All language subtitles for Air.Disasters.S20E15.Mountain.Impact.1080p.SMIT.WEB-DL.AAC2.0.H.264-maldini_track3_[eng]

[dramatic music]

NARRATOR: A passenger
jet that's supposed

to be flying south
of Kathmandu crashes

into a mountain north of it.

Terrain.

Pull up.

It's false, it's false.

Oh, my god!

[dramatic music]

NARRATOR: Investigators
are baffled.

It immediately
created some mysteries

as to why the airplane got to
a place that was not expected.

Air China 1-2-9, are
you able to land?

NARRATOR: A plane that should
be high above a mountain

slams into it instead.

Ah!

[dramatic music]

NARRATOR: The
investigation uncovers

a shocking series of mistakes.

Why didn't they turn here?

But they just
keep going, and then

they crash into the mountain.

NARRATOR: And a jetliner goes
down in a high altitude forest.

The crash reveals a
technological blind spot

that could have
prevented the tragedy.

The common thread
is a combination

of loss of situational
awareness and lack

of understanding of where you
are in relation to the terrain.

Ladies and gentlemen, we
are starting our approach.

We lost both engines.

Emergency responding.

Mayday, mayday.

Brace for impact!

[chattering]

[theme music]

NARRATOR: Thai
Airways Flight 311

cruises over Nepal
just above the peaks

of the mighty Himalayas.

The pilot flying is
Captain Preeda Suttimai.

His first officer is
Phunthat Boonyayej.

Combined, they have close
to 30,000 flying hours.

I will never get tired
of seeing these mountains.

Kathmandu is a popular
tourist destination.

It's a mountainous airport,
and a lot of tourists

fly in just for that,
for the scenery.

NARRATOR: The
flight's 99 passengers

left Bangkok, Thailand, and
are bound for Nepal's capital

Kathmandu on an Airbus A310.

The approach requires navigating
above some of the world's

highest mountains
and then making

a steep descent to the runway.

Kathmandu, Thai 311.

Request descent.

Thai 3-11, you have
negative traffic.

You are cleared for descent.

Thai 3-11, request 0-2.

Thai 3-11, confirm
requesting 0-2.

Confirm.

NARRATOR: The
pilots are expecting

to make the usual
straight-in approach

from the south to runway 0-2.

That will keep them clear of the
mountains north of the airport.

But then--

Kathmandu, Thai 3-11.

We cannot make a approach now.

We will need to
turn back to Romeo

and climb to one, 8,000 feet,
and start our approach again.

NARRATOR: They realize they're
too close to the runway

to safely commence the descent.

The pilots will
need to circle back

to the south of the airport
and make a second approach.

[dramatic music]

You would be challenged
at doing a circling approach

in a mountainous environment.

It's one thing to do it
without any obstacles.

It's another thing to do
it surrounded by mountains.

NARRATOR: Using the
flight management system,

the first officer enters the
navigational waypoint called

Romeo to restart their approach.

But the system won't lock
in a flight path to Romeo.

It disappeared.

It's Romeo 27, isn't it?

Row me oh.

NARRATOR: The pilots keep trying
to program the Romeo waypoint.

Thick clouds limit their
view from the cockpit.

Then--

Pull up.

Airspeed low.

Sink rate.

Pull up.

Turn back, turn back.

It's false, it's false.

Pull up.

Obstacle, obstacle.

Pull up.

Oh, my god!

[dramatic music]

Thai 3-11, please
report your position.

NARRATOR: The search for
Thai Airways Flight 311

begins immediately.

Nepal sets up a royal
commission of local and

international investigators.

This is the airport.

Flight 3-11 was coming
in from the south here.

Until the airplane is found,
it's difficult to figure

out what might have happened.

And the search, of course,
started in the south

because it came
in from the south.

Where is this thing?

[sighs]

NARRATOR: As the
search continues,

investigators interview
the air traffic controller,

hoping for any kind of lead.

We don't have radar
here, so can't help you

with the airplane's location.

I just rely on what
the pilots tell me.

The nonradar environment,
they have a mental picture.

And that picture is really
painted by the words

that the pilot reports to them.

They don't have
an exact location.

Almost like working blindfolded.

Kathmandu, Thai 3-11.

We cannot make approach now.

We will turn back to Romeo
and climb to one, 8,000 feet,

to start our approach again.

To redo the approach,
they would have

to turn back towards the south.

NARRATOR: 48 hours
after Flight 311

disappeared, villagers report
finding aircraft debris

to the north of Kathmandu.

The reported crash
site is nowhere near

the area they'd been searching
south of the airport.

[dramatic music]

North of Kathmandu, the
Himalayan peaks soar

to a height of 20,000 feet.

For that reason,
almost all planes

approach the airport from the
south, where the mountains

are closer to 8,000 feet.

Send out the helicopters.

Tell them to start searching
to the north of the airport.

NARRATOR: Later that day,
27 miles north of Kathmandu,

search crews find the remains
of Thai Airways Flight 311.

The point of impact
is a steep rock

face more than 11,000 feet up
the side of a remote mountain.

None of the 113 people
on board survived.

It immediately
created some mysteries

as to what might
have happened or why

the airplane got to a place
that was not expected.

The level of
destruction was enormous.

You couldn't tell that
you had an Airbus A310.

You couldn't even tell
you had two engines.

NARRATOR: The location of the
crash puzzles investigators.

The plane should never have been
this far north of the runway.

So why was it there?

[dramatic music]

It's hard to tell what
we're even looking at here.

NARRATOR: Incredibly, the team
is able to retrieve the plane's

flight data recorder and
cockpit voice recorder

from the fragments of debris.

Investigators hope
they'll provide

crucial evidence that will
reveal how the plane ended up

so far off course.

OK.

Start it up.

NARRATOR: While the
FDR is being processed,

investigators analyze the
cockpit voice recorder.

Lots of clouds.

Thai 3-11, Kathmandu.

Go ahead.

Would you request for
runway 0-2, please?

Thai 3-11, request 0-2.

Everything seems
normal until they begin

to restart their approach.

Thai 3-11, report 2-5 DME.

Wait a minute.

Kathmandu to Thai 3-11.

Confirm runway 0-2 available.

Wait a minute.

Stop.

NARRATOR: The pilot
talking to the controller

is not the first officer.

It's the captain.

Why was the captain speaking
with the controller?

NARRATOR: As the pilot flying,
the captain should have focused

solely on flying the
plane, not communicating

with air traffic control.

In this case, I noticed that
the captain was, in many cases,

taking over the radio
transmission work.

NARRATOR: The CVR reveals
another surprising detail.

Dammit.

The flaps cannot be extended.

[beeping]

The flaps would not extend
to full configuration,

which for the Kathmandu
approach, is essential.

NARRATOR: The recording
leads investigators to ask

some troubling questions.

Did malfunctioning flaps
and a distracted pilot

send the plane into
the path of a mountain?

[dramatic music]

Investigators listening to the
cockpit voice recorder for Thai

Airways Flight 311 learned
that the pilots were concerned

with an issue with their flaps.

Dammit.

The flaps cannot be extended.

[beeping]

NARRATOR: But just as suddenly
as the problem with the flaps

began, it resolves itself.

Back to normal now.

Can we make a left
turn to Romeo?

Understand.

Operation normal.

And you'd like to
make an approach?

Affirm, affirm.

NARRATOR: But the delay
in extending the flaps

meant that the pilot couldn't
descend toward the airport

in time to make a safe landing.

We can't land at this time.

We have to make a left
turn back to Romeo

and start our approach again.

NARRATOR: But other air traffic
coming in from the south

prevented Flight 311
from turning left.

Thai 3-11, we have negative
traffic at or above flight

level 1-5-0 within
our jurisdiction

except Royal Nepal at 2-0-6.

You are cleared for descent.

NARRATOR: Then something
perplexing happens.

The controller goes silent.

Answer, please.

Answer, please.

NARRATOR: Flying
north toward mountains

and with traffic to his left,
the captain makes a decision--

without waiting
for ATC clearance.

We'll climb and
turn to the right.

NARRATOR: He begins
turning right,

back toward the start of his
approach south of the airport.

Moments later, investigators
hear a strange question

from the first officer.

Are we going north?

NARRATOR: The recording exposes
a troubling inconsistency.

It seems the captain
intended to turn

south, away from the mountains.

Yet minutes later,
they're still flying

north toward the mountains.

Terrain, terrain.

Pull up.

Turn back, turn back.

It's false, it's false.

Pull up.

Pull up.

Oh, my god!

[explosion]

Is that the data from the FDR?

Great.

NARRATOR: Fortunately, the
flight data has been recovered.

And investigators have their
first chance to analyze

the plane's flight path.

What the team finds
raises more questions.

And right here, they're
looping around to restart

their approach, right?

All right.

But they just kept turning.

NARRATOR: Instead of
turning 180 degrees,

straightening out, and
heading south to Romeo,

Flight 311 turns
a full 360 degrees

and continues heading north.

It just doesn't
make any sense.

Fire it up.

NARRATOR: In search
of answers, they

turn to a flight simulator.

The captain just took
over communications

with the controller.

Yeah, OK.

In terms of simulator
testing, the one thing

that it allows the
investigators to do

is replicate what
happened and understand

what the crew would see
or any other problems

that they would face.

Kathmandu, Thai 3-11.

Confirm runway 0-2 available.

NARRATOR: At this point, they
know the captain is flying

the aircraft, monitoring
the instruments,

and communicating
with the controller,

all on one of the steepest, most
difficult descents in aviation.

OK, there is a lot
going on here right now.

[beeping]

Dammit.

The flaps cannot be extended.

NARRATOR: The time spent
resolving the flaps

issue creates a delay.

They can no longer make
the straight-in landing.

Only choice is to circle
back and do another approach.

NARRATOR: The
captain can program

the flight computer to take the
plane to a selected heading.

In this case, that means
turning to the left,

but there's a problem.

This controller said
there's traffic to the left.

NARRATOR: To avoid the
traffic, the captain decides

to adjust the autopilot.

We'll climb and
turn to the right.

NARRATOR: To make
the turn, he rotates

the heading knob to the right.

[dramatic music]

Watch your turn.
You're starting to level off.

- Oh.
- Here, right.

He's trying to do too
many things at once.

He's asking to go to a point.

He's trying to fly the
airplane on the autopilot.

And he's starting to lose
the 3D picture that he

has in his head, where he is.

NARRATOR: Investigators
make a critical discovery.

The distracted pilot
makes a fatal mistake.

He turned it one too many
times and did a full circle.

NARRATOR: Investigators now
understand how the crew, facing

nearly zero visibility
and thick clouds,

had no idea they
were actually turned

back toward the mountains.

Pilots are trained to fly
using only their instruments.

Why did these pilots fail
to notice their mistake?

Take a look at this.

There are no cardinal
points on the compass.

NARRATOR: A close look
at the cockpit compass

reveals a possible explanation.

The instrument lacks
the usual direction

markers, N, S, E, W,
indicating north, south, east,

and west headings.

I think if there had been
a big N on top of the compass

for the whole time, it might
have made a difference.

Are we going north?

We will turn back soon.

And then, only many
miles north of the airport,

does the first officer
say, hey, my display

says I'm north of the airport.

But by then, it's too late.

Terrain, terrain.

Turn back, turn back.

Ah!

No!

NARRATOR: The accident
underscores the need

for more advanced air
traffic control technology

at Kathmandu's airport.

In an environment like
Kathmandu, radar was essential.

And radar was put into
Kathmandu so this-- this

would not happen again.

NARRATOR: The disaster also
reinforces the importance

of teamwork among pilots.

I'm flying the airplane.

You're doing this.

You communicate with me.

I communicate with you.

And we, as a team, maximize
to a greater extent

than the individual capability
everything we can do,

including knowing
situational awareness

and escaping when we have to.

NARRATOR: 10 years
later, a plane

crashes into a mountain it
should have flown high above--

Must go around!
Pull up!

Pull up!

NARRATOR: --in a tragedy that is
a simple matter of bad timing.

No!

Terrain, terrain.

Pull up.

[dramatic music]

NARRATOR: Air China Flight
129 has been in the air

for almost two hours.

Captain Wu Xinlu heads
the Chinese flight crew.

He has more than
6,000 flying hours.

18 degrees, dew point 1-6.

NARRATOR: First Officer Gao
Lijie has more than 1,200

hours flying the Boeing 767.

Expect radar vectors
to left downwind.

NARRATOR: The most
junior member of the team

is Second Officer Hou Xiangning.

There was a third pilot in
the cockpit, a second officer,

who was doing the radio calls.

And that was because of
his proficiency in English.

NARRATOR: Flight 120
is flying southeast

from Beijing to
Gimhae International

Airport in Busan, South Korea.

It should be on the ground
in less than 30 minutes.

Good morning.

With you now.

NARRATOR: 15 minutes
before landing,

the crew contacts the
airport's approach controller.

Air China 1-2-9,
Gimhae approach.

Fly heading 1-9-0.

Descend to 6,000.

In this case, the copilot
started the approach.

He was actually
the pilot flying.

The captain was
really monitoring

his altitude because the
captain is a nonflying pilot.

Air China 1-2-9, turn
left, heading 1-6-0.

Descend to 2,600.

Visibility is not very good.

NARRATOR: Gimhae
Airport is near Korea's

southern coast, an area known
for unpredictable weather.

Visibility is spotty
as the passenger jet

descends through thick clouds.

I don't think there
was anything unique

about this particular day.

There were clouds.

There was rain.

Runway in sight.

NARRATOR: But strong winds
are making the current landing

approach difficult.
So the controller

gives Flight 129 new
landing instructions.

Air China 1-2-9,
Contact Tower 118.1.

Circle west.

Circling approach
is a visual approach.

The pilot has to maintain sight
of the runway the entire time.

The wind is too strong.

It's very hard to fly.

NARRATOR: The thick
clouds obscure

the tower controller's view.

Cleared to land Runway 1-8.

Reduce speed.

OK.

NARRATOR: The controller
can't see the incoming 767.

Air China 1-2-9,
are you able to land?

Let's go around!

Pull up!

Pull up!

Ah!

[dramatic music]

[screaming]

NARRATOR: Air China
Flight 129 crashes

into the side of
Mount Dotdae just

three miles from the runway.

Of the 166 people on
board, only 37 survive.

[non-english speech]

The crash site was horrible.

It was truly a
miracle that they were

able to find a
number of survivors

from such a massive crash.

NARRATOR: Korean Air
accident investigators

arrive on the scene along
with American investigators

from the National
Transportation Safety Board.

You could tell the whole
area had been cleared out

by the impact of
the whole aircraft,

and pieces were spread
all over the place.

I was surprised that
anybody survived.

Let me borrow those.

NARRATOR: From the crash
site, investigators

can see the airport
nearly three miles away.

There is no simple
explanation why Flight 129 hit

terrain so far from the runway.

What are they doing
here, when they should

be all the way down there?

What would cause them to
get this low this far out?

He should have been
well above this terrain.

You wondered, why did
this guy hit a hill?

NARRATOR: Investigators
concentrate

on what the crash site can
tell them about the accident.

Debris is scattered for hundreds
of yards down the hillside.

[non-english speech]

Normally, when
an incident occurs,

the wreckage and debris
tend to stay within the area

of their initial impact point.

NARRATOR: The positioning
of the fuselage

gives the team its first clue.

The back of the
plane must have landed

first, meaning the
pilot was pulling

up as the plane hit the ground.

NARRATOR: This suggests the
crew made a last-ditch effort

to clear the mountain.

Investigators are faced
with a troubling question.

In the midst of a
routine landing,

why did the pilots fly so low?

[dramatic music]

To understand why Air
China Flight 129 crashed

into the side of a mountain,
puzzled investigators

look at the airliner's
speed, altitude,

and heading parameters obtained
from the flight data recorder.

Around here, they should
be circling in for a landing.

But they just keep going.

Then they crash
into the mountain.

Why didn't they turn here?

How were they supposed
to do this approach?

NARRATOR: The landing procedure
for a circling approach

to Busan requires pilots to
use a cockpit chronometer,

a type of stopwatch.

Flight crews time maneuvers
to the precise second

using headings and
visual landmarks

as references, a challenging
task in thick clouds.

So gear down here.

Then they're supposed to do a
45-degree turn for 20 seconds

here.

Then they're supposed to make
a 45-degree turn for 20 seconds

here.

You time it out 20 seconds.

You turn parallel to
the runway, and then you

make what's called a base
turn, a 180-degree turn,

and land on the runway.

Here, they should see
the end of the runway.

20 seconds later, they should
have made their base turn.

NARRATOR: Investigators
realize the pilot

got off to a bad start
making his approach.

The flight recorder
showed that the pilot

did not execute his first turn.

He's supposed to make
a left turn to begin

the circling approach,
and he's supposed

to do that aggressively.

He didn't turn with
a steep enough bang.

What were the pilots thinking?

Let's start from the very
beginning of their approach.

NARRATOR: The team hopes
the cockpit voice recorder

will answer that question.

Air China 1-2-9,
you may approach.

Fly heading 1-9-0.

Descend to 6,000.

Roger.

Up till now, they think
they're coming in here.

But the controller
changes it to here.

NARRATOR: The crew's plan
for a straight-in approach

is changed to a
circling approach

that will take them to the
other side of the airport.

We are using Runway 1-8 right.

So they immediately
acknowledged

their circling approach.

NARRATOR: But it soon
becomes clear that they

missed a critical step.

So we exit on
this side taxiway?

What's it called?

Charlie 6.

After we land, we
can leave directly

using taxiway Charlie 6.

Stop.

They're talking about
what to do after they land.

But they never did
a proper briefing

for the actual
landing, let alone

what they had to do to
execute their final approach.

When that didn't happen,
somebody is supposed to speak

up and say, hey, Captain
or First Officer,

we didn't do approach briefing.

That's crew coordination.

NARRATOR: The missed briefing
helps explain why the crew

made the mistake of
turning much less

than the required 45 degrees.

Timing.

Wait, where's my stopwatch?

NARRATOR: A circling approach
demands precise timing

of the turning maneuvers.

Here's where they
start the timer.

Timing.

NARRATOR: The captain
must begin his final turn

in exactly 20 seconds.

They had to fly the aircraft
away from the airport, and then

a turn onto final, and then
land in the opposite direction.

The wind is too strong.

It's very hard to fly.

I have control.

NARRATOR: Investigators are
stunned by what they hear.

In the middle of
the 20-second timer,

he decides to take control.

That's crazy.

It confused the first officer,
made the division of duties

uncertain.

They didn't know what person
was supposed to do what.

NARRATOR: A critical mistake--

the captain fails to
tell the first officer

to monitor the chronometer.

The U-turn back to
the runway should

begin at the 20-second mark.

But with the captain now flying,
and the first officer still

looking through the
clouds for the runway,

they don't initiate the turn.

Do you have the
runway in sight?

No, I can't see out.

Turn!

Turn now!

Then why he didn't say,
let's get out of here,

let's miss the approach,
we don't understand that.

NARRATOR: The captain
continues to descend

for almost 30 seconds before
the first officer finally

speaks up.

Must go around!

Pull up!

Pull up!

Ah!

NARRATOR: But it's too late.

No!

[screaming]

Pull up!

Terrain, terrain.

Terrain, terrain.

Pull up.

[dramatic music]

This is a botched approach.

You never fly into a cloud
on a circling approach.

If you do, you should be
initiating a go around

to get out of that situation.

NARRATOR: Why did these
experienced pilots

attempt a visual approach in
the worst possible conditions?

The cause of this accident
are basically human factors.

There was poor
crew coordination.

There was poor communications
intracockpit and between

the tower and the cockpit.

NARRATOR: In the aftermath
of the Flight 129 disaster,

Air China classifies Gimhae
as a special airport.

All pilots are now
taught how the area's

challenging mountainous terrain
can affect takeoffs, landings,

and go arounds.

If you're worried about
visibility, if you're worried

about the mountainous terrain,
don't do a circling approach

unless you're absolutely
sure you can keep the runway

environment in sight.

You have to be on
your best game when

you fly in mountainous terrain.

NARRATOR: But when a jet
goes down in the mountains

of Alsace, investigators observe
how even the most sophisticated

navigational equipment
can't guarantee

disasters will be averted.

[dramatic music]

Air Inter Flight 148 has just
taken off from Lyon, France.

1-2-4, decimal 9-0-5.

Thank you.

NARRATOR: Captain Christian
Hecquet and First Officer Joel

Cherubin are experienced
pilots with more than 12,000

flying hours between them.

The flight is a
short, one-hour hop

between Lyon, France
and Strasbourg

in the mountainous
Alsace region.

The French airline prides
itself on quick turnarounds

between cities.

And crews are encouraged
to avoid delays.

We were famous for our
very short turnaround.

And the faster we flew,
the better wages we got.

NARRATOR: But the pilots,
already under time pressures,

receive an unwelcome
communication

from Strasbourg Airport,
which changes their plans.

They are told to
land on Runway 0-5.

0-5?

No chance.

NARRATOR: Captain
Hecquet was hoping

to use Runway 23,
an approach that

provides the plane's autopilot
with a precise navigational

fix.

But the new runway, 0-5,
requires the aircraft

to circle around for landing.

A new approach could take
longer and affect flight time

and delay their turnaround.

If they had warned
us in advance-- cripes!

[sighs]

Get down fast.

I hear you.

NARRATOR: Picking up on
the captain's frustration,

the air traffic controller in
Strasbourg offers assistance.

I can take you with the radar
to lead you to ANDLO at 5,000.

NARRATOR: The controller will
guide the flight crew step

by step towards ANDLO, an
electronic waypoint on the

approach path to Runway 0-5.

That will help the pilots line
up for landing on that runway.

That's good.

Oh, yeah.

OK, then.

Turning left to heading.

2-3-0 degrees.

NARRATOR: Since Runway
0-5 doesn't allow

for a full autopilot
approach, the captain

must make his own calculation
for the angle of descent.

That makes three
decimal three degrees.

3.3 degrees is a normal
flight angle that provides

a good slope for landing.

[non-english speech]

NARRATOR: The controller
talks Flight 148

through the last turn, which
will align it with the runway,

now 15 miles away.

Authorized for
final approach 0-5.

NARRATOR: The captain configures
the plane for landing.

Flaps towards two.

Flaps towards two.

Flaps at two.

Gill down.

We have to watch our descent.

The approach axis.

NARRATOR: The first
officer notices a problem

with how the plane is lining up
horizontally with the runway.

It was 60.

Check it out.

Right away.

NARRATOR: But before the crew
can adjust their course--

[screaming]

the A320 has flown into
the side of a mountain.

The crash is catastrophic.

An emergency is declared
at Strasbourg Airport.

I immediately called my
two main investigators,

and we organized the go team.

NARRATOR: It takes rescuers
more than three hours

to get to the crash
site, which is located

12 miles from the runway.

Eight passengers have survived.

But 87 passengers and
crew, including the pilots,

have died.

[non-english speech]

A group of international
accident investigators

joins the French Accident
Investigation Bureau, the BEA,

in the search for clues
to explain the crash.

[dramatic music]

In the BEA lab,
technicians discover

that the flight data recorder
is completely destroyed.

They try to salvage data
from another recorder

used by maintenance crews.

It's known as a QAR.

It took about a day to
read a second of recording.

NARRATOR: Their
painstaking efforts

could take a month or more.

Any additional
second recovered

could reveal something that
would make a difference.

NARRATOR: In the meantime,
the focus of the investigation

shifts to the cockpit voice
recorder, which has survived.

Runway 23.

Otherwise, I can't make--

NARRATOR: The recording
reveals the captain's

anxiety early in the flight.

You are taking 23, then?

Yes!

0-5.

What sort of wind
are they giving us?

Turn left, steer 90.

NARRATOR: While they
listen, investigators

also plot the plane's trajectory
using radar information.

They discover that as
it circled the mountain,

the plane began a dangerously
steep and rapid descent.

Store at two.

NARRATOR: The voice recording
reveals a remark from the crew

about the speed of the descent.

It comes just 16 seconds
before the crash.

The aircraft was
accelerating abnormally.

We have to watch our descent.

The captain started to
realize there was something

wrong with the descent rate.

NARRATOR: But then the first
officer changes the subject.

The approach axis.

We're hitting the
axis a half point off.

There.

It was 60.
Check it out.

Where?

Where?

He refocused the
captain's attention

on the lateral
situation rather than

the vertical situation,
which was the main problem,

of course.

NARRATOR: But the
biggest mystery remains.

What caused the rapid
descent in the first place?

After months of work, all flight
data from the damaged recorder

becomes available.

The data reveals that
the angle of descent

was far greater
than the 3.3 degrees

calculated by the captain.

Three decimal three degrees.

That's quite a difference.

NARRATOR: As he
studies the data,

Perries discovers
an important clue

involving two key numbers--

the plane's vertical speed,
3,300 feet per minute,

and the intended flight
path angle, 3.3 degrees.

Coincidence?

NARRATOR: Perries uses a flight
simulator to test a new theory.

Can you show me a descent
of 3,300 feet per minute?

NARRATOR: He believes
that the similarity

is more than a coincidence.

On the autopilot, there
are two descent modes--

flight path angle
and vertical speed.

They are both displayed
on the same window.

So 3,300 is shortened to 33.

Now, show me a flight
angle of -3.3 degrees.

And the problem
on this aircraft

was that the two values were
visible on the same window

and controlled by the same knob.

NARRATOR: Perries suspects the
display confused an already

anxious Captain Hecquet.

The confusion is quite
easy between the two modes

if you don't do it carefully.

This is his.

NARRATOR: If the captain failed
to push the mode selector knob,

then entering 33 would not
have initiated a safe angle

of descent of 3.3 degrees.

Instead, it would
have put the plane

into a deadly rate of descent
of 3,300 feet per minute.

But when tested
in the simulator,

this theory still doesn't
result in a crash.

We are missing something.

Every approach would
overfly this obstacle

by a significant margin.

NARRATOR: To understand
why, Parries turns

to an airbus engineer, who
explains a little known element

of the autopilot's design.

In emergency situations where
the A320 needs to change

direction quickly,
the autopilot is

programmed to reverse
the plane's direction

at twice the normal rate.

We immediately went
back to the data at the--

the very second at
which the descent

was commanded by the crew.

Gear down.

NARRATOR: Parries discovers
a tragic coincidence.

We found that at this very
second, there was turbulence.

NARRATOR: The momentary
turbulence caused

the plane to climb slightly.

It was during that
second that the crew

commanded the plane to descend.

It was 60.
Check it out.

Where?

NARRATOR: The autopilot
read this as an emergency

requiring a rapid descent.

That could be it.

NARRATOR: Parries goes
back to the simulator

to test his theory.

Here it comes.

And we got a crash.

NARRATOR: Perries's
theory now explains

fully how the crew's confusion
with the autopilot display--

Three decimal three degrees.

NARRATOR: --caused the
plane to descend dangerously

close to the mountain.

Then, how turbulence,
an obscure safety

feature, and catastrophic
timing combined

to bring it even closer.

Half a second before,
half a second later, they

wouldn't have the accident.

The report will
list these causes--

flight deck--

NARRATOR: The investigators'
conclusions highlight

weaknesses in the Airbus
320 cockpit design.

Airbus responds immediately.

The main change, which
was very quickly made,

was to change the
display window.

NARRATOR: With the
new design, when

a pilot selects
a vertical speed,

the entire four-digit
number is displayed.

The confusion between an
angle and a vertical speed

was no longer possible.

If I hear there's a smoking
hole in the side of a mountain,

I'm almost certain to say, OK,
we've got a controlled flight

into terrain situation.

The common thread is a lack
of understanding of where you

are in relation to the terrain.

[dramatic music]