Original subtitles

Every passenger jet is a travelling life-support system,

carrying inside it the highly pressurized oxygen that keeps us alive.

If it ever 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.

Aloha 243...

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

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!

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.

Mayday, mayday!

This is the assembly plant for the Airbus A320.

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

Almost 2000 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 two millimeters 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.

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 air stream out there,

it would kill you almost immediately.

It's not natural for people to travel through this killer atmosphere.

But, every day, millions of us fly easily

some 3,000 meters 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.

But taking oxygen with us up to 11,000 meters

is potentially dangerous.

The air inside an airplane is pressurized, so passengers can breathe easily.

As 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 meter of the fuselage

must support more than 5,000 kilograms of force.

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

But only because airplane designers have learned tragic lessons.

We have concentrated, in the past, on changing things.

But, unfortunately, we wait until we have enough bodies.

In the 1950s, a series of shocking accidents

triggered changes that are still seen today.

The Comet has blazed new trails, achieving new speeds,

setting a new standard.

The passenger jet era began in the 1950s,

with the introduction of the De Havilland Comet.

For the first time, jet engines were being used

to push commercial planes higher than ever before.

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.

But less than two years after its maiden flight,

the glittering jewel of British aviation disintegrated in mid-air.

It would have been horrible.

It would have been a horrible situation,

but mercifully, it would have been quick.

What they had found with the bodies that they had recovered,

was 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 that many of these people actually smashed

their heads against the structure.

Three months later, another Comet ripped apart in flight.

Officials feared that every single Comet could be a flying time bomb.

The entire fleet is 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.

Unknown to engineers,

there was a deadly flaw in The Comet's basic 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 flights

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 travelled rapidly through

the rest of the structure, causing a massive decompression.

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.

Every plane that's built today is safer

because of the disaster that struck The Comet.

Like other passenger planes, the windows on the A-320 are rounded,

so that pressure doesn't build up around the corners.

Perhaps even more importantly, extra rivets reinforce the skin of today's planes

to contain cracks that might start anywhere on the fuselage.

It's designed to go to that first row of rivets and absolutely be stopped.

It's a healthy structure.

It can never unzip itself.

But 34 years after The Comet crash,

aircraft manufacturers were faced with another tragic disaster.

The extra rivets that were supposed to save lives

failed to withstand the relentless power of air pressure on metal.

One of the fellows that I knew at the FAA,

he said, "You know, the day after this accident,

I had to throw away most of what I knew about metallurgy and start over."

At this Airbus factory in Toulouse, France,

A320s roll out at about the rate of one every working day.

Titanium rivets, lightweight and extremely tough,

hold the fuselage together.

Three thousand are used to join the separate sections.

Another 3,000 can be found 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 the 28th, 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 the planes of Aloha.

Protected within the jet, passengers give little thought to the fact

that the cabin is filled with pressurized oxygen.

It's constantly pushing against the fuselage,

trying to escape into the surrounding atmosphere.

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.

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.

There's an internal structure to a modern all-metal airplane.

The skin, without the structure, would collapse easily. It would buckle easily.

It would be sort of like a, you know, a paper bag

without any structure inside to hold it.

If you remove the skin of a passenger jet,

you'll find hoop-shaped bulkheads and formers

supporting the width of the aircraft.

Stringers run the length of the plane.

They all help 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 centimeter.

On this day in April, 1988,

passengers are about to learn what happens when that air suddenly escapes.

I saw a brilliant flash of light. And, boom.

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

Aloha Airlines 243 has just suffered what

experts call an "explosive decompression."

he air inside the plane that makes jet flight possible

escapes in a sudden, horrifying moment,

Thirty-five square meters 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 in that cabin.

That's three times hurricane-force winds.

Those people were dressed for Hawaii in the springtime,

not minus fifty-degree temperatures.

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

What was that? We have to get down!

Captain Bob Schornstheimer begins an emergency descent,

dropping 20 meters a second.

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

The woman that was sitting next to me, and her husband,

he was on the other side in the next row.

And she was next to me, and they were reaching their hands out,

and they were trying to touch fingers to say goodbye.

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.

Running through 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.

Metal fatigue is something that sounds exotic,

but it really is easy to understand.

Any piece of metal has a certain breaking point.

All of us have tried to open a tin can

and not quite gotten the opener all the way around.

We work it back and forth until that last portion breaks.

You just demonstrated metal fatigue.

A plane isn't a rock solid 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.

If you could stand at the back

of a 250-foot-long jetliner and just sight along that fuselage,

you'd see it begin to puff up a little bit.

And as a plane lands, the pressure differential

between the inside and the outside of the plane disappears.

The fuselage returns to normal.

So the airplane is constantly cycling.

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

Records show that the Aloha jet was 19 years old.

Boeing 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? - 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.

But something doesn't make sense.

The Aloha jet lost 35 square meters of its fuselage.

In the years after The Comet disaster,

Boeing and other companies had designed a safety feature

that should have kept any tearing to an absolute minimum.

Inside the fuselage of every 737,

Boeing has installed a series of tear straps.

If any kind of tear develops in the fuselage,

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

never more than 13 centimeters away,

before shooting off at a 90-degree angle.

This would have prevented the sort of

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 ten inch square basically.

The ten-inch square allows controlled decompression,

and confines any structural damage to a very small area.

But for Aloha 243,

the tear straps did not contain the rupture caused by the metal fatigue.

The NTSB believes there were so many cracks in the fuselage

that they eventually joined together,

tearing an enormous hole in the plane.

But jets aren't held together by rivets alone.

The Comet disaster had also highlighted the need to reinforce the fuselage.

The skin of an airplane is built from separate panels, which overlap.

These panels are bonded together by a powerful adhesive, known as epoxy.

As the epoxy hardens, the panels are locked together by rivets.

And during his investigation on the Aloha fuselage,

Jim Wildey finds discoloration inside some of the overlapping joints.

You can see 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.

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 written warnings about the epoxy.

If it isn't applied at the right temperature,

if the panels have moisture or dirt on them, the bonding 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 un-repaired,

and now you have an airplane that is a ticking time bomb.

The fuselage on Aloha 243 was seriously compromised

by several factors.

Poor maintenance, the age of the aircraft and by the 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.

The Aloha accident was another step towards 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.

The Aloha story 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.

Ladies and gentlemen, this is your captain speaking.

My name is Tim Lancaster.

Welcome aboard this British Airways Flight to Malaga.

June the 10th, 1990.

British Airways Flight 5390 is leaving Birmingham, England, for Spain.

Eighty-seven people are on board.

Eighty knots.

Two minutes into the climb, the flight crew switch

the flight crew switch on the autopilot,

Captain Tim Lancaster takes off his shoulder straps.

I went into the flight deck

to ask Tim and Alistair what they would like to drink.

Would you gentlemen like a tea?

Please, the usual.

Minutes later, at 5,200 meters,

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

close to 550 kilometers an hour.

The temperature is minus 17 degrees centigrade and there's very little oxygen.

Co-pilot Alastair Atchison is alone at the controls.

Ordinarily, cockpit windows cannot budge from their frames.

The force of the air, as the planes soars through the sky,

pushes the windshield onto the plane.

But on Flight 5390, something has gone terribly wrong.

Flight attendant Nigel Ogden rushes in to help.

And I looked in. The flight deck door was resting on the controls,

and all I could see was Tim hanging out the window.

I just grabbed him before he went out completely.

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.

All I remember is Tim's arms flailing out.

His arms seemed about six foot long.

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 think he's dead."

- "I think he's dead." -

Just 35 minutes after taking off,

Atchison gets his jet safely back on the ground.

But the most unbelievable chapter of this entire story

is the fact that captain Tim Lancaster survives his incredible 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 remember seeing the tail of the aircraft.

I can remember the engines going around.

And then I don't remember much more.

Tim Lancaster was pinned to the outside of the plane

for over 20 minutes.

His injuries were surprisingly minor.

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 papers scattered around inside.

The maintenance log is recovered from the plane.

Stuart Culling learns the windscreen had been replaced,

just hours before takeoff.

Everything okay? - Fine.

She's just come out of maintenance, by the look of it.

Nothing much, though. Just changed the windscreen.

I wanted to find out exactly what had happened

to the aircraft before it took off.

Early in the investigation, the missing windscreen is found.

It contains a curious piece of evidence.

There was something like 30 bolts found with it,

most of which were one size short in diameter, one size too small in diameter.

On many planes,

windscreens are fitted from inside the cockpit.

Internal cabin pressure pushes against them, keeping them in place.

But on the BAC 111, the windscreen is bolted from the outside.

The pressurized oxygen inside the jet pushes out against the windscreen.

The bolts must resist this pressure.

There are enough of them there that they simply can't pull out of the structure.

But, of course, if you then violate the very premise of that

by putting the wrong bolts on, all bets are off.

You're now a test pilot.

During his interview with the ground engineer who repaired the plane,

Culling gets a major break.

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 we rushed across to the waste bin

and found something like 80 discarded bolts.

The old bolts are the proper size.

Why were smaller bolts used to replace them?

And these are the ones you checked against the new ones?

- That's right. Yeah. I took those... - From the carousel?

It was really excellent evidence.

Gold, as far as I was concerned.

Instead of using the old bolts to put the new window on,

the ground engineer decided to replace them.

He did not check the parts catalogue to verify

which bolt he needed for the job.

Morning.

The bolts he chose seemed the same,

but in fact, were just over half a millimeter smaller.

They were too thin to do the job.

Early in the morning, working against the side of a hanger,

the engineer couldn't tell the difference.

Hours later, the window gave way.

Faced with a challenge they weren't trained for,

the crew still managed to pull their plane back from the brink.

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.

The Airbus A320, one of the most popular passenger jets in the sky.

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.

Any hole in the fuselage is a potential danger,

so engineers designed passenger doors that can't be opened in flight.

It is virtually impossible.

I don't use that word very easily,

but it is impossible for a passenger

to open a plug type door in flight.

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.

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 the 24th, 1989. Honolulu Airport.

United Airlines 811 is bound for Auckland, New Zealand.

Expected flying time,

nine-and-a-half 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. - Okay.

We did notice that there were thunderstorms a 100 miles south,

right on course,

which was rather unusual, for that time of night.

So I left the seat belt sign on.

Captain Cronin's decision to keep that sign on

will save lives.

As the 747 climbs past 7,000 meters, 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? - And the next nanosecond,

it was pure, unadulterated pandemonium.

The next thing I knew, I found myself

on the stairwell

hanging onto 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.

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.

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 out of the hole.

And I first thought we weren't going to make it,

you know? I just didn't think there was any hope.

The situation is desperate, but by itself,

an explosive decompression won't bring a plane down.

In 811, there's a whole as big as Tulsa on the side of this thing.

I mean, it's an aerodynamic disruption of massive proportions.

But if it was designed the way we had designed things a long time ago,

it would have unzipped.

After the door came off,

eventually, a row of rivets held,

keeping the plane from pulling itself apart.

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

The flight crew needs to descend as fast as possible.

CO-PILOT: Left right valves on.

Start dumping the fuel.

I am dumping.

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."

Probably the best landing I've ever made.

When we finally stopped on the runway,

we deployed all ten chutes,

and the flight attendants evacuated all of the passengers.

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 open,

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.

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.

Lee can't have died for nothing.

You know, you've got to find out why he died.

And you've just got to make sure that it never happens again.

Two months after the accident,

the National Transportation Safety Board holds preliminary hearings.

During a break, the Campbells take matters into their own hands.

They remove several boxes full of files.

So we quickly realized we'd 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 in earnest at that stage.

The unpublished documents reveal a disturbing catalog of problems

with the 747's forward cargo door,

going right back to its original design.

Passenger doors are plug doors, but most cargo doors on jets open outward.

This increases the space for luggage and other cargo.

As the plane gains altitude, the pressure inside the jet

presses outwards against the door.

To prevent the door from opening,

Boeing had installed what it believed was a foolproof locking system.

What they do is they build in multiple redundancies

to make sure the door is properly latched

and does not open. And you build it into a point of law

that is extremely improbable that the door would ever open.

The Campbell's research uncovers two major flaws

with the 747 cargo doors.

The first involved the locking system,

To lock the cargo doors,

electric motors turn C-shaped latches around pins in the doorframe.

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 supposedly foolproof system had failed.

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

It showed the first deadly flaw in the locking system.

Aluminum locking sectors could not hold the C-latch in place

if the latches started to open on their own.

With the aluminum locking sectors,

if the C-locks tried to backwind 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.

But what would cause the C-lock to backwind?

The Campbells didn't have the answer,

but they knew they were onto something.

During their research, they learned that two years before Flight 811,

a Pan Am 747 out of Heathrow had pressurization problems

as it climbed to cruising altitude.

The pilot was forced to turn back.

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.

The passengers on this flight were lucky.

They had survived the faulty locking system.

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

As the Campbells continue 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 placed in the closed position,

a master lock switch should disconnect the power supply.

This would 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 locks started to move

and it started to force the locking sectors out of the way.

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 know

that you're right, and had been all along.

And the support that they had given you was, you know, was vindicated.

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.

After United Flight 811,

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

Inspections were increased. Another potential cause

of explosive decompression had been found and eliminated.

That is an amazing accolade to what we've learned, not just Boeing,

but what we've learned about maintenance,

about structures, maintaining them and inspecting them.

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.

But more than 15 years after United 811,

another deadly lesson is learned.

Helios five-two-two, do you read? Over.

It's August the 14th, 2005.

For almost an hour,

Helios Flight 522 has been circling the skies over Athens.

Helios 522, over.

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.

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.

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.

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.

So we climbed over the hill, and there we were. You know,

facing this situation, which was beyond any description.

I saw

a great area in front of me, which was burning. It was black,

burning, people spread, pieces of the airplane.

The autopsies add more mystery to the case.

Everyone on board the Helios flight was alive up to the moment of the crash.

He did not die from inhaling a toxic substance

in the airplane or from an explosion.

These people died on impact.

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.

Before it took off on its last flight,

the Helios jet arrived in Cyprus with a problem.

During the trip, the cabin crew had heard loud banging

and saw ice on a rear service door.

To make sure there's nothing wrong with the seal on the door,

the 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.

The jet's engines are turned off, so the engineer uses the plane's

auxiliary power unit to force air into the cabin.

Just 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.

A normally well-maintained jetliner of any age

is simply not going to be completely airtight.

You're going to have leaks. As a matter of fact, as pilots,

we know that

certain airplanes are going to leak more than others,

and you've really got to crank the pressurization up.

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.

If the flight crew fails to see that the switch is on manual,

their plane won't properly pressurize.

The oxygen available inside the plane

will be just as thin as the outside atmosphere.

The passengers will be directly exposed

to a deadly environment in which they cannot survive.

August the 14th, 2005.

The worst airline disaster in Greek history has stunned the nation.

Investigators are sifting through the gruesome wreckage.

A few minutes after 9:00 a.m.

Helios Flight 522 left from Cyprus bound for Athens.

The crew has no idea that hours before takeoff during a maintenance test,

a flight engineer has left a pressurization switch set to manual.

Both the captain and co-pilot miss 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 takeoff config warning?

It's a non-pressurization warning.

but it sounds identical to another alarm.

The pilots confuse the two.

It's a critical mistake.

The alarm sounded.

And that alarm was misinterpreted.

Most of

the flight crew, they will never

face an alarm

with non-pressurization

in all their flight career because it's a rare event.

Operations, this is Flight 522, over.

Flight 522, what can I do for you?

We have a takeoff config warning on.

Sorry, can you repeat?

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 is

there's simply too much up there.

And if you have things popping out,

they're going to hit switches that they shouldn't hit.

The crew don't realize they have a pressurization problem.

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

The oxygen is too thin to breathe.

We're the ones that

should be trained consistently to understand

that the ears popping, anything that indicates pressurization,

you don't even talk to each other before you grab that mask and put it on.

The passengers are unaware that the plane

is now flying itself.

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 survive until the pilots

get the airplane down to a low altitude.

But with both pilots already unconscious,

the Helios jet did not descend so passengers could breathe without assistance.

Instead, the plane flew on autopilot to Athens.

When the oxygen supply stopped, the passengers passed out.

By the time the Greek Airforce intercepted the Helios jet,

only one person was still moving.

Likely surviving with bottled oxygen,

flight attendant Andreas Prodromou

was still conscious when the fighters approached.

He made it to the cockpit, but he couldn't save the plane.

Athinai control.

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

Eventually, when its fuel ran out,

Helios 522 crashed.

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?

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,

fifty years in which the industry has learned, sometimes painfully,

how to safely fly more than ten kilometers in the sky.

When you look back at all the other accidents over the last 20 years,

in most cases, we were pushing the frontier of knowledge.

Unfortunately, when you're pushing the envelope,

you're pushing the boundaries of design.

You're going to encounter problems that you hadn't anticipated.

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.