Original subtitles

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.