All language subtitles for Mayday.S04E05.Hidden.Danger.1080p.AMZN.WEB-DL.DD+2.0.H.264-playWEB_track3_[eng]

The Boeing 737 is
one of the most popular

passenger jets in the world.

Around the globe,

the plane has carried
more than 12-billion passengers.

It's the backbone
of the aviation industry.

But in 1991, something happened
onboard a 737 that sent

shudders through the world
of aviation.

Oh my lord.

A deadly crash has
investigators scrambling.

There was a time

when I had doubts that we'd
be able to solve it.

It was like he was
tracking a serial killer.

The hunt for answers

will take ten
long and grueling years.

The fate of the airline
industry hangs in the balance.

And the mystery isn't solved,

until more than 1-hundred
and 50 people are dead.

9:40 AM,
March the 3rd 1991.

After a short 17-minute trip
from Denver,

United Airlines Flight 585
is on final approach

into Colorado Springs.

It looks like a perfect
day for flying.

But there's trouble in the air.

Nice looking day.

Hard to believe
the skies are unfriendly.

There's been heavy
turbulence during the flight,

and violent gusts of wind are
forecast over Colorado Springs.

Never driven to Colorado
Springs and not gotten sick.

At the controls
is 52-year-old

Captain Harold Green,

a pilot with
20 years' experience

and a sterling reputation.

Green's co-pilot
is Patricia Eidson.

At 42, she is one of the first
female flight officers

in United's history.

Flight attendants
prepare for landing.

At Colorado Springs
Municipal Airport,

air traffic controller
James Rayfield

is ready to bring Flight 585 in.

United 585 report
the airport in sight.

Got it?
Yup.

Airport in sight
United 585.

Lower landing gear.

United 585 is cleared for
a visual approach to runway 35.

Weather conditions, wind 320
degrees at 16, gusting at 29.

As its speed
decreases,

flight 585 becomes more
vulnerable to the turbulence.

Eidson wants to know
what other planes have

experienced on landing.

Any reports lately
of any loss or gain of airspeed?

Yes ma'am. Five hundred
feet a fifty knot loss,

at four hundred feet
a fifteen knot gain

and at a hundred and fifty feet
a gain of twenty knots.

Sounds adventurous.
Thank you.

Starting on down.

Less than three
kilometers from the airport,

retired policeman Harold Darnell

is on his way to
a local flea market.

A kilometer overhead,
Green and Eidson

focus on keeping their speed
constant as they descend.

We had a
ten knot change here.

Yeah I know. Awful lot
of power to hold that airspeed.

As United 585
approaches the runway,

Darnell feels something strange.

Whoa!
What the heck was that?

Out of nowhere,
a powerful gust of wind

strikes his vehicle,
almost blowing him off the road.

Another ten knot gain.
Thirty flaps.

From
the control tower,

James Rayfield can now see
flight 585's final approach.

As the aircraft
closes in on the airport,

the ride gets even bumpier.

Wow.
We're at a thousand feet.

Then, without warning,

the 737 starts to spin
out of control.

Oh god. Flip!

Fifteen flaps.
Fifteen.

Oh no

Oh my god!
Oh my god! Oh my god!

Crash! Crash!

Rescue workers
arrive within minutes -

but there is almost no
sign of the 737.

The shattered remains
of the 38-ton jet

lie buried in a fire-blackened
impact crater.

The plane didn't,
it didn't skate or bounce,

you know like, like when a plane
comes in normally and lands?

It just nosed right in and where
it hit is where it stayed.

And I came down
here and this is

when I saw all of the
huh it was horrible.

There are
no survivors.

All 20 passengers
and 5 crew members are killed

instantly by the high-speed
impact and exploding jet fuel.

In 10 violent seconds, Colorado
Springs has become the site

of one of the most mysterious
air crashes in aviation history.

By nightfall, investigators
from the National Transportation

Safety Board
descend on Colorado Springs.

Known to insiders as tin
kickers, NTSB investigators

examine over 2-thousand aviation
accidents a year,

at times by picking through the
metal debris of fallen aircraft.

While coroners mark the location
of human remains in red,

NTSB investigators mark
scraps of metal in yellow,

looking for clues to help them
solve the mystery of flight 585.

Like investigating
a mass murder,

it's a tough job
walking onto a crash site.

Among the investigators
assigned to the case is

Malcolm Brenner, a specialist
in human performance.

His job will be to find out

if the crash was
caused by pilot error.

The area was
cordoned off by police

and there were
Salvation Army trucks.

I got a cup of coffee, a cup
of hot chocolate or something

and I thanked them for it

and they said no,
no thank you and they had this

look in their eyes like my god
you have to go into this site.

Clues to the fate
of United 585 lie

mangled in a deep, black hole.

The fuselage is crushed like an
accordion in the impact crater.

The rest of the plane
is in pieces spread over an area

smaller than a football field.

There was
a lot of fire damage.

There'd been a fire afterwards.

And it was all contained in a
relatively small area which,

just initial impression,

it can be a sign that
the airplane was intact.

If there was a midair
explosion or something

came off the airplane,

you would expect that
to be a much larger site.

My first sense that it was
going to take some time to

investigate the accident was the
damage that we saw to the parts.

When they're burnt and broken,
the process always takes longer.

But with their work
just beginning, members of

the NTSB have no clue that the
case of United 585 will become

the longest crash investigation
in aviation history.

Fifteen flaps.
Fifteen.

A clear,
late-winter day in 1991

turned deadly in
just ten short seconds.

Moments from landing in Colorado
Springs, United Airlines

flight 585 fell out of the sky

at 3-hundred and 70 kilometers
per hour.

Crash! Crash!

All twenty passengers
and five crew members died.

The National Transportation
Safety Board begins

a painstaking
investigation into the crash.

Engine turbines,
hydraulic pressure gauges,

the cockpit voice recorder

and in-flight data recorder
are all carefully extracted

from the site, photographed, and
sent to the lab for analysis.

An important first
step in the investigation is

the analysis of the cockpit
voice recorder.

With pilot error a factor
in seventy percent of air

disasters, Malcolm Brenner's job
is to see what role pilots

Green and Eidson
played in the crash.

This crew was,
and I felt this at the time,

was one of the more impressive
crews I had ever dealt with.

There was a little
bit of tension release,

a little bit of humour.

The captain said...

Never driven to Colorado
Springs and not gotten sick.

The first officer
suggested that they add

extra speed as a safety margin.

The captain agreed with it.
It was good interaction.

Got it?
Yup.

Airport in sight
United 585.

Lower landing gear.

This was a sense
of an excellent crew

caught randomly, if anything.

So again that was my first
impression is that this

would be consistent
more with a hardware situation.

As more about flight
585 becomes known,

mechanical failure becomes
a serious suspect.

Just seconds before it crashed,

the plane rolled onto its back
and spun wildly out of control.

Investigators wonder
if the sudden motion was

caused by the plane losing
an engine or a wing.

From the state of
the aircraft onsite,

it's clear that it was
intact at the time of the crash.

What investigators don't know is

if the engines were still
working at impact.

Technicians examine
the engine turbines.

They discover dirt has been
drawn deeply into every crevice.

These blades were
clearly spinning

and sucking in air
at the time of impact.

The engines may have
been running

but technicians aren't
sure how well.

The plane's hydraulic pressure
dials are destroyed,

their glass covers broken.

The indicator needles
have been snapped off

by the force of the impact.

But even these ravaged
dials tell a tale.

Fifteen flaps.
Fifteen.

On close inspection,

investigators find
a critical mark.

At the moment of impact,

a dent was made on the faceplate
by the jarred indicator needle.

It proves that
when United 585 crashed,

its engines were running
normally.

With engine failure ruled out,

there seems to be only one
other mechanical explanation

for why 585 suddenly rolled over
and then fell out of the sky.

It appears increasingly likely
that the plane had suffered

a catastrophic problem
with its flight controls.

Investigators quickly become
suspicious of the rudder

at the back of the tail.

We focused in after
eliminating other flight

control surfaces that we thought
could contribute to the roll.

We started
looking at the rudder.

Bring that up
so I can take a look at it.

Investigators
begin their examination

of the rudder on site but the
violence of the crash makes

the job extremely difficult.

Almost nothing left.

Most of
the plane's parts are

too crushed or burned
for testing.

But a vital component is
still reasonably intact:

the power control unit or PCU.

Used constantly during flight,
especially during landings,

the PCU performs like
a car's power steering.

When the pilot pushes
on a rudder pedal the PCU uses

hydraulic fluid to convert
gentle movements of a pilot's

foot into the pressure needed to
move the 737's enormous rudder.

The heart of the PCU

is something called
the dual servo valve.

Shaped like a soda can,
it has two slides,

which glide past one another,
directing the flow

of pressurized hydraulic fluid
that moves the rudder.

The servo valve is,
is very unique.

That it is in
effect two valves in one

and that that creates a whole
range of interactions that

don't occur in a, in a more
conventional hydraulic valve.

When a technician
opens the power control unit,

chips of metal are found
floating in the hydraulic fluid.

It's a disturbing find.

These particles could cause
the servo-valve to jam,

making it impossible to work
the plane's rudder.

It's a chilling prospect.

Could a microscopic fault
bring down a 38-ton jet?

It's difficult
for Phillips to tell.

While more intact than much
of the wreck,

the PCU and dual servo
valve are both damaged.

The airplane crashed

and burnt in a pretty confined
area and there was a lot

of damage to the flight control
components and the things

we were testing and needed
to test and look, look at.

And the more damaged
the components are,

the harder it
is to take measurements

and do functional testing.

To test what
he does have,

Phillips travels
to California to the labs

of Parker Hannifin where
the rudder control unit is made.

The curious metal chips
floating in the PCU's

chambers are dismissed.

Phillips is told that filters
keep them out of the delicate

servo valve that directs fluid
and moves the rudder.

Nothing else is found that
could explain any sudden

movement of the rudder
on flight 585.

We didn't have any
absolute indication or

information that we could
point to that said the rudder

valve control unit,
the servo valve or any,

any part of that flight control
system caused that accident.

Phillips still
suspects a mechanical problem,

but with no conclusive evidence
that the PCU or servo valve

caused the crash, he is forced
to sign off on the tests.

It's a pass.

With Phillips
at a dead end,

only bad mountain weather
remains as a primary suspect.

An expert on weather-related
aviation accidents,

Greg Salotollo is
trying to determine

if heavy winds on the day
of the crash were a factor.

There is a history
of events where there've been

airplane accidents, attributed
to mountain waves and rotors.

In 1966,
a BOAC 707 near Mount Fuji

disintegrated in a mountain wave
and rotor.

High winds crashing
over mountain peaks leave

so-called wind
rotors in the lee side.

Invisible, highly turbulent
downdrafts that come

plunging down with
devastating power

and are extremely
dangerous to aircraft.

We found
a great deal of evidence,

looking at the surface
and upper air data and talking

to witnesses in the area that
rotors were a possibility.

Salotollo hears
several eyewitness

reports of bizarre mountain
weather on the day of the crash.

One of the most intriguing is
from Harold Darnell...

Oh, what the heck
was that?

...whose truck was
struck by a powerful

gust of wind just
moments before 585 crashed.

But as Greg Salottolo
combs through his evidence,

the theory that a wind
rotor knocked

the plane from the sky is
getting less and less likely.

Nice looking day.

Hard to believe
the skies are unfriendly.

Wind rotors are areas
of extremely low barometric

pressure, so if flight 585 did
pass through one,

its altimeter reading
would have spiked

as the plane was
blown suddenly upwards.

There was no
evidence that we saw

that on the flight
data recorder of 585.

What the flight
recorder did show was a fast

and deadly drop in altitude
as the plane fell to earth.

It's been 21 months
since the investigation

into the crash
of United Flight 585 began.

Almost two years, in which
the NTSB has studied the crew,

the weather, the rudder,

and thousands of other
pieces of evidence.

They've come up empty handed.

For only the fourth
time in its history,

the NTSB releases a report which
doesn't reach a conclusion.

The cause of the crash
of flight 585 is undetermined.

We had put a lot of time
and effort in into the

investigation and we just
weren't sure what had happened.

It was like he was
tracking a serial killer.

He was frustrated that they
had not solved 585.

He did not want
that to happen again.

But, almost 2 years
after the report

on 585 is released,
the killer strikes again.

At 7 pm on a clear,
windless day,

US Air Flight 427 is
nearing Pittsburg.

Captain Peter Germano
and First Officer Chuck Emmett

are getting
ready for their final approach.

Folks,
from the flight deck,

we should be on the ground in
'bout ten more minutes.

Sunny skies, little hazy.

Flight attendants please
prepare for landing.

I ask you to check the security
of your seatbelts. Thank you.

US Air 309, descend
and maintain 6000.

As they close
in on the airport,

the pilots are on the lookout
for another

flight about 10 kilometers
ahead of them.

Looking for the traffic.

Turning one zero zero US Air
four twenty-seven.

I see zuh jet stream.

As they pass
through the turbulence

left behind by the other flight,
their jet suddenly

and alarmingly rolls left.

Hold on, hold on.
Shoot!

Nothing the pilots do
seems to have any effect.

What the hell is this?
What the hell?

Shoot! 427 emergency!

Oh shoot!
Pull! Pull!

Oh god no! No!

Rescue crews
arrive quickly,

but the fate of flight 427 is
tragically clear.

There is no hope for the 132
passengers and crew.

The human carnage is so bad

authorities declare
the crash site a biohazard.

US Air 427 accident was
the first US accident where

biohazard suits were used
and it made it more difficult.

They were uncomfortable,
they were hot and to this day

when I put on a pair of rubber
gloves for any reason I'm

instantly transformed
back to the site in Pittsburgh.

Captain John Cox,
a 737 pilot and flight systems

specialist with the Airline
Pilots Association, is asked

to join the team investigating
the crash of Flight 427.

As coroners attempt to collect
human remains,

NTSB lead investigator
Tom Haueter already

knows his hunt for clues will
be long and painstaking.

When we first arrived
to the crash site, uh, well,

first of all,
there was no aircraft there.

There were only bits
and pieces of the airplane.

It wasn't really
recognizable as an airplane.

With the help
of eyewitnesses,

information from the flight data
recorder and the cockpit voice

recorder, investigators begin
to quickly see some striking

similarities between 427 and the
unsolved case of United 585.

In fact, they seem to be
mirror images of each other.

On final approach,
United 585 rolls right,

while US Air 427
rolls to the left.

Both crews are caught
by surprise.

And after just a few
terrifying seconds,

both aircraft plummet
straight into the ground.

Certainly, the whole team
was aware of the previous

accident with United
585 in Colorado Springs.

We tried to keep
that in the back of our minds

and take a look at this one
as to what it presents to us.

As the investigation
continues,

the list of similarities grows:
427's engines were also

attached and functioning
at the time of impact.

But for all the similarities,

there is one important
difference.

Unlike United 585, as US Air
427 approached Pittsburg

weather conditions
were dead calm.

Folks,
from the flight deck

we should be on the ground in
'bout ten more minutes.

Sunny skies, a little hazy.

Taking a look at the flight
data recorder information,

the aircraft's approaching
Pittsburgh.

It's an extremely smooth night.

Uh there's just no turbulence at
all. Uh the pilots are relaxed.

They're talking
about the landing.

Flight attendants
please prepare for landing.

Ask you to check the security
of your seatbelts. Thank you.

As he did
in the case of United 585,

Greg Phillips will
once again lead

the investigation into the
mechanical aspects of the crash.

Almost immediately,
he makes a promising discovery.

Miraculously,
much of US Air 427's tail

and rudder appear intact.

The hydraulic devices
inside the tail

have also sustained
very little damage.

Phillips and Haueter
prepare to send the parts to the

manufacturer Parker Hannifin
for testing as soon as possible.

They need answers.

Pressure on the NTSB to solve
the accident is growing quickly.

Not only are the crashes
of flights 585 and 427

disturbingly similar,
both of them

involve the same kind
of airplane-- a Boeing 737.

But with serious questions
being raised

about the plane's safety,
billions of dollars

and perhaps the airline
industry itself, are at risk.

We couldn't live
with the fact as investigators

of having two
unsolved 737 accidents.

The airplane is in too much use,
too wide a use around the world.

It carries too many
people every day.

Unsolved was not
an acceptable answer.

If 427 was
an undetermined accident,

we could not find
the cause of this accident,

there was a great
chance that there was a,

if there was a third accident
with a 737 fleet under similar

circumstances that the 737 fleet
would have been grounded.

Careful with that.

The investigation
into the crash of flight 427

may be the most important
in the history of the NTSB.

But it won't be easy.
Answers are still years away.

In just three years,
two Boeing 737s

have crashed in the United
States with no survivors.

Airport in sight
United 585.

In both cases,

the planes were
moments from landing.

Flight attendants,
please prepare for landing.

And in both cases, the
planes were under full power,

but the pilots were
unable to control their jets.

Pull! Pull!

In the crash of United
flight 585,

NTSB investigators were
unable to find a cause.

Now, they're searching for clues
into the second crash -

US Air flight 427.

As the investigator in
charge, I never allowed myself

to think this investigation
could go undetermined.

Just we kept on pushing
and kept on researching.

As long as we had things
to research we kept on going.

To find their killer,

the NTSB can't afford to rule
anything out.

From the possibility that
a collision with birds

brought 427 down to strange,
even bizarre theories.

They looked
at electromagnetic interference.

They got calls from people

saying it might be
Russian death rays.

They considered everything.

There were a couple
of witnesses who gave reports of

the aircraft suddenly descending
and hovering before it blew up.

We discounted those.

But the
investigation's primary

suspect is the dual servo valve,

part of the power control unit
that moves the 737's rudder

and a suspect in the crash
of United 585.

Parker Hannifin made the valve.

At it's lab in California,
investigators

look inside the main cavity of
the US Air power control unit.

Just like in the earlier crash,

they find tiny chips of metal
floating in the hydraulic fluid.

But once again, Parker
and Boeing repeat their claim.

Filters designed to stop any
debris from interfering

with the delicate metal
slides have done their job.

Investigator Greg Phillips
wants to be absolutely sure.

If the chips were blocking
the slides, they would have left

tiny scratch marks behind where
they rubbed against the metal.

But Phillips can't find any.
Another pass. Okay.

Phillips has
technicians put the servo

valve from flight 427 through -

as many tests as he can think
of trying to find a weakness.

If he can find one,

it could explain why two planes
were ripped from the sky.

But he comes up empty.

That, that unit passed
all its operational tests.

There wasn't any indication
that it had failed and it,

it operated within the
parameters we expected it to.

Once again,
the investigators are forced

to shift their focus
back to the pilots.

By studying the plane's
flight data recorder,

investigators know that the
jet's rudder was deployed

fully to one side,
what's called rudder hard-over.

We were definitely
focused on a rudder,

on hard over rudder, full rudder
input for about 20 seconds.

It can be caused
either by hardware,

something unknown
in the hardware

or it can be caused
by pilot input.

First Officer
Chuck Emmett, who was

flying 427 did indeed step
down hard on his rudder and then

held it there while the plane
plummeted toward the earth.

It raised a grizzly question,

was he trying to fly
the plane into the ground?

In looking at this and,
being a pilot myself,

it's like this doesn't
seem like rational behaviour.

What the hell is this?

Human performance
specialist Malcolm Brenner

listens closely for evidence
on the cockpit voice recorder.

What the...? No. Shoot.

In this case, they had
microphones right by their

mouths and you can hear as well
as in real life, or better.

You can hear breathing sounds.

Ooh ya.
I see zuh jet stream.

The cockpit recordings
indicate that flight 427's

troubles began at the moment it
flew through the jet wake

of a Delta Airlines 727 that had
just passed in front of them.

Both pilots
are startled by the wake.

I see zuh jet stream.

The first officer
breaks off

at the end of a sentence.

I see the jet stream. Zuh.

And there's no more discussion

of the jet stream
or anything else.

They both focus.
Something happened here.

Captain says sheez.
Sheez.

It was such a smooth
flight that it was a momentary

jolt that they just
hadn't anticipated.

And, with that, the, the pilots
got on the controls

and immediately, you know,
put in a rudder input.

Sheez.

The cockpit recorder
even records the thumping sound

of the jet stream turbulence
as 427 flies through it.

As flight 427 encounters
the turbulence,

Brenner hears something unusual.

First Officer Emmett
begins to grunt.

The grunting
is unusual.

The controls are designed so
that pilots don't need to grunt.

They're specially designed
around human capabilities,

so to have someone
grunting is typically

a sign of an emergency.

By matching
data from the flight

recorder with the crew's voices,

Brenner is able to confirm
that Emmett's grunts begin

a split second after he pushed
down on the rudder pedal and

3 to 4 seconds after the wake
turbulence affected flight 427.

What the...? Help!

On their own,

the cockpit voice recordings
prove very little.

But it seems clear
that the crew

weren't trying
to crash their plane.

Something happened which
took them by surprise.

They reacted as quickly
as they could,

but nothing
they did seemed to help.

What the hell is this?

It's been almost two years
since the crash of flight 427,

and the investigation
has stalled.

Now, two 737s have gone
down in startlingly similar ways

and investigators still
don't know why.

We were all frustrated
as months wore into years.

What were we missing?

It definitely took
a toll in their personal lives.

They worked incredibly
long hours.

They never stopped
thinking about it.

We were going
up against an aircraft

that had an incredible
safety history.

It was really everything you
could see for 30 years

this had been a great airplane.

Um we were trying to prove
that there was

something wrong with
the straight A student.

It clearly
was on his mind.

He at one point had a nightmare
about it, where he dreamed

that he was in front of a
congressional committee that

was grilling him on, now there
had been a third 737 crash.

And, in Tom's dream, all
the cameras were pointing to him

and a congressman asked,
why didn't you ground the fleet?

Unsure of where
to look next,

and with the trail of evidence
getting colder,

investigators need
a break in the case, and fast.

Eastwind 517,
you're clear for landing.

On June 9th 1996,

they finally get the break
they've been looking for.

Captain Brian Bishop is on final
approach to Richmond Virginia

when, without warning,

his Eastwind jet rolls
sharply to the right.

We didn't
know to what extent,

but we knew we had
a problem with the rudder.

I turned the yoke
the opposite direction

and stood on the opposite
rudder pedal.

The pedal didn't move for me.

For over 30 seconds,
the 737 flies in a precarious

right bank as Bishop fights
to keep it from rolling over.

Then suddenly, the unknown
forces holding the jet let go,

snapping the wings
back to horizontal.

In a matter of seconds,
it released itself

and went back to normal.

We had started the checklist.

Almost before I could
finish the sentence,

all of a sudden there
was just a wham.

For a second time,
the 737 is pushed on its side.

For 30 harrowing seconds, the
737 takes on a life of its own.

Then, once again, as quickly
as it began, the rollover stops.

After the second time,
I looked at the first officer

and I said,
declare an emergency.

Tell the controller
we have flight control problems.

As they slow down to
land, the risks increase.

If a third rollover occurs,

they won't have enough
airspeed to recover.

I did, at some point,
tell my first officer

to look out the window
and find a dark spot.

It was nighttime
and we were looking to avoid

a neighbourhood or a populated
area and he very calmly

responded that hey
here's a spot over here.

But there is
no third rollover.

Bishop brings Flight 517 in high
and fast, and lands safely.

Taxiing in is when
I realized my legs were shaking.

We got the aircraft to the gate

and I did pick up the PA to make
an announcement to try

to explain what had just
happened to these people.

And on picking up the microphone
I realized there was

nothing I could say to make
this any better.

And probably for the first
time in a long time

I was at a loss of words.

So I simply put the microphone
down and let it go at that.

But Bishop won't
remain speechless for long.

By the next day,

the investigation team has
arrived in Richmond.

Flight data
recorder was undamaged,

the airplane undamaged.

We launched to the scene.

The airplane literally
didn't move,

stayed at its location at the
airport 'til we got down there.

There were a lot of FAA,
a lot of NTSB

and they all
wanted to talk to us very badly.

It gave the NTSB
a tremendous break

because suddenly
they had a 737

that had had a rudder incident,

that was intact, and
they had a pilot who was alive

and who could talk about it.

I think they were
much happier

to have the airplane than me.

I think the airplane
probably gave them

more to research
than I ever could.

NTSB investigators
quickly determine that what

happened onboard Eastwind
flight 517 is alarmingly

similar to events
on flights 427 and 585.

If they can discover why
Bishop's 737 rolled over,

they may be able to crack two
mysterious and fatal accidents.

And when we
said what happened?

They said there was something
wrong with the rudder pedal.

The pedal wouldn't go down.

I was standing on
the rudder pedal

and I couldn't get it
to go down.

My god!

With Bishop's first
person testimony, investigators

immediately zero in on
Eastwind's rudder controls.

The power control unit
is removed, inspected

and then tested again and again.

To the frustration of everyone,
the unit performs perfectly.

We tested that
aircraft as is. It was intact.

We went through it completely,
did flight tests with it.

And it passed all tests.

After a five-year hunt
for clues, a third mysterious

rudder event on a 737 and a live
pilot as a witness, Tom Haueter

still lacks the evidence
he needs to crack his case.

He decides to push
his chief suspect,

Flight 427's rudder controls,
a little harder.

One fellow mentioned a test

they had done in the military
of a thermal shock where

if you had the actuator being
very cold and put in very hot

hydraulic fluid, it would cause
it to react in strange ways.

So we put together
the thermal shock test.

And this test was extreme,
to say the least.

On August 26th 1996,

in Valencia California,
NTSB investigators

gather to watch the torture
test of US Air 427's PCU.

After soaking it in dry ice,

the PCU is blasted with nitrogen
gas to simulate

the minus 40-degree temperatures
at 10-thousand meters.

Then, it's quickly injected with
super-heated hydraulic fluid

and given the command
to start working.

As we were standing there
listening to the actuator

move left and right,
left and right it stopped,

and it was not
commanded to stop.

It just jammed, stopped
working completely. Take a look.

Systems investigator
Greg Phillips now

asks that the valve be taken
apart and scanned for scratches.

They find none.

Look at that.
Doesn't leave a trace.

It is
a crucial breakthrough

to solving an
almost perfect crime.

Not only have they proven
that the valve which controls

the rudders can jam,
no evidence is left behind.

Tom Haueter and his team have
now found that a small

hydraulic valve that controls
the rudder of the world's most

popular commercial jetliner can
jam in the right circumstances.

It's an ominous discovery
but incredibly, there's another

shocking surprise in store
for the investigators.

October the 15th,
1996.

For the last five years
the National Transportation

Safety Board has been struggling
to crack its toughest case--

two completely separate
but seemingly linked accidents.

The crash of United 585 killed
25 passengers and crew.

Three years later, the
US Air Flight 427 disaster took

another 1-hundred and 32 lives.

Now, after examining
hundreds of clues,

investigators have made
a surprising discovery.

A new test has revealed that
under the right circumstances,

the hydraulic valve that moves
the 737's rudder can jam.

It just jammed.

But the surprises
aren't over.

The most important
breakthrough came

when a Boeing engineer,

examining the data from that
test, discovered some numbers

that indicated the valve at that
point had actually reversed.

Whoa. Sheez.

It is
a stunning revelation.

Not only can
the servo-valve jam,

but it can then
function in reverse.

It means that anytime
a pilot tries to correct

a rollover by pushing
on the rudder, the rudder might

turn in the opposite direction,
causing a fatal accident.

And the reversal
is like driving your car.

You turn to the right,
it goes left.

You're not gonna figure
out this failure mode

until you go off the road.

And, in these cases, that's what
the pilots were faced with,

something so unusual

that they didn't understand
what was happening.

What the hell is this?

They had evidence
now that the valve was unique,

that the valve not only could
jam, but would reverse.

427 emergency.

That would explain
why the first officer,

Chuck Emmett, would keep
his foot on the rudder pedal,

because he's thinking why
isn't the plane going right?

And he's feeling
the plane go to the left.

To the very end,
Chuck Emmett pushes hard,

hoping his rudder will help him
pull out of his deadly spiral.

Tragically, he has no
way of knowing that he's

steering his aircraft
straight into the ground.

Never driven to Colorado
Springs and not gotten sick.

Flight attendants
prepare for landing.

Satisfied that
they've determined

the cause of the crash
of US Air 427,

the NTSB turns its attention to
the unsolved case of United 585.

Another ten knot gain.

Going
back to Colorado Springs,

you could follow a progression
of what the captain was doing.

He's close to the ground
and suddenly under rudder

reversal he puts in a little
bit of pedal.

The pedal violently
pushes his leg back.

Oh god, flip.
Fifteen flaps.

Fifteen.

Rudder reversal
certainly fits what

I know about this crew
and how it fits.

We were able to show
the failure mode.

It matched the flight data
recorder for each aircraft.

It fit like a glove.

So we now had
a lot more information

we could apply
to United Flight 585 and,

based on that, we redid
the accident report.

Oh my god, oh my god!

From rudder
reversal to impact took

less than ten seconds.

585's flight crew had no chance

to save their
plane or passengers.

In the aftermath
of the investigation,

sweeping changes were
made to improve

the safety of the 737 and
the entire aviation industry.

New training protocols were
designed to help pilots

react to unusual
in-flight events.

In the 737 fleet, pilots
are now trained on how to react

to both rudder hard-overs
and reversals.

The scenario
of the US Air 427 accident,

if the crew had the information
that we have today I believe

they would have landed safely
in Pittsburgh that evening.

The FAA also directed
Boeing to redesign the rudder's

dual servo valve to eliminate
the potential for reversal.

Boeing spent hundreds
of millions of dollars to

replace the valves on thousands
of 737s around the world.

One thing we don't
like at the Safety Board

is to have an undetermined
accident,

because then we can't make
a change to improve safety.

So out of US Air 427,
United 585,

we have a much safer 737 fleet.

It took NTSB tin
kickers ten years to solve

the mysterious crashes
of Flights 585 and 427,

the longest investigation
in aviation history.

There are still some
people in aviation who

don't think the NTSB
got it right.

But I became convinced after
talking to many, many,

many people, pilots, engineers,
people at Boeing uh

and spending a lot of time

with the investigators
that they did get it right.

Since the replacement
of the 737 servo valves,

there hasn't been a similar
crash of the most popular,

most profitable
plane in the world.