Original subtitles

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.