Original subtitles

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!

Help me.

Help me hold it.

Hold me hold it!

FLIGHT ATTENDANT: Ladies and gentlemen, we

are starting on our approach.

We lost both engines.

MAN (ON RADIO): Mayday.

Mayday.

WOMAN: Brace for impact!

MAN: He's gonna crash!

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.

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.

Push the blue side up!

Here we go.

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!

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.

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.

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.

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!

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.

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.

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!

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

NARRATOR: Near Carrollton, Georgia,

a resident sounds the alarm about Flight 529.

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.

NARRATOR: But 10 passengers eventually

succumbed to their injuries.

Called into action, the NTSB creates teams to examine

various parts of the plane.

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.

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.

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.

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.

Autopilot disconnect!

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.

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.

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.

Oh, god!

Give me 15 flaps.

15.

Go up.

Oh, my god.

Oh, my god!

NARRATOR: All 20 passengers and five crew members 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--

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!

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.

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.

NARRATOR: The 737 is once again out of control.

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.

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.