Original subtitles

NARRATOR: Every passenger jet is a traveling life support

system filled with highly pressurized oxygen

to keep us alive.

If the oxygen escapes, a simple flight

becomes a living nightmare.

United 811.

There was nothing in front of us or to the side of us.

The whole side of the plane was gone.

NARRATOR: Aloha 243.

Everything was going, was being sucked out of the plane.

NARRATOR: British Airways 5390.

I'll never forget.

His face was hitting the side screen, but he didn't blink.

Mayday! Mayday!

Mayday!

Declaring an emergency!

NARRATOR: Sometimes it takes a terrible accident

to expose hidden dangers and change

the way airplanes are built.

Unfortunately, we wait until we have enough bodies.

Too many of the changes have been,

in effect, written in blood.

FLIGHT ATTENDANT (ON INTERCOM): Ladies and gentlemen,

we are starting our approach.

PILOT: We lost both engines.

PILOT: Mayday.

Mayday.

WOMAN: Brace for impact!

MAN: He's gonna crash!

NARRATOR: This is the assembly plant for the Airbus A320.

After the Boeing 737, it's the most popular jet ever built.

More than 9,000 of them are flying

for airlines around the globe.

It's safe and dependable--

the airline equivalent of a minivan.

The aluminum skin on the top of an A320 is less than half

an inch thick--

about as thick as a coin.

But this slender piece of metal helps keep passengers alive.

Because the skies aren't nearly as friendly as they seem.

JOHN NANCE: Most people take aviation

absolutely for granted.

The difference between being on a commercial airliner

at 35,000 feet and being in a space capsule in orbit

is really not all that different.

They're both life support systems.

The reality is it's a hostile environment.

The reality is it's 50 degrees below zero outside.

The reality is that jet stream or that air stream

out there would kill you almost immediately.

NARRATOR: It's not natural for people

to travel through this killer atmosphere.

But every day, millions of us fly almost 10,000 feet higher

than the top of Mount Everest.

All our life support that's natural

for us is down here at the bottom of this sea of air.

And if we swim up too high, however we get there, if we're

not protected we can't live.

NARRATOR: But taking oxygen with us up to 36,000 feet

is potentially dangerous.

The air inside an airplane is pressurized so passengers

can breathe easily.

As the planes climb, the pressure outside decreases.

The tightly packed air in the cabin

begins exerting tremendous pressure on the fuselage.

On an average jetliner, it means that every square yard

of the fuselage must support almost 6,000 pounds of force.

And on almost every flight, the fuselage wins the battle--

but only because airplane designers

have learned tragic lessons.

In the 1950s, a series of shocking accidents

led to modifications that are still seen today.

ANNOUNCER: The Comet has blazed new trails,

achieving new speeds, setting a new standard.

NARRATOR: The passenger jet era began in the 1950s

in Great Britain, with the introduction

of the de Havilland Comet.

For the first time, jet engines took commercial planes higher

than they'd ever gone before.

JAMES DELAURIER: What Great Britain had at stake

with the Comet was enormous.

They wanted to really declare their place in Civil aviation

by having the first successful jet transport aircraft.

NARRATOR: But less than two years after its maiden flight,

the glittering jewel of British aviation

disintegrated in mid-air.

JAMES DELAURIER: It would have been horrible,

but mercifully it would have been quick.

What they had found with the bodies that they had recovered

is the massive decompression, of course,

caused the air inside your lungs to burst your lungs.

At the same time, the out rush of air

would tear you from your seat.

And many of these people actually smashed their heads

against the structure.

NARRATOR: Three months later, another Comet

ripped apart in flight.

Officials feared that every single Comet

was a flying time bomb.

The entire fleet was grounded.

The design of the Comet was actually a very sound design.

There was only one thing that they didn't do,

and it's because nobody knew.

NARRATOR: Unknown to engineers, there was a deadly flaw

in the Comet's design.

To find the jet's fatal weakness,

investigators built a massive water tank.

They immersed a stripped down Comet.

The pressure in the tank was increased and decreased,

simulating the strains of flight.

The experiment ran 24 hours a day, seven days a week.

After the equivalent of some 3,000 flights,

the Comet's Achilles heel revealed itself--

its square windows.

You have a rapid change of direction in the shape,

essentially a corner.

You have a high stress concentration.

It gave rise to a fatigue crack, which then traveled rapidly

through the rest of the structure

causing a massive decompression.

NARRATOR: The most advanced passenger jet in the world

had succumbed to metal fatigue.

The fuselage simply could not handle the force of the air

inside pressing out.

The airplane, with all that force behind it,

suddenly unzipped itself.

NARRATOR: Every plane that's built today

is safer because of the Comet disasters.

Like other passenger planes, the windows in the A320

are rounded so that pressure doesn't

build up around the corners.

Perhaps even more important-- extra rivets reinforce the skin

of today's planes to contain cracks that might

start anywhere on the fuselage.

This Airbus factory in Toulouse, France

rolls out one new A320 just about every working day.

A big part of the job is strengthening

the fuselage with lightweight and extremely tough

titanium rivets.

3,000 rivets join the separate sections together.

Another 3,000 are used on each wing.

Without them, the fuselage couldn't

contain the pressurized air that's

forced inside during flight.

But even these rivets aren't foolproof.

April 28, 1988--

Aloha Airlines Flight 243 is traveling from Hilo Airport

on the Big Island to Honolulu.

With this island hop, Aloha 243 is making

its ninth flight of the day--

a normal schedule for Aloha's planes.

In the cabin, the pressure is kept at a constant level

so passengers feel like they've never left the ground.

But as a plane rises to its cruising altitude,

the air pressure outside the cabin is dangerously low.

Well, what we do is extract air from the engines

and use that to pressurize the airplane.

And what we can do then is control the pressure

inside by a series of valves.

NARRATOR: The air moving through the cabin

creates constant pressure on the jet's fuselage,

keeping it inflated like a balloon.

Every modern jet is built to withstand this pressure.

JAMES DELAURIER: There's an internal structure

to a modern all metal airplane.

The skin without the structure would collapse easily.

It would buckle easily.

It'd be sort of like a--

you know, a paper bag without the--

without any structure inside to hold it.

NARRATOR: Beneath the skin of a passenger jet,

hoop shaped bulkheads and formers

support the aircraft's width.

Stringers run the length of the plane,

further helping to support the fuselage.

And the cabin needs all the help it can get.

Because as the plane gains altitude,

that pressurized oxygen inside the plane

is pushing against every square inch.

Passengers aboard Aloha 243 are about to learn

what happens when that pressurized air suddenly

escapes.

NARRATOR: April 1988--

Aloha Flight 243 is en route to Honolulu

when the plane rips open.

I saw a brilliant flash of light, then boom.

Everything was going--

was being sucked out of the plane.

NARRATOR: Aloha Airlines 243 has just suffered what experts call

an explosive decompression.

The air inside the plane escapes in a sudden horrifying moment.

370 square feet of the fuselage are gone.

Just imagine the scene up there.

The top of the airplane broken off--

you now have 300-mile-an-hour winds blowing into that cabin.

That's three times hurricane force winds.

Those people were dressed for Hawaii in the springtime, not

minus 50 degree temperatures.

Any period of time at 24,000 feet and those people will die.

What was that?

We have to get down!

NARRATOR: Captain Bob Schornstheimer

begins an emergency descent, dropping 75 feet a second.

The stress on the damaged craft threatens to tear it apart.

The woman that was sitting next to me--

her husband, he was on the other side in the next row up.

She was next to me.

And they were reaching their hands out,

and they were trying to touch fingers to say goodbye.

NARRATOR: Against incredible odds,

the flight crew land their bruised and battered airplane.

Even with this explosive decompression,

there's only one death on Aloha Flight 243--

a flight attendant who was pulled out of the plane.

Jim Wildey investigates the crash for the National

Transportation Safety Board.

In his laboratory, Wildey makes a disturbing discovery.

In some of the pieces of the plane's fuselage

he finds a series of hairline cracks.

They're right beside the holes created by rivets,

and barely visible to the naked eye.

But they're classic signs of metal fatigue.

A plane isn't a rigid tube.

To maintain the pressure passengers

need to enjoy a flight, it's designed

to be much more flexible.

The fuselage of the airplane is actually breathing.

It expands and contracts depending on altitude.

When it's on the ground, it's in a contracted status.

When it's at altitude, 24,000 feet, the fuselage expands.

So the airplane is constantly cycling.

That's pressurization.

That will weaken the structure over a long period of time.

NARRATOR: Records show that the Aloha jet was 19 years old.

737S are designed for a 20-year service life, and

a recommended 75,000 flights.

But as investigators take a closer look,

they discover that the Aloha jet had

logged an astonishing 89,000 separate flights.

The short hops between the Hawaiian islands

meant that the planes in the Aloha fleet

went through more pressurization cycles than any other aircraft.

You saw something as you got on this airplane.

What did you see?

NARRATOR: Investigator Jim Wildey

gets a lead when he interviews one of the Aloha passengers.

She says she saw a small crack in the fuselage

just to the right of the door.

The witness saw cracking in this area,

and we found fatigue cracking back in here.

So this is the line where the fatigue cracking joined up.

One piece came down this way and folded off,

and the other piece went across the top

and came off to the right side.

NARRATOR: But something doesn't make sense.

The Aloha jet lost 370 square feet of its fuselage.

In the years after the Comet disaster,

Boeing and other companies designed

a safety feature meant to keep any tearing to a minimum.

Inside the fuselage of every 737,

Boeing installed a series of tear straps.

If a tear develops in the fuselage,

it should only run as far as the next tear strap--

never more than five inches away--

before being redirected.

This is meant to prevent the catastrophic disintegration

that ripped apart the Comet.

The purpose of the tear strip is to confine any kind

of rip or tear in the fuselage skin to a 10-inch square,

basically.

The 10-inch square allows a controlled decompression

and confines any structural damage to a very small area.

NARRATOR: But on Aloha 243, the tear straps

did not contain the rupture caused by the metal fatigue.

The NTSB believes that the many cracks in the fuselage

eventually joined together, allowing

an enormous hole to open under the stress of flight.

But jets aren't held together by rivets alone.

The Comet disaster had also highlighted

the need for redundancy.

The skin of an airplane is built from separate overlapping

panels.

These panels are bonded together by a powerful epoxy.

Then the panels are locked together by rivets.

During his investigation of the Aloha fuselage,

Jim Wildey defines discoloration inside some

of the overlapping joints.

You can see it now where the dark material is

the epoxy that was used to bond the two layers of the lap joint

together.

The white material you see here is corrosion damage

of the aluminum fuselage skin.

NARRATOR: The Hawaiian climate is great for tourists,

but it's tough on airplanes.

The ocean air is humid and heavy with salt.

It can corrode even industrial epoxy.

Investigators learned that Boeing--

the company that built Aloha 243--

had issued numerous warnings about the epoxy.

If it isn't applied at the right temperature--

if the panels have moisture or dirt on them--

the bonds can fail.

Boeing recommended regular detailed inspections.

But workers at Aloha didn't report

any problems with the epoxy.

They either never saw the compromised epoxy

or, if they did, it wasn't repaired.

The stress that's trying to pull one skin away

from the other skin piece--

the stresses would go through the bonding and not

through the rivets.

Of course, as this thing becomes disbonded,

now the rivets themselves are loaded,

and especially this top row of rivets.

And this is the row of rivets we think that had

the fatigue cracking in it.

These cracks go unrepaired, and now you have an airplane

that is a ticking time bomb.

NARRATOR: The fuselage on Aloha 243

was seriously compromised by several factors--

age, poor maintenance, and heavy tours of duty.

Since 1988, we have come light years in understanding this.

And we no longer leave ourselves the tolerance that

used to be left to airlines to just go out

and take a look at the airplane and sign it off.

NARRATOR: The Aloha accident was another step toward making

passenger jets safer.

It's important to always learn from your mistakes.

It's important to learn lessons from that.

And that has been the case with aeronautical engineering.

NARRATOR: Aloha 243 was a brutal lesson in the dangers of metal

fatigue, but it wasn't the last example

of the power of cabin pressure.

Two years later, the industry would get

another terrifying reminder.

PILOT (ON INTERCOM): Ladies and gentlemen,

this is your captain speaking.

NARRATOR: June 10, 1990--

British Airways Flight 5390 is leaving Birmingham, England for

Spain with 87 people on board.

80 knots.

NARRATOR: Two minutes into the climb, the flight crew

switches on the autopilot.

Captain Tim Lancaster removes his shoulder straps.

I went into the flight deck to ask Tim and Alastair

what they would like to drink.

You gentlemen like a tea?

Please, the usual.

NARRATOR: Minutes later, at 17,000 feet,

the plane is very close to its assigned altitude.

And then, like a cork out of a champagne bottle,

the windshield bursts from its frame.

Captain Tim Lancaster is sucked out of his seat,

and is pinned to the fuselage by blistering winds roaring

more than 340-miles-an-hour.

The temperature is just above zero degrees Fahrenheit,

and there's little oxygen. Co-pilot Alastair Atchison

is alone at the controls.

Ordinarily, cockpit windows cannot budge.

The force of the air as the plane

soars through the sky presses the windshield

against the fuselage.

But on Flight 5390, something has gone terribly wrong.

Flight attendant Nigel Ogden rushes in to help.

NIGEL OGDEN: When I looked in, the flight deck door

was resting on the controls.

And all I could see was Tim out the window.

I just grabbed him before he went out completely.

NARRATOR: Other flight attendants do what they can.

Co-pilot Alastair Atchison reduces speed

and descends quickly.

But as he slows the plane down, the drop in wind pressure

lets the captain slide around on the side of the plane.

NIGEL OGDEN: All I remember is Tim's arms flailing out.

His arms seemed about 6-foot long.

And I'll never forget that his eyes were wide open.

I mean, his face was hitting the side of the side screen,

but he didn't blink.

And I thought to myself and I said to John--

I said, I think he's dead.

I think he's dead.

NARRATOR: Just 35 minutes after taking off,

Atchison gets the jet safely back on the ground.

But in the most unbelievable chapter to end this story,

Captain Tim Lancaster survives the horrific ordeal.

But I remember watching the windscreen

move away from the aircraft.

And then it had gone like a bullet.

It disappeared into the distance.

And I was very conscious of going upwards.

And, well, the whole thing became completely surreal then,

as it would.

And I was aware of being outside of the airplane.

I can remember seeing the tail of the aircraft.

I remember the engines going around.

And-- and then I don't remember much more.

NARRATOR: Tim Lancaster was pinned

to the outside of the plane for more than 20 minutes.

His injuries were surprisingly survivable--

bone fractures in his right arm and wrist,

frostbite, and shock.

Within five months, Tim Lancaster was flying again.

In the immediate aftermath, investigators

have very little to go on.

Windscreen was missing.

There was a certain amount of blood around.

There were some minor dents and scrapes on the fuselage,

as you'd expect if the window had gone past.

And, really, that was about it apart from a lot of paper

scattered around inside.

NARRATOR: The maintenance log is recovered from the plane.

Stuart Culling learns the windscreen had been replaced

just hours before takeoff.

Early in the investigation, the missing windscreen is found.

It contains important evidence.

There were something like 30 bolts found with it,

most of which were one size short in diameter, one size

too small in diameter.

NARRATOR: During his interview with a ground

engineer who repaired the plane, Culling gets a major break.

STUART CULLING: One thing that came out was that he said, oh,

the old bolts went into a waste bin in the hangar--

where he did the job--

and they may still be there.

So he rushed across to the waste bin

and found something like 80 discarded bolts.

NARRATOR: The old bolts are the proper size.

Why were smaller bolts used to replace them?

These are the ones you checked against the new ones?

That's right. Yeah.

I took those off. - From the carousel?

Yeah.

STUART CULLING: This was really excellent evidence-- gold,

as far as I was concerned.

NARRATOR: Instead of using the old bolts

to put the new window on, the ground engineer replaced them.

He did not check the parts catalog to verify which

bolts he needed for the job.

Morning.

NARRATOR: The bolts he chose looked similar,

but were 0.2 inches or 20% smaller.

They were too thin to do the job.

Early in the morning, working in the shadow of a hangar,

the engineer couldn't tell the difference.

Hours later, the window gave way under the force

of the pressurized air inside the plane.

Faced with a challenge they weren't trained for,

the crew still managed to avoid disaster.

5390!

NARRATOR: But the massive pressure inside an airplane

doesn't need bad maintenance to rip a jet apart.

That pressure can also find a tiny flaw

somewhere in the design and cause a nightmare in the sky.

NARRATOR: The Airbus A320--

one of the most popular passenger jets.

Every day around the world thousands of passengers

board this plane.

When they do, they walk through what

would seem to be an obvious weak spot in the fuselage--

the door.

Passenger doors are plug type doors.

They're built to be slightly larger than their frames.

When a plane takes off and pressurizes,

the atmosphere inside the aircraft seals the door shut.

That door probably has 10,000 or more

pounds of pressure holding it firmly

in place in that door frame.

And you have to pull it out of that door frame to get it open.

NARRATOR: But not all doors on an airplane are built the same.

Even designs that seem flawless on paper

can rip apart in the real world.

February 24, 1989, Honolulu Airport.

United Airlines 811 is bound for Auckland, New Zealand.

Expected flying time--

9 and 1/2 hours.

There are 355 people on board, plus a full load of cargo.

The doors close on time, and the plane leaves the gate

just after 1:30 in the morning.

Tell them we can handle 33 if it's available.

OK.

DAVID CRONIN: We did notice that there were thunderstorms

100 miles south, right on course, which was rather

unusual for that time of night.

So I left the seatbelt sign on.

NARRATOR: Captain Cronin's decision to keep

that sign on will save lives.

As the 747 climbs past 2,300 feet,

passengers sitting just above and behind the cargo door

begin to hear a strange noise.

Kind of a grinding noise.

I heard a-- like, a thud.

What the hell?

The next thing I knew, I found myself on the stairwell hanging

on to the rungs.

And I immediately knew it was an explosive decompression.

Everything on the airplane that

wasn't fastened down, tied down, or secured became airborne.

The noise was incredible.

NARRATOR: The 747's cargo door had torn off, ripping

away a section of the fuselage.

The pressurized oxygen in the cabin

shot out with explosive force.

And as I looked up, that was the first time

I saw this tremendous hole on the side of the aircraft that

was just a void.

And seats were missing, and I immediately knew

that we had lost passengers.

BOB BAHNA: Everything in front of us was gone.

Where we were sitting, we were about 6 inches from the hole.

So there was nothing in front of us or to the side of us.

The whole side of the plane was gone.

Actually, our feet were dangling on the hole,

and my first thought, we weren't gonna make it.

You know, I just didn't think there was any hope.

NARRATOR: The situation is desperate.

An explosive decompression won't bring a plane down by itself,

but the gaping hole is putting massive stress on the aircraft.

The flight crew needs to descend as fast as possible.

Left-right valves on.

Start dumping the fuel.

I am dumping.

NARRATOR: Struggling to fly their badly damaged jet,

the crew turned back to Honolulu Airport.

And, all of a sudden, we were slowing down, slowing down.

And I said, oh, my god.

We've landed.

We're on ground.

DAVID CRONIN: Probably the best landing I've ever made.

When we finally stopped on the runway

we deployed all 10 chutes, and the flight attendants

evacuated all the passengers.

NARRATOR: Thanks to the experienced flight

crew, United Airlines 811 landed with everyone on board alive.

But nine passengers were missing--

sucked out of the plane when the fuselage tore apart, taking

with it five rows of seats.

One of those passengers was a New Zealander

on his way home, Lee Campbell.

We got a phone call from Chicago.

And they just said that they regret

to inform us that our son was missing, presumed dead.

NARRATOR: In the wake of their son's tragic death,

Kevin and Susan Campbell embark on an international mission

to discover exactly why the door had come off the plane.

Two months after the accident, the National

Transportation Safety Board holds preliminary hearings.

During a break, the Campbells remove

several boxes full of files.

SUSAN CAMPBELL: So we quickly realized

we had got a really good set of papers

with a lot of things that hadn't been released to the public.

We were able to really start our investigation

and learn most at that stage.

NARRATOR: The unpublished documents

reveal a disturbing catalog of problems

with the 747's forward cargo door,

going back to its original design.

Unlike doors for passengers, most cargo doors open outward.

This increases the space for luggage and other cargo.

The Campbell's research uncovers two major flaws with the 747

cargo door locking system.

To lock the cargo doors, electric motors

turn C-shaped latches around pins in the door frame.

A handle then moves arms or locking

sectors over the top of the C latches

to prevent them from reopening.

But on Flight 811, the system had failed.

Kevin Campbell built a model of the 747 cargo door latch.

It showed the first deadly flaw in the locking system.

The aluminum locking sectors did not hold if the C latches

started to open on their own.

With the aluminum locking sectors--

if the C locks tried to back wind, open electrically,

it would just push the locking sector out of the way.

It just simply wasn't up to the job that it was designed for.

NARRATOR: During their research, the Campbells

learned that two years before Flight 811, a Pan Am 747 out

of Heathrow was forced to turn back

when its cargo doors opened and prevented

the cabin from pressurizing.

When they got back to Heathrow,

they found that the door was hanging open an inch

and a half at the bottom.

And all of the locks were open.

When it got to the maintenance base,

they found that all of the locking sectors

were either bent or broken.

NARRATOR: The passengers on this flight were lucky.

They survived the faulty locking system.

But why had the C latches turned and bent the locking sectors?

As the Campbells continued to search,

a Pan Am report surfaces that lays

out a critical issue with the cargo door's electrical system.

When the cargo door's outer handle is closed,

a master switch is supposed to disconnect the power supply

and stop the C latches from turning.

But something was wrong with the switch.

There was power to the door locks with the-- with

the outer handle closed.

And the lock started to move, and it

started to force the locking sectors out of the way.

NARRATOR: The faulty power switch and weak locking sectors

were no match for the pressurized oxygen

inside the plane.

After years of being pushed by the Campbells,

the NTSB produces a report that agrees.

There was an inadvertent failure of either the switch

or the wiring that caused an uncommanded opening

of the door.

It's nice that other people will know that you're right,

and had been all along, and that the support

that they had given you was-- you know, was vindicated.

SUSAN CAMPBELL: I couldn't have lived

with myself if we had done no investigating ourselves.

It was just something we both felt we needed to do.

We didn't even discuss it.

We just knew that's what we would do.

NARRATOR: After United Flight 811,

the locking system on the Boeing 747 cargo doors was changed.

Inspections were increased.

Another potential scenario for explosive decompression

had been found and eliminated.

Since the first jet engines pushed planes higher

in the sky, the aviation industry has struggled

to harness and contain the deadly power

of pressurized oxygen. They know all too

well that a single flaw can lead to a terrifying decompression.

And more than 15 years after United 811,

another deadly lesson is learned.

Helios 522, do you read?

Over.

NARRATOR: August 14, 2005--

for almost an hour, Helios Flight 522 has been

circling the skies over Athens.

AIR TRAFFIC CONTROL: Helios, 522, over.

NARRATOR: Its flight crew has stopped communicating

with air traffic control.

Fearing a terrorist attack, the Greek Air

Force scrambles two fighter jets to circle the mystery aircraft.

FAITHON KARAIOSOFIDES: One of them

was actually in a shooting position behind the 737.

The other one was nearby the cockpit,

and he was trying to communicate visually

with the person in the cockpit.

NARRATOR: The fighter pilots can't

see any damage to the jet, no holes in the fuselage.

There is no structural failure.

There is no fire.

There is no problem--

obvious problem from the external view with the plane.

NARRATOR: Someone in the cockpit waves at the fighter pilot.

But all too soon, the jet loses altitude

and falls towards the ground.

All 121 people on board are killed.

It's the worst air crash in the history of Greece.

Within minutes, investigators are on the scene.

I saw a great area in front of me, which was burning.

It was black, burning, people spread, pieces of--

of the airplane.

NARRATOR: The autopsies add more mystery to the case.

Everyone on board the Helios flight was alive up

to the moment of the crash.

INTERPRETER: They did not die from inhaling

a toxic substance in the airplane,

but from an explosion.

These people died on impact.

NARRATOR: But if the passengers were alive until impact,

why didn't the fighter pilots see more activity on the plane?

Akrivos Tsolakis is the lead investigator.

He begins to dig through maintenance records.

He learns that on the day of the crash

the rear door had been inspected for leaks in Cyprus.

After landing in Cyprus, the cabin crew

reported the problem.

They had heard loud banging and saw

ice and the rear service door.

To make sure there's nothing wrong with the seal

on the door, an engineer runs a pressurization test.

He's looking for a leak.

So explain again how you tested the pressure.

I went into the cockpit.

I turned the pressurization switch to manual.

Switching digital pressure control

unit from auto to manual.

NARRATOR: The jet's engines are turned off,

so the engineer uses the plane's auxiliary power unit

to force air into the cabin.

BILL WALDOCK: It's like looking for a leak in a tire.

In this case, what you're having to do

is pressurize the aircraft, use a barometer

essentially to monitor the pressure inside

and look for leaks that way.

NARRATOR: After completing the pressurization test,

the ground engineer reports that the jet

is in good working order.

But the digital pressure control is left in the manual position.

They were supposed to return the selector

to the auto position.

NARRATOR: Both the captain and co-pilot

missed the fact that the plane is not set

to pressurize automatically.

As Helios 522 climbs, an alarm blares in the cockpit.

What is it?

A take-off config warning?

NARRATOR: It's a non-pressurization warning,

but it sounds identical to another alarm.

The pilots confuse the two.

It's a critical mistake.

FAITHON KARAIOSOFIDES: The alarm sounded, and that alarm

was misinterpreted.

Most of flight crew, they will never

face an alarm with no pressurization in all

their flight career.

Because it's a rare event.

PILOT (ON RADIO): Operations, this is Flight 522.

Over.

Flight 522, what can I do for you?

We have a take-off config warning on.

Sorry.

Could you repeat?

NARRATOR: As the pilots troubleshoot with ground

engineers, life sustaining oxygen

is slowly seeping out of the plane.

Eventually, oxygen masks drop in the cabin.

They do not fall in the cockpit.

The reason that we don't have automatically

deploying oxygen masks in a cockpit-- there's simply

too much up there.

And if you had things popping out,

they're gonna hit switches that they shouldn't hit.

NARRATOR: The crew don't realize they

have a pressurization problem.

Eventually, both the captain and the co-pilot collapse,

unconscious from a lack of oxygen.

We're the ones that should be trained consistently

to understand that ears popping--

anything that indicates pressurization,

you don't even talk to each other before you

grab that mask and put it on.

NARRATOR: The passengers are unaware there

is no one at the controls.

In emergency situations, chemical

generators above the seats pump out oxygen.

But there's a catch.

These generators only produce enough oxygen

for about 12 minutes.

Well, the problem with the passenger masks is,

for one thing, they're not designed to keep you

oxygenated at a high altitude.

What they're designed to do is give you enough oxygen

so that you can't survive until the pilots get the airplane

down to a low altitude.

NARRATOR: But with both pilots already unconscious,

the Helios jet does not descend so passengers

can breathe without assistance.

Instead, the plane flies on autopilot to Athens.

When the oxygen supply stops, the passengers pass out.

By the time the Greek Air Force intercepts the Helios jet,

only one person is still moving.

Likely surviving with bottled oxygen, flight attendant

Andreas Perdomo is still conscious

when the fighters approach.

He makes it to the cockpit, but he can't save the plane.

FIGHTER PILOT (ON RADIO): Athens control,

there is one person moving in the cockpit of Helios 522.

NARRATOR: Eventually, when its fuel runs out,

Helios 522 crashes.

Investigators eventually find the panel

with the pressurization switch.

Are you sure this is the way it was found?

It hasn't been moved at all?

NARRATOR: All 121 people on the Helios flight

died because their plane didn't carry

enough life sustaining oxygen as it climbed into the sky.

It's been more than 50 years since the beginning

of the passenger jet era--

50 years in which the industry has learned, sometimes

painfully, how to safely fly more

than six miles up in the sky.

Unfortunately, when you're pushing the envelope

and you're pushing the boundaries of design,

you can encounter problems that you hadn't anticipated.

NARRATOR: In search of the safest plane imaginable,

the history of aviation traces a flight

path through tragic accidents to technological breakthroughs.

Many of these accidents display the incredible power

of explosive decompression.

The Airbus A320, and every other passenger plane built today,

is infinitely safer than the first jets

that flew in the 1950s.

They have to remain safe and get even safer,

because we rely so heavily on this incredible mode

of transportation that takes us somewhere

we were never meant to be.