL.AAC2.0.H.264-maldini_track3_[eng]

[ominous music]

NARRATOR: 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.

NARRATOR: 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.

NARRATOR: Constant
checks keep planes

flying and passengers safe.

A single oversight
can end in tragedy.

MAN (ON RADIO): Mayday!

Something exploded.

WOMAN: Oh, my god!

[screams]

Help me.

Help me hold it.

Hold me hold it!

[crash]

FLIGHT ATTENDANT:
Ladies and gentlemen, we

are starting on our approach.

We lost both engines.

[radio chatter]

MAN (ON RADIO): Mayday.

Mayday.

WOMAN: Brace for impact!

[radio chatter]

MAN: He's gonna crash!

[somber music]

NARRATOR: A hot summer
night in Phoenix, Arizona--

it's 11:00 PM, but
the maintenance

workers at Southwest Airlines
are just getting started.

Tonight they're going to
open up a Boeing 737-700.

Almost 40 inspectors
and mechanics

are going to spend
the night making

sure the plane is fit to fly.

GREG FEITH: 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.

NARRATOR: To operate
in peak condition,

an airplane will get more health
checks than most passengers.

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 the B and C checks.

Tonight workers are
performing a C check.

From start to finish, it can
require hundreds of man hours.

It all has to get done
tonight so the plane

can fly 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.

GREG FEITH: 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.

NARRATOR: It's a lesson
the aviation industry

has learned the hard way.

January 31, 2000.

[suspenseful music]

On board Alaska
Airlines Flight 261,

the situation is desperate.

Operating a damaged
plane, the captain

is trying to land at Los
Angeles International Airport.

But the aircraft is not
responding to controls.

The MD-83 is plunging
toward the Pacific Ocean.

Other pilots flying nearby
report the nightmare scene back

to LAX air traffic control.

[beeping]

Push the blue side up!

Here we go.

[beeping]

[crash]

[somber music]

NARRATOR: Flight 261 crashed
off the coast of California

at almost 250 miles an hour.

All 88 passengers
and crew are killed.

Investigators from the National
Transportation Safety Board

quickly get to work.

The cockpit voice recordings
provide some early 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.

There's something
was wrong back there.

NARRATOR: 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
jacks screw on the tail

moves the stabilizer
up and down.

When investigators recovered
the tail from the crash site,

they make a puzzling discovery.

The jack screw wasn't
mated with the knots

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 jack screw was.

To have a screw separate
itself from a nut with very

thick threads surprised us.

NARRATOR: Without
the jack screw,

the stabilizer was
beyond control.

Without the stabilizer,
the plane was doomed.

Investigators very
quickly figure

out how the accident happened.

Next, they want to know why.

And soon they find another clue.

There was no lubrication or
visible grease on the working

area of the screw.

That was surprising and strange.

NARRATOR: The Federal
Aviation Administration

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

At Alaska Airlines, the jack
screws 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're under
tremendous pressure

to cut corners to
keep planes flying.

RICHARD RODRIGUEZ:
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.

NARRATOR: To survive an
economic recession in the 1990s,

Alaska Airlines slashed
its maintenance regime.

With air carriers, especially
those that may be economically

strapped, they're gonna
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 gonna 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 gonna operate

at the highest levels
of safety, I'm gonna

typically exceed the minimums.

It's gonna cost more,
but I'm gonna 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 gonna do.

NARRATOR: Jack screws
in the company's fleet

had been inspected every
5 to 700 flight hours.

But in 1996, to cut
costs, Alaska Airlines

began checking the jack
screws 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 out 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.

NARRATOR: Proper
maintenance becomes

even more critical
for components

with zero redundancy.

On the MD-83, there
was no alternative

if the jack screw 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.

GREG FEITH: 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.

NARRATOR: To stay
afloat financially,

Alaska Airlines put
countless lives at risk.

But disaster can erupt even
when an airline doesn't

cut back on maintenance.

Gonna hit the mountain!

Higher!

Keep trying!

[music playing]

NARRATOR: 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 undergo.

We work overnight, 'cause
that's when nobody flies.

It's better for the airline
to keep the airplane on ground

overnight to fix 'em up.

NARRATOR: Tonight
they're scheduled

to make 339 individual
inspections,

each one tracked by computer.

The yellow entries are
unscheduled procedures-- items

that have just been spotted.

Unscheduled maintenance are
those kinds of things typically

that people will 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.

NARRATOR: Tonight,
the inspectors

discover a worn tire on one
of the main landing gears.

They add it to the list of
unscheduled maintenance items.

They'll replace it 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 to make sure
everything's in working order

so the people get to
where they need to go.

NARRATOR: But sometimes,
despite all the maintenance,

the worst case
scenario comes true.

A simple repair can
unexpectedly lead to disaster.

[ominous music]

August 13, 1985,
Mount Osutaka, Japan.

This is the wreckage from
the deadliest single plane

disaster in aviation history--

the crash of JAL
123, and the loss

of 520 passengers and crew.

Amazingly, four people survived.

Because the 747 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.

[chatter]

NARRATOR: 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?

That would explain everything.

NARRATOR: 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.

Now the pressure bulkhead
becomes a prime suspect

for investigators.

They find a piece
of the panel that

had been spliced into the
bulkhead seven years earlier.

It shows a fault in
the bulkhead repair.

RON SCHLEEDE: The repair had, in
fact, not been done correctly.

There was only one row of rivets
holding that joint together

where there should have
been two rows of rivets

holding the joint together.

NARRATOR: With a
single row of rivets

holding the new
panel in place, this

was a disaster
waiting to happen,

especially for such a busy jet.

RON SCHLEEDE: 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.

NARRATOR: 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 flights.

On 747s, the cabin
is pressurized

but the tail is not.

During flight, the pressurized
air presses against the tail

bulkhead.

Over the course of
some 12,000 flights,

this pressure has
stretched the faulty repair

to the breaking point.

[intense music]

Then the cabin's pressurized
air that into the hollow tail

fin and blew it off.

Flap up!

Flap up!

NARRATOR: The loss
of the tail crippled

the plane's hydraulic systems.

RON SCHLEEDE: The Boeing 747
had four independent hydraulic

systems to power its systems.

So it had quadruple redundancy.

Unfortunately, these
four lines came together

on the lower part of the spar.

And when it separated, it
sheared those four lines.

All four hydraulic
systems were depleted.

Both hands!

NARRATOR: 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.

Brace it off!

NARRATOR: Despite
their heroic efforts,

it was a losing battle.

Brake loose!

[somber music]

NARRATOR: All this
death and destruction

from a missing row of rivets
that had gone undetected.

Two decades after
JAL Flight 123,

airlines are constantly
looking for hidden flaws.

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 blades,

the rotor blades.

We're looking for missing
material off of 'em,

you know, any hot
spots that have

worn through the metal, cracks,
radial and axial cracks.

NARRATOR: 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've got your lights.

NARRATOR: August 21, 1995.

Atlantic Southeast Airlines
Flight 529, an Embraer

Brasilia, is about to take
off for Gulfport, Mississippi

with 29 people on board.

It was, at the
time, the fastest,

sleekest turboprop around.

NARRATOR: Before the plane even
reaches its cruising altitude,

something seems to
explode outside.

[explosion]

AUTOMATED VOICE: Autopilot.

Engine control.

Oil.

MATT WARMERDAM: 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.

And they were immediately
lurched to the left.

NARRATOR: No matter what
the flight crew try to do,

the plane pulls
violently to the left.

AUTOMATED VOICE: Autopilot.

Engine control.

Help me hold it.

Help me hold it.

Help me hold it.

Over there.

[beeping]

NARRATOR: 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!

[crash]

NARRATOR: ASA Flight 529
crashes on a farmer's field.

[intense music]

NARRATOR: Near
Carrollton, Georgia,

a resident sounds the
alarm about Flight 529.

[phone dialing]

911 OPERATOR (ON
PHONE): Emergency.

WOMAN (ON PHONE): Yes.

We have a plane crashed
in our backyard.

911 OPERATOR (ON
PHONE): A plane crashed?

NARRATOR: All 49 aboard
survived the crash landing.

[crying]

NARRATOR: But 10
passengers eventually

succumbed to their injuries.

[sirens wailing]

[radio chatter]

Called into action, the NTSB
creates teams to examine

various parts of the plane.

[chatter]

Jim Hookey, an
aerospace engineer,

heads the propeller
maintenance group.

We came along a lot of pieces
of the wing, came along the--

the propeller assembly that was
missing one part of the blade.

NARRATOR: 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.

NARRATOR: Hookey had
good reason to focus

on the broken propeller blade.

17 months before Flight
529, identical blades

made by Hamilton
Standard broke on flights

over Canada and over Brazil.

In both cases, the aircraft
managed to land safely.

Hookey and his team start
combing through Hamilton

Standard's maintenance records.

JIM HOOKEY: We found out
that that propeller blade had

actually been removed
from service once

already for a crack indication.

And that became the
first clue about there

may be a problem
with that propeller

blade and those inspections.

NARRATOR: Inside the
propeller, investigators

find what they're looking for.

In the hollow interior,
or taper bore,

weights are inserted
to balance the prop.

They are kept in place by cork.

The simple cork was the trigger
in a deadly chain of events.

JIM HOOKEY: 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.

NARRATOR: Moisture
inside the propeller

caused the chlorine
in the cork to leach

out and corrode the aluminum
alloy of the propeller.

And there's something else.

On the inner surface, extending
about an inch and a half

from the fracture, investigators
find sanding marks.

Going through the
blade's repair records,

Hookey notices the
initials CSB, Christopher

Scott Bender, a
technician at a Hamilton

Standard repair facility.

When Christopher binder
watches news of the accident,

he learns that the
investigators are examining

the Hamilton propeller.

CHRIS BENDER: As soon as I
heard that, my heart just sank.

And 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 something
I had put my hands on,

the procedure somebody
trusted me to do failed.

And because of that,
somebody had died.

NARRATOR: Investigators
asked 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 fiber optic

bore scope and look for cracks.

And therein lied one of
the primary problems.

The bore scope that he was
using had a bright white light

that would put a lot of glare
back into the inspector's eyes.

Really did not lend
itself to the inspection

that was required.

NARRATOR: When he examined
the propeller blade,

Bender did not detect any
cracks or evidence of corrosion.

He then did what he'd
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.

NARRATOR: The NTSB
finds that by polishing

the blade Hamilton Standard
unwittingly removed

all traces of the crack.

If anomalous--

NARRATOR: Even a later,
more thorough ultrasound

examination did not detect it.

--unlimited life
with no inspection.

NARRATOR: The company
that manufactured

Flight 529's propeller is now
part of United Technologies.

Its inspection
and repair process

was made more stringent,
and in some cases

exceeding FAA requirements.

[beeping]

Flight 529 was the last
time one of its propellers

failed in flight.

Out of the thousands of parts
on board and Embraer Brasilia,

a small cork was the key
to a horrific accident.

PSA Flight 529 underlines
the critical need

for proper maintenance.

[crash]

[music playing]

At Southwest in Arizona,
maintenance work continues

into the early morning.

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 newer 737s.

But the company's very first 300
model, bought in the mid-1980s,

is still flying.

GREG FEITH: 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.

NARRATOR: Updating older
planes is a standard practice.

But sometimes installing a new
component in an older plane

can lead to tragedy.

Inside this hangar are
the shattered remains

of Swissair Flight 111.

On September 2, 1998,
the passenger jet

crashed off the coast
of Nova Scotia, Canada,

killing everyone on board.

The amount of debris
recovered from the seabed

is overwhelming.

There is more than 150
miles of wiring alone.

In Swissair we had about 2
million pieces of airplane.

And we pretty much almost
had to look at 'em all.

In the business
we refer to often

finding the golden nugget.

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

NARRATOR: Somewhere in this
wreckage investigators hope

to find that golden nugget--

the one piece that
will reveal the reason

why Swissair 111 crashed
into the Atlantic Ocean.

[crash]

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.

NARRATOR: The first officer
checks the area around the air

conditioning vent.

He finds nothing wrong.

I don't see any
thing there, and

there's nothing up there now.

NARRATOR: Captain Zimmerman is
troubled by the smell of smoke.

There it is again.

NARRATOR: He starts
to divert the plane

to the nearest airport.

Find the closest
place to land, Stephan.

NARRATOR: He radios
air traffic control

in Moncton, New Brunswick.

PILOT: Moncton
Center, Swissair 111

heavy is declaring PAN-PAN-PAN.

We have smoke in the cockpit.

NARRATOR: PAN-PAN-PAN
is an international term

used to notify air traffic
control of an urgent situation.

It is one step below
declaring mayday.

I guess Boston.

We need--

NARRATOR: Swissair 111
is directed to Halifax

and starts its descent.

OK.

Can I vector you to set up
for runway 06 at Halifax?

NARRATOR: 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.

NARRATOR: But the seemingly
controlled situation

on board Flight 111--

Turn to the south.

NARRATOR: --escalates into
a full-scale emergency.

[sirens sounding]

Autopilot disconnect!

[intense music]

We are declaring emergency now,
Swissair 111, at time 0124.

All my screens are down.

I'm flying on standby
instruments, maintaining 300.

NARRATOR: Shortly after
declaring an emergency,

the plane goes silent.

[beeping]

[ominous music]

BILL PICKRELL: 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.

NARRATOR: At 10:31 PM Atlantic
time, residents of Peggy's Cove

hear a devastating explosion.

NARRATOR: Investigators
work their way

through the wreckage
of Swissair 111,

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--

Dumping fuel now!

NARRATOR: --directly
behind the first officer.

Following this trail leads the
team to an unlikely suspect--

the entertainment
system in first class.

The Swissair
MD-111's first class

entertainment system was
among the most sophisticated

in the world.

Passengers could choose
their own movies,

access the internet,
and even gamble.

This system was not part of
the original MD-11 design.

Any time you have
an electrical system

or you're putting an aftermarket
install into an airplane,

you run the risk of compromising
the integrity of the aircraft

itself as it was
originally designed.

NARRATOR: When informed
about the system's flaws,

Swissair immediately disabled
it on the rest of its fleet.

Meanwhile,
investigators find out

why the fire spread so quickly.

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

metallized polyethylene
terephthalate--

covering material that covers
the insulation blankets.

NARRATOR: This
polyethylene insulated

was commonly used by commercial
airlines around the world.

It had somehow
passed the industry's

flammability tests,
which require materials

to self-extinguish quickly.

VIC GERDEN: This thermal
acoustical material

that was in this aircraft
was very flammable.

Even though it passed
a test, it does sustain

and it does propagate flame.

NARRATOR: The fire spread
quickly from the cockpit back

into the first class galleys.

Less than 12 minutes
after the crew

declared a PAN-PAN-PAN,
the fire disabled

all electronics in the cockpit.

[crash]

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.

They're now testing
the plane's rudder.

One of the jet's most
vital control surfaces,

the rudder allows a plane
to turn left and right.

OK, Seabass,
rudder should turn.

MAN (ON RADIO): All
right, go ahead.

NARRATOR: A problem here could
have terrifying consequences.

MAN (ON RADIO): You're
clear on the left.

NARRATOR: In fact, despite
years of proper maintenance,

a problem with a
component in a 737 rudder

killed more than 100 people.

Not even the most
diligent maintenance

workers could have spotted it.

March 3, 1991.

United Flight 585 begins
its final approach

into Colorado Springs.

Another 10-knot gain.

30 flaps.

[alarm sounding]

Oh, god!

Give me 15 flaps.

15.

Go up.

Oh, my god.

Oh, my god!

[somber music]

NARRATOR: All 20 passengers and
five crew members are killed.

[radio chatter]

Investigators from the National
Transportation Safety Board

descend on Colorado Springs.

[radio chatter]

My first sense
that it was going

to take some time to
investigate the accident was--

was the damage that
we saw in the parts.

NARRATOR: 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.

NARRATOR: 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--

the rudder's 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 a hydraulic fluid

that moves the rudder.

When a technician
opens up the PCU,

the valve 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.

NARRATOR: Less than two
years later, Greg Phillips

and the NTSB will revisit
the mysterious disaster

after the crash of another 737.

Hold on.

Hold on.

Hold on.

Shoot!

What the hell is this?

Oh, god no!

[screaming]

NARRATOR: September 8, 1994--

US Air Flight 427
crashes near Pittsburgh,

killing everyone on board.

Investigators learn 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 servo valve

of the rudder's PCU.

They come up
empty-handed again--

until almost two years later
they get a third chance

to solve this deadly mystery.

NARRATOR: In the early
1990s, two 737s crashed

in mysterious accidents.

In both cases, the jets
spiraled out of control.

In 1996, the 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.

[beeping]

BRIAN BISHOP: I turned the
yoke the opposite direction

and stood on the
opposite rudder pedal.

The pedal didn't move for me.

We didn't to what
extent, but we knew we

had a problem with the rudder.

NARRATOR: For more
than 30 seconds,

Bishop struggles to
control his renegade plane.

And then, just as suddenly,
the 737 calms down

and goes back to horizontal.

BRIAN BISHOP: We had
started the checklist.

Almost before I could finish
the sentence, all of a sudden

there was just a wham.

[rumble]

NARRATOR: The 737 is once
again out of control.

[intense music]

Then, out of the blue,
it's back on track.

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

BRIAN BISHOP:
Taxiing in is when I

realized my legs were shaking.

I launched to the scene.

The airplane
literally didn't move.

It stayed in its
location in 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.

NARRATOR: 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.

NARRATOR: The power control
unit is soaked in dry ice

and blasted with nitrogen gas
at minus 40 degrees Celsius.

Then it's injected with
superheated hydraulic fluid.

It's then given a
command to start working.

[clicking]

JOHN COX: 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.

It stopped working completely.

NARRATOR: When investigators
double check their results,

they discover
another major flaw.

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 into places

that it shouldn't have gone.

TOM HAUETER: And the reversal
is like driving your car.

You turn it 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--
the pilots were faced with

something so unusual that
they didn't understand

what was happening.

NARRATOR: 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 rudders' servo

valves 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.

NARRATOR: It's 7:00 AM.

After an 8-hour shift, the
maintenance is finished

on the Southwest Airlines 737.

According to their
maintenance reports,

the team has conducted
78 unscheduled procedures

and 339 scheduled inspections.

Despite the horror of
airplane disasters,

they are still extremely rare
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.

NARRATOR: This system depends
on the dedicated teams

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.