All language subtitles for Air.Disasters.S07E05.Hidden.Danger.720p.AMZN

narrator: 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.

[passengers screaming]

but in 1991, something happened
on board a 737

that sent shudders
through the world of aviation.

man: oh, my lord.

narrator: a deadly crash
has investigators scrambling.

man: there was a time
when i had doubts

that we'd be able to solve it.

man: it was like he was
tracking a serial killer.

narrator: the hunt for answers

will take ten long,
grueling years.

the fate of the airline industry
hangs in the balance.

and the mystery isn't solved

until more than 150 people
are dead.

flight attendant:
ladies and gentlemen,
we are starting our approach.

pilot: we lost both engines!

flight attendant:
put the mask over your nose.

emergency descent.

pilot: mayday, mayday.

flight attendant:
brace for impact!

controller: i think i lost one.

man: investigation starting...

man: he's gonna crash!

narrator: 9:40 a.m.,
march 3, 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.

harold green: nice-looking day.

hard to believe the skies
are unfriendly.

narrator: there's been heavy
turbulence during the flight,

and violent gusts of wind are
forecast over colorado springs.

green: never driven
to colorado springs

and not gotten sick.

narrator: 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.

green: flight attendants,
prepare for landing.

narrator: at colorado springs
municipal airport,

air traffic controller
james rayfield

is ready to bring flight 585 in.

james rayfield: united 585,
report the airport in sight.

patricia eidson: got it?

green: yup.

eidson: airport in sight,
united 585.

green: lower landing gear.

rayfield: united 585 is cleared

for a visual approach
to runway 35.

weather conditions,

wind 320 degrees at 16,
gusting to 29.

narrator:
as its speed decreases,

flight 585 becomes more
vulnerable to the turbulence.

eidson wants to know

what other planes
have experienced on landing.

eidson: any reports lately of
any loss or gain of airspeed?

rayfield: yes, ma'am.

500 feet, a 15-knot loss,

at 400 feet, a 15-knot gain,

and at 150 feet
a gain of 20 knots.

eidson: sounds adventurous.
thank you.

green: starting on down.

narrator:
just a mile from the airport,

retired policeman harold darnell

is on his way
to a local flea market.

half a mile overhead,
green and eidson focus

on keeping their speed constant
as they descend.

eidson: we had
a 10-knot change here.

green: yeah, i know.

awful lot of power
to hold that airspeed.

narrator: as united 585
approaches the runway,

darnell feels something strange.

harold darnell: whoa!
what the heck was that?

narrator: out of nowhere,
a powerful gust of wind

strikes his vehicle,
almost blowing him off the road.

eidson: another 10-knot gain.

green: 30 flaps.

narrator:
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.

eidson: wow.

we're at 1,000 feet.

narrator: then, without warning,

the 737 starts to spin
out of control.

eidson: oh, god. flip!

[passengers screaming]

green: 15 flaps!
eidson: 15!

green: oh, no.

eidson: oh, my god!

[screaming]

oh, my god!

oh, my god!

[screaming]

[explosion]

darnell: oh, my lord!

rayfield: crash! crash!

narrator: 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.

man: the plane didn't, it didn't
skate or bounce, you know,

like when a plane comes in
normally and lands?

it just nosed right in,

and where it hit
is where it stayed.

narrator:
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
2,000 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.

malcolm brenner: 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.

narrator: 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.

brenner:
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.

greg phillips:
my first sense that it was
going to take some time

to investigate the accident

was the damage that we saw
in the parts.

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

narrator: the national
transportation safety board

begins a painstaking
investigation into the crash.

engine turbines,
hydraulic pressure gauges,

the cockpit voice recorder
and inflight data recorder

are all carefully extracted
from the site, photographed,

and sent to the lab
for analysis.

an important step
in the investigation

is the analysis of
the cockpit voice recorder.

with pilot error a factor
in 70% of air disasters,

malcolm brenner's job
is to see what role

pilots green and eidson
played in the crash.

brenner: 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 humor.

the captain said...

green:
never driven to colorado springs
and not gotten sick.

brenner:
the first officer suggested

that they add extra speed
as a safety margin.

the captain agreed with it.

it was good interaction.

eidson: got it?

green: yup.

eidson: airport in sight,
united 585.

green: lower landing gear.

brenner: 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.

narrator: 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 on site,

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.

green: 15 flaps!
eidson: 15!

narrator: 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 flight 585 crashed,

its engines
were running normally.

with engine failure ruled out,

there seems to be only one other
mechanical explanation

for why flight 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.

phillips: we focused in

after eliminating other
flight control surfaces

that we thought could
contribute to the roll.

we started looking
into the rudder.

bring that up
so i can take a look at it.

narrator: investigators
begin their examination

of the rudder on site.

but the violence of the crash

makes the job
extremely difficult.

phillips: almost nothing left.

narrator:
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.

bill adair: the servo valve
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 more
conventional hydraulic valve.

narrator: 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.

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.

phillips: we didn't have
any absolute indication

or information
that we could point to

that said
the rudder power control unit,

the servo valve or any, any part
of that flight control system

caused that accident.

narrator: 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.

phillips: it's a pass.

narrator:
with phillips at a dead end,

only bad mountain weather
remains 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.

greg salotollo:
there is a history of events

where there've been
airplane accidents

attributed to mountain waves
or rotors.

in 1966, a boac 707
near mount fuji disintegrated

in a mountain wave and rotor.

narrator: 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.

salotollo: we found
a great deal of evidence,

looking at the surface,
upper air data

and talking to witnesses
in the area

that rotors were a possibility.

darnell: an explosion.
it was right over there.

narrator: salotollo hears
several eyewitness reports

of bizarre mountain weather
on the day of the crash.

one of the most intriguing
is from harold darnell...

darnell: oh, what the heck
was that?

narrator:
...whose truck was struck
by a powerful gust of wind

just moments before 585 crashed.

but as greg salotollo
combs through his evidence,

the theory that a wind rotor
knocked the plane from the sky

is getting less and less likely.

green: nice-looking day.

hard to believe
the skies are unfriendly.

narrator: 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.

salotollo: there was no evidence
that we saw that

on the flight data recorder
of 585.

narrator:
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.

phillips: we had put a lot
of time and effort in

into the investigation,

and we just weren't sure
what had happened.

adair: 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.

narrator: but almost two years

after the report on 585
is released,

the killer strikes again.

at 7 p.m. on a clear,
windless day,

u.s. air flight 427
is nearing pittsburgh.

captain peter germano
and first officer chuck emmett

are getting ready
for their final approach.

chuck emmett:
folks, from the flight deck,

we should be on the ground
in about ten more minutes.

sunny skies, a little hazy.

flight attendants,
please prepare for landing.

i ask you to check the security
of your seatbelts.

thank you.

controller: u.s. air 309,
descend and maintain 6,000.

narrator: as they close in
on the airport,

the pilots are on the lookout
for another flight

about six miles ahead of them.

peter germano:
looking for the traffic.

turning 1-0-0, u.s. air 427.

emmett:
i see zuh jet stream.

narrator: as they pass
through the turbulence

left behind the other flight,

their jet suddenly
and alarmingly rolls left.

emmett: hold on, hold on.

hold on.

germano: shoot!

narrator: nothing the pilots do
seems to have any effect.

[screaming]

germano: what the hell
is this?

what the hell?

shoot!

427, emergency!

[screaming]

emmett: oh, shoot!

germano: pull! pull!

emmett: oh, god, no!

no!

[screaming]

narrator:
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.

john cox: u.s. air 427 accident
was the first u.s. 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.

narrator: captain john cox,

a 737 pilot and
flight systems specialist

with the air line 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.

tom haueter: when we first
arrived at the crash site,

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.

narrator: 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 u.s. air 427 rolls
to the left.

both crews are caught
by surprise.

and after just a few
terrifying seconds,

both aircraft plummet
straight to the ground.

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 u.s. air 427
approached pittsburgh,

weather conditions
were dead calm.

emmett: folks,
from the flight deck

we should be on the ground
in about ten more minutes.

sunny skies, a little hazy.

narrator: 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
u.s. 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,
untold lives

and perhaps the airline industry
itself are at risk.

cox: 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.

narrator: to find their killer,

the ntsb can't afford
to rule anything out...

from the possibility
that a collision with birds

brought flight 427 down

to strange,
even bizarre theories.

adair: they looked at
electromagnetic interference.

they got calls from people

saying it might be
russian death rays.

they considered everything.

haueter: there were a couple
of witnesses who gave reports

of the aircraft suddenly
descending and hovering

before it blew up.

we discounted those.

narrator:
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 its lab in california,

investigators look
inside the main cavity

of the u.s. 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.

phillips: another pass.

okay.

narrator:
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.

phillips: that unit passed
all its operational tests.

there wasn't any indication
that it had failed,

and it operated within the
parameters we expected it to.

narrator: 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.

brenner: 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.

narrator:
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 grisly question--

was he trying to fly the plane
into the ground?

haueter: in looking at this,
and being a pilot myself,

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

germano: what the hell is this?

narrator: human performance
specialist malcolm brenner

listens closely for evidence
on the cockpit voice recorder.

germano: what the...no!

shoot.

brenner: 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.

emmett: hmm, yeah.

i see zuh jet stream.

narrator:
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.

brenner: both pilots
are startled by the wake.

emmett: i see zuh jet stream.

brenner:
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."

germano: sheez.

haueter: it was
such a smooth flight

that it was a momentary jolt

that they just hadn't
anticipated.

and, with that, the pilots
got on the controls

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

germano: sheez.

narrator: 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.

[grunts]

brenner: 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.

narrator: 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.

germano: what the...

help!

narrator: on their own,

the cockpit voice recordings
prove very little.

but it seems clear that the crew

wasn't trying to crash
their plane.

something happened
which took them by surprise.

but nothing they did
seemed to help.

germano: what the hell is this?

narrator:
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.

cox: we were all frustrated
as months wore into years.

what were we missing?

haueter: we were going
up against an aircraft

that had an incredible
safety history.

it was really...everything
you could see for 30 years,

this has been a great airplane.

we were trying to prove
that there was something wrong

with the straight-a student.

narrator:
unsure of where to look next,

and with the trail of evidence
getting colder,

investigators need a break
in the case, and fast.

controller: eastwind 517,
you're clear for landing.

narrator: on june 9, 1996,

captain brian bishop
is on final approach

to richmond, virginia,
when, without warning,

his eastwind jet rolls
sharply to the right.

brian bishop: 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.

narrator: 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.

bishop: 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.

narrator:
for a second time, the 737
is pushed on its side again.

for 30 more harrowing seconds,

the 737 takes on
a life of its own.

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

bishop: after the second time,
i looked at the first officer

and i said declare an emergency.

tell the controller we have
flight control problems.

[sirens]

narrator: as they slow down
to land, the risks increase.

if a third rollover occurs,

they won't have enough airspeed
to recover.

bishop: 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 neighborhood

or a populated area,

and he very calmly responded

that hey, here's a spot
over here.

narrator: but there is
no third rollover.

bishop brings flight 517 in
high and fast and lands safely.

bishop: taxiing in is when
i realized my legs were shaking.

we got the aircraft to the gate,

and i did pick up the p.a.
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.

narrator: but bishop won't
remain speechless for long.

by the next day,

the investigation team
has arrived in richmond.

bishop: there were a lot of faa,
a lot of ntsb,

and they all wanted
to talk to us very badly.

adair: 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.

narrator: ntsb investigators
quickly determine

that what happened
on board 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.

brenner: 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!

[passengers screaming]

narrator: 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.

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.

haueter: one fellow mentioned

a test they had done
in the military

of a thermal shock,

where if you had the actuator
be very cold

and put in very hot
hydraulic fluid,

it would cause it to react
in strange ways.

so we put together
a thermal shock test.

and this test was extreme,
to say the least.

narrator: on august 26, 1996,
in valencia, california,

ntsb investigators gather
to watch the torture test

of u.s. 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 32,000 feet.

then it's quickly injected with
super-heated hydraulic fluid

and given the command
to start working.

cox: as we were standing there
listening to the actuator

move left and right,
left and right...

it stopped, and it was not
commanded to stop.

narrator: systems investigator
greg phillips now asks

that the valve be taken apart
and scanned for scratches.

they find none.

phillips: look at that.

doesn't leave a trace.

narrator:
it is a crucial breakthrough to
solving an almost perfect crime.

they have proven that the valve
which controls the rudders

can jam, but 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.

adair: 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.

germano: whoa. sheez.

narrator:
it is a stunning revelation.

not only can
the servo valve jam,

but it can then function
in reverse.

it means that any time a pilot
tries to correct a rollover

by pushing on the rudder,

the rudder might turn
in the opposite direction,

causing a fatal accident.

haueter: 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.

germano: what the hell is this?

adair: they had evidence now
that the valve was unique,

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

germano: 427, emergency!

adair: 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.

narrator: 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.

[screaming]

green: never driven
to colorado springs

and not gotten sick.

flight attendants,
prepare for landing.

narrator: satisfied that they've
determined the cause

of the crash of u.s. air 427,

the ntsb turns its attention to
the unsolved case of united 585.

eidson: another 10-knot gain.

brenner: 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.

eidson: oh, god, flip.

green: 15 flaps!
eidson: 15!

brenner: rudder reversal
certainly fits what i know

about this crew
and how it fits.

haueter: we were able to show
the failure mode.

it matched the flight data
recorder from each aircraft.

it fit like a glove.

so we now had a lot more
information we could apply

to united 585,

and, based on that, we redid
the accident report.

eidson: oh, my god!
oh, my god!

narrator:
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 inflight events.

in the 737 fleet,

pilots are now trained
on how to react

to both rudder hard-overs
and reversals.

cox: the scenario
of the u.s. air 427 accident,

if the crew had the information
that we have today,

i believe they would have
landed safely

in pittsburgh that evening.

narrator:
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.

haueter: one thing we don't like
at the safety board

is to have
an undetermined accident,

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

so out of us air 427,
united 585,

we have a much safer 737 fleet.

narrator: it took
ntsb tin kickers ten years

to solve the mysterious crashes
of flights 585 and 427,

the longest investigation
in aviation history.

adair: 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,

and spending a lot of time
with the investigators

that they did get it right.

narrator: 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.