Original subtitles

NARRATOR: January 2008, the Boeing 777 is the gold standard

for commercial aviation.

For more than 10 years, the plane

has logged 2 million flights without a single major

accident, but less than 1,000 feet above London's Heathrow

Airport, all that changed.

Hey, I can't get power on the engines.

What do you mean?

It's not giving me power.

This can't be happening.

I thought if I don't do anything,

then everybody will die.

Mayday.

Mayday.

Speed bird, speed bird.

NARRATOR: Whatever brought down one of the world's most

technologically advanced airplanes

can't be found at the crash site.

And they had the pilots.

They had the passengers.

They had the aircraft.

They had everything there except the thing that caused it.

That had disappeared as if by magic.

WOMAN: Ladies and gentlemen we are starting on our coach--

We lost both engines.

WOMAN: Put the mask over your nose.

Emergency dispatch.

Mayday, mayday.

WOMAN: Brace for impact!

MAN:

MAN: He's gonna crash!

NARRATOR: British Airways Flight 38, a 10 and 1/2 hour

journey from Beijing to London.

Captain Peter Burkill has made this trip many times before.

I was operating that really a lot.

It was one of my favorite flights

because it was daylight.

You could see the views the whole way along the route.

NARRATOR: Burkill and first officer John

Coward both have thousands of hours flying

the 777 under their belts.

PETER BURKILL: It's a joy to fly.

She handles really well.

She's got modern equipment.

The computers are easy to use.

She's got the range, the ability to do short haul

as efficiently as long haul.

NARRATOR: For these two as for many pilots, a day in the job

consists of sitting in the cockpit for long hours

as computers fly the plane.

Powered by two massive Rolls Royce engines,

the Boeing 777 is one of the safest planes in the industry.

Nick Harris is flying back to London after a business trip

to China.

The good thing was that it seemed to me that we were

coming in early into Heathrow.

I was looking forward getting home to the family.

NARRATOR: Flight 38's destination

is Heathrow Airport, one of the busiest international airports

in the world.

Nestled in the southwest corner of London,

it's bordered by the A30 motorway.

People who live in the nearby neighborhood of Hounslow

are well accustomed to the sound of jets taking off and landing.

The area is tightly packed houses come into distant view

for British Airways Flight 38.

The autopilot is in control as the crew

lines up with runway 2-7 left.

As it nears the ground, the plane

is buffeted by some high winds.

Just a little turbulence.

NARRATOR: It's the first wrinkle in an otherwise smooth journey.

It's not me.

That's the auto throttles doing their thing.

PETER BURKILL: We were picking up wind gusts for about 20

to 30 knots, and we were fully aware that the auto throttles

would be able to cope with that so they'd be

moving up and down quite a lot.

You might want to keep the autopilot on

a little longer than usual to let the wind calm down.

Will do.

NARRATOR: In turbulent weather, the autopilot

can make faster adjustments than the pilots

to keep a plane level.

1,200 feet above the ground--

You have to turn that off for now, sir.

Sure.

Thank you.

NARRATOR: Two minutes before landing, Coward

takes over flying the plane.

You have control.

NARRATOR: Just as the two pilots had planned.

I have control.

PETER BURKILL: I had a few seconds to spare

before clearance to landing, and I was

just tidying up my map charts.

Speedbird 38, prepare to land 27 left.

Clear to land 2-7 left, speed bird 38.

PETER BURKILL: I remember looking out to see

if our gate was available.

500 feet.

500 feet.

Stable.

Well, sort of--

NARRATOR: Suddenly, there's a problem.

Hey, I can't get power on the engines.

It's not giving me power.

What's going on?

What do you mean?

PETER BURKILL: I was looking at the engine instruments,

and they didn't make sense to me because we were asking

for full power but the engine instruments were

not giving us any power at all.

What's going on?

NARRATOR: Flight 38 is crippled in the worst way imaginable.

It looks like we have double engine failure.

NARRATOR: With neither engine providing enough power,

the jet won't make it to the airport.

With 152 people on board, Flight 38

is falling toward London with only seconds

until it hits the ground.

PETER BURKILL: This can't be happening.

This is one of the most modern jets in the world.

NARRATOR: In the cabin there's no indication

that anything's gone wrong.

NICHOLAS HARRIS: On approach to land,

everybody seemed relaxed in the cabin.

We're just looking forward to landing.

Airspeed low.

Airspeed low.

Airspeed low.

Airspeed low.

PETER BURKILL: And now I was looking at impact point.

I could see a set of buildings around the hat and cross

area and a petrol station.

I just knew if we were hitting those,

then it was certain 100% fatalities.

That point I felt the weight of my four bars on my shoulder.

I am the captain, if I don't do anything,

then everybody will die.

NARRATOR: This close to the ground,

Burkill doesn't have many options.

He could take back control of the plane from Coward

but doesn't think that's the right move.

First decision really was to not take control.

I remember looking over at John.

He was still flying the plane well.

Good.

He's doing a good job.

He's doing what I need him to do.

NARRATOR: Burkill knows that even if they manage to clear

Hounslow, the jet could still smash

into the busy A30 motorway or the antennas

at the edge of the airport.

PETER BURKILL: I thought about raising the gear.

That's the biggest drag on on a commercial jet,

but I needed the gear to crash on.

We were going to impact, and that was going to take

the brunt of the impact.

So I had to leave the gear down.

NARRATOR: With a crash imminent, Burkill has only one move left,

but it comes with enormous risk.

PETER BURKILL: I knew who I had about 15 seconds

to make a huge decision.

I needed to get past those buildings.

It was obvious to me that I needed to raise the flaps.

NARRATOR: Retracting the flaps will reduce drag but also lift.

The plane will fly further but drop faster.

Burkill must determine which is more important.

I want her to discuss it with the crew,

but this was all in a matter of seconds.

I remember holding that lever for a fraction of a second.

NARRATOR: He hopes this move will help

his plane avoid catastrophe.

The effect was immediate.

NARRATOR: Now Burkill makes the announcement

every pilot dreads.

Mayday.

Mayday.

Speed bird, speed bird.

It was hard, very hard.

I remember the couple of impacts and the noise.

Good God.

The landing was hard.

Just a huge bang.

The noise, bits falling off the ceiling.

Then I also became a passenger because we were now

in an uncontrolled aeroplane.

We were sliding along the ground,

and I didn't know what we were going to hit next.

And then I thought about my wife and kids,

and I said goodbye to them.

The lights went out.

I could hear the wreckage breaking up.

And then we stopped.

When we actually stopped, I was surprised

that I was still there.

And then I look around the flight deck,

and I was more surprised to see that all three of us

were unscarred.

Everybody stay seated with your seatbelts fastened.

PETER BURKILL: And I quickly became the captain again.

NARRATOR: The crew switches off the fuel--

APU fire switch.

NARRATOR: Hydraulic and electrical systems--

Control switches, cut.

Checklist complete.

PETER BURKILL: I then had to look after my survivors.

And I thought 20% were dead because this aircraft was

broken up.

This is an emergency.

Evacuate.

Evacuate.

NICHOLAS HARRIS: The air stewardess

then took control completely.

Everybody unfasten your--

NICHOLAS HARRIS: As she opened the door, I smelt the fuel--

We should hurry.

NICHOLAS HARRIS: And realize that we

need to get off this aircraft.

NARRATOR: The big danger now is fire.

Having survived the landing, passengers

could be killed if flames sweep through the jet.

Jump.

Jump.

I left all my luggage and just got myself down the slide.

Jump.

Let's get out of here.

I went through the galley and checked the right hand aisle,

and I was surprised to see nobody there.

NARRATOR: Despite Burkhill's fears, there are no casualties.

Outside in the cool January air, the scale of the disaster

is clear.

The right wheel had actually been

ripped off during the impact.

The engines were-- were just sad.

They were ripped apart.

The cowlings were-- were ripped off,

and the engines were half buried.

It wasn't an aircraft anymore.

I now start to realize that we've been

involved in a major accident.

I saw the damage to the undercarriage and to the wing.

NARRATOR: The plane narrowly missed the A30 and the antennas

at the foot of the runway.

It could have been total devastation.

The impact had been on anything other than soft mud and grass,

the likelihood of a fire was immense.

NARRATOR: Given the amount of damage,

it's incredible that only 47 people were injured.

I felt quite lucky that I was still alive.

It was quite a surreal feeling.

NARRATOR: Investigators have some early hunches about what

caused the engines to fail.

They also have plenty of clues, but a key piece of evidence

is already missing.

NARRATOR: The crash of British Airways Flight 38

dominates the news.

It's the most serious accident at Heathrow in 30 years.

Heathrow Airport is a very busy airport.

It is the main airport in the UK,

so anything that happens in Heathrow Airport

immediately hits the media.

NARRATOR: More significantly, the 777 is one of the world's

most modern and reliable jets.

The accident unsettles the airline industry.

The 777 is as safe as an aeroplane can be.

It had such a superb safety record.

What on Earth happened?

You don't like not knowing whether it might happen

to the rest of your fleet.

NARRATOR: Investigators quickly arrive on the scene.

Phil Sleight is the lead engineer

for Britain's AAIB, the Air Accidents Investigation Branch.

PHILIP SLEIGHT: When you first approach an accident

site like this, the first thought you've got

is, where do I start?

The plane landed about 1,000 feet short of the runway.

The landing gear is pushed up through the wings.

The nose landing gear collapsed.

NARRATOR: Just hours after the crash, investigators interview

the crew.

Captain Peter Burkill has a firsthand account

of what happened.

I think I wanted to talk about it

because I knew of the innocence of the whole crew.

This isn't our fault. We didn't do anything wrong.

But the engines weren't giving us any more power.

Well, it looks like we have double engine failure.

We tried to give them more power manually,

but there was nothing.

NARRATOR: Burkill explains that something had caused both

of the jet's Rolls Royce engines to roll back at the same time,

robbing the aircraft of power.

PHILIP SLEIGHT: When you hear something

of a double engine failure, you start looking at what's common.

What could cause both engines to fail at the same time?

So our first thoughts were that it may have run out of fuel.

NARRATOR: A fuel shortage is rare,

but it has happened before.

In 1983, a Boeing 767 ran out of fuel

flying over central Canada.

A mistake in converting between and imperial measurements

left the plane with much less fuel than the crew thought.

At 26,000 feet, they lost power to both engines.

The captain was able to glide the plane to a safe landing

at an abandoned airbase.

In the case of Flight 38, this theory

is immediately questioned.

PHILIP SLEIGHT: Jet fuel has a certain odor.

Anyone who has worked with an aircraft

will know the distinctive smell of jet A1 fuel.

There was certainly a lot of fuel leaking from the aircraft

from the rupture to the bottom of the engines

when we arrived on site.

NARRATOR: A check of the plane's dipsticks

confirms two of the tanks did in fact have fuel.

PHILIP SLEIGHT: So we knew that there

was plenty of fuel on board this aircraft

to complete the flight.

NARRATOR: There remain several other possible explanations

for such a loss of power, and investigators

must tackle them all.

There is great confidence that the mystery can be solved

as any and every piece of evidence investigators

could ever want is readily available.

They have access to the pilot--

I have control.

NARRATOR: The crew and most importantly the entire plane.

PHILIP SLEIGHT: We were quite fortunate to have the amount

of data that we had on this aircraft

because the aircraft remained intact.

We were able to interrogate lots of computers

and also get lots of data from the recorders.

Additionally, we had data external to the aircraft

such as radar data and also the radio transmissions as well.

NARRATOR: Mark Ford retrieves the plane's flight data

recorders which promise to give Investigators important clues.

He has another resource, the QAR or quick access recorder.

While the black boxes are in the rear of the plane,

the QAR is a data recorder in the front

used mainly for diagnostics.

MARK FORD: It does have an advantage over the flight

recorder though in that it has a greater memory capacity

and can record additional parameters

over and above those available on the flight recorder.

You can recover the data from a quick access

recorder within a matter of minutes in some cases.

NARRATOR: Speed, altitude, control settings,

cockpit conversations.

In all the various recorders have preserved

1,400 different pieces of data, which

should help the investigators close the case.

But when they study the QAR what they find is chilling.

MARK FORD: The quick access recorder

data stopped about 45 seconds prior to the accident itself.

Initial thoughts were that there was an electrical problem

possibly that had affected not only the QAR but possibly

the engines as well.

NARRATOR: Pilots don't directly control the 777.

Instead their inputs are sent to a computer.

Those signals are then relayed to the engine's flaps and

other systems on the aircraft.

Pete, I can't get power on the engines.

NARRATOR: A massive failure of a plane's information systems

could explain why the engine stopped working.

Perhaps the computers that control them froze.

Farnborough is home to the AAIB.

The flight data recorder has been sent here for analysis.

An electrical problem would be recorded on this device,

but interpreting the information will take some time,

time investigators don't have.

There are hundreds of 777s landing every day.

They need to find answers before whatever brought

down Flight 38 strikes again.

While they wait authorities turn their attention to the fuel

recovered from the ruined plane.

A bad batch of fuel could have deprived the aircraft of power

just when it needed it most.

BRIAN MCDERMID: A fuel can be contaminated

in many different ways.

It can be particle contamination.

It can be biological contamination,

and they can get into filters to restrict the flow.

PHILIP SLEIGHT: We took several samples from various points

within the fuel system so not only from the fuel tank

itself but also from remnants within fuel lines

and also within various fuel components.

BRIAN MCDERMID: We managed to trace the fuel back

through the documentation to a shipment of jet A1, which came

from South Korea, and that was shipped across in a tanker

to China and was then transported

by pipeline to Beijing airport.

We compared it with over 1,200 other batches of fuel

in the UK, and it came out to be very good.

NARRATOR: Investigators next consider the possibility of

a blockage in the fuel tanks.

BRIAN MCDERMID: When we're in the fuel tanks,

we did find a few small articles.

One of them was a red scraper.

NARRATOR: The items were likely left over from when the plane

was built seven years ago.

Obviously it's not ideal to find

anything within a fuel tank.

We obviously looked at to see whether this

would have had an effect on the fuel flows of the engines.

The items were so small, they would

not have caused a restriction.

NARRATOR: As the search for answers continues,

a puzzling find surfaces.

Less than three years before the Heathrow crash,

another 777 suffered a serious mid-air problem.

After taking off in Perth, Australia,

a Malaysian passenger jet was climbing through 38,000 feet

when suddenly the plane's autopilot pitched the nose up

and climbed steeply.

Calamity was only averted when the pilot

took manual control of the jet.

They did manage to get it safely back to base.

The investigators established that this

was a computer problem.

NARRATOR: British aviation authorities

consider that the electronic brain

of one of the world's most reliable jets might be faulty.

The Malaysian Airlines aircraft was

really quite an unnerving one.

The investigators sort of pulled out the files on it

and started looking over it to try and get some clues.

PHILIP SLEIGHT: Did we have a problem

with the electronic control system causing

the engines to roll back?

NARRATOR: If flaws are found with Flight 38's computers,

it means 777s around the world could

all be disasters in the making.

This thought hangs heavily over the industry.

When the readout of the plane's flight data recorder

is conducted in Farnborough, investigators

learn that the crash could have been much worse.

There was very little I could actually have done.

It looks like we have double engine failure.

There was only 30 seconds between when they were fully

aware of what was going on--

Air speed low.

PHILIP SLEIGHT: And the accident was inevitable.

NARRATOR: Records show that soon after the engines failed,

the plane started falling faster than 1800 feet a minute,

a very steep descent.

But lack of power was only one factor

working against the crew.

They had extended their flaps on descent.

It helps control the plane at lower speeds

by increasing lift, but the extended flaps create drag,

which slows the plane down.

It takes more power to keep the plane flying with a wider wing.

Air speed low.

PETER BURKILL: I needed to raise the flaps.

I knew that by raising it one notch to flap 25

would be the right thing to do to reduce the drag.

That's what I had to do.

And what he did here gave them a few extra feet.

Had the flaps been left at flaps 30,

the aircraft would have struck the ground just

before a set of ILS antennas.

NARRATOR: The plane then would have slammed into the antennas,

sustaining even more damage and increasing the chances

of serious injuries.

For their efforts, Burkill and his team are considered heroes.

PETER BURKILL: Flying is about teamwork,

and we had an outstanding team.

NARRATOR: There is no doubt without the flight

crew's actions, Flight 38 landing

would have been a catastrophe.

Now the recordings--

NARRATOR: As they continue examining data from the flight

data recorders--

Continue right until the moment of impact.

NARRATOR: Investigators are no closer to solving the case.

MARK FORD: Analysis of the flight recorder

didn't identify any particular fault with the aircraft

electrical system that would have resulted

in both engines rolling back.

NARRATOR: They are at least able to determine why no data was

found on the quick access recorder for the last 45

seconds of the flight.

It's not an electrical fault as they first feared.

MARK FORD: The 45-second delay in the QAR recording

was a result of the fact that the QAR

doesn't record in real time.

It buffers the data and then will record

approximately 45 seconds later.

NARRATOR: With all the information from the plane's

various data recorders now collected,

investigators have a precise picture of what happened.

The jet was on the proper course as it

made its approach to land when things went horribly wrong.

PHILIP SLEIGHT: At 720 feet, the right engine rolled back.

7 seconds later the, left engine rolled back.

The autothrottle attempted to command greater thrust.

It's not me, that's the auto throttle.

But the engines didn't respond.

I can't get power on the engines.

NARRATOR: Well, electrical and computer problems are ruled out

as causes of the crash.

The flight data recorders do point investigators

to the likely source of the problem.

Two most significant parameters

from the quick exit recorder with the fuel metering valves.

Both those valves indicated that they had

opened the fully open position.

NARRATOR: The plane's computers were calling for as much fuel

from the tanks as possible.

The valves were fully open, but not nearly enough fuel

was reaching the engines.

A plane with plenty of fuel and a working computer

system still crash landed, and investigators

can't determine why.

They focus on the weather on the day of the accident.

PETER BURKILL: It was a very cold air mass over Siberia,

so we were aware of the cold conditions.

But the flight was expected to be smooth and also quick.

NARRATOR: The plane's path took them high over Russia.

The outside temperature went as low as -74 degrees

Celsius, -101 Fahrenheit.

It's a potentially dangerous temperature

if not monitored closely.

With cold air masses, you have to be very

aware of the fuel temperatures.

PHILIP SLEIGHT: When fuel starts to freeze, it produces wax.

As the fuel temperature drops, the wax

becomes greater within the fuel itself

until it comes to a point where it can no longer flow.

PETER BURKILL: We have a fuel temperature gauge

on on the flight deck, which is monitored at all times,

and I personally look at that every time I do a fuel check

which minimum is once an hour.

Basically you don't want to get it

below -34 degrees centigrade.

NARRATOR: While they were cold, the temperatures never

dipped into the danger zone where the fuel could wax

and clog the system, and the fuel

temperature increased significantly

as the jet approached London.

PETER BURKILL: We were coming through 20,000 feet,

and I remember seeing -20 degrees centigrade.

So it had already started to warm up.

NARRATOR: Another potential cause of the crash

is ruled out.

Investigators are now running out of possible explanations.

And the expectation from the media

and also from aviation experts was that we would

have the answer within days.

REPORTER: But there was no comment today

about the ongoing investigation.

PHILIP SLEIGHT: It very quickly became apparent that that

would not be the case.

DAVID LEARMOUNT: They had the pilots.

They had the passengers.

They had the aircraft.

They had everything there except the thing that caused it.

That had disappeared as if by magic.

NARRATOR: They turned to a failure of the fuel

delivery system as a culprit.

To prove it was responsible, they need to pinpoint anything

out of the ordinary about Flight 38 journey on January 17, 2008.

PHILIP SLEIGHT: It's a daily flight operating

from Beijing to Heathrow.

We wanted to work out what was unique about this flight.

Why this flight in particular?

If you've got a very puzzling incident like the 777 accident

at Heathrow, you're going to mine all the data you can.

Just put them over there for now.

PHILIP SLEIGHT: So we approached many operators

to obtain some data of previous flights

so that we could then do a comparison.

NARRATOR: Flight data is collected from companies

around the world.

PHILIP SLEIGHT: And we had something

like 144,000 flights in all.

NARRATOR: But it will take months of work

to analyze this enormous volume of information.

To study Flight 38's fuel system more closely,

huge sections of it are brought to the AAIB hangar

and reassembled.

BRIAN MCDERMID: It allowed us to basically just sit there

and look at it and to brainstorm and come up

with ideas as to what might have caused it.

NARRATOR: McDermid and his team consider all possibilities

from design flaws to a malfunction specific

to this aircraft.

The examination comes up empty.

We carried out an exhaustive and very

thorough inspection and testing of the fuel system,

and we find nothing wrong.

NARRATOR: Frustrated, investigators

are at a dead end.

PHILIP SLEIGHT: We kept going through this cycle.

We kept going back over what we'd done before.

Had we'd missed something?

And every time we went back over,

we kept coming back to no.

NARRATOR: There was no trouble with the plane's computers,

it had plenty of fuel and no problems with the fuel itself,

but somehow when it was most vital--

Mayday, mayday--

NARRATOR: The engines didn't get the fuel they needed.

The AAIB detectives have run out of suspects.

DAVID LEARMOUNT: There was no clear explanation.

Nothing that you'd seen before--

It's not giving me power.

In any airplane, let alone a 777, could have explained this.

NARRATOR: Now they turn away from the clues

they have to those they don't.

Phil started coming out of a mantra, which was

a quote from Sherlock Holmes.

PHILIP SLEIGHT: Whatever is left, however improbable,

must be the cause.

Has to be.

PHILIP SLEIGHT: In this case, we always keep coming back

to what's there, what could have been

there, that's not there now.

And we come back to ice.

Suspicion that the cause of the accident was ice

grew stronger and stronger.

NARRATOR: But investigators are faced

with a nearly impossible task.

How can they prove that ice had brought down the plane when

the evidence they need would have melted

by the time they arrived?

to work with engineers at Boeing,

the company that built the 777.

If ice brought down Flight 38, they need to prove how and why.

PHILIP SLEIGHT: We took the pipes from the right side

of the fuel system, and those pipes and those couplings

were used on the test rig at Boeing.

NARRATOR: Ice in the fuel lines has long been a concern

for jets of all kinds, but decades ago, engineers

found an ingenious way to deal with the ice forming

in the fuel of passenger jets.

The heart of the system is the fuel

oil heat exchanger or FOAG.

Cold fuel runs through thin tubes

which are surrounded by the hot oil

used to lubricate the engines.

The purpose of a fuel heater is

to heat the fuel up to prevent icing of the delicate fuel

control system.

The FOAG is designed to prevent exactly the problem

Investigators suspect crippled Flight 38,

but it's also a potential bottleneck in the system.

BRIAN MCDERMID: During the fuel testing,

we were looking to establish where we could get

a restriction that would restrict the fuel flow to 6,000

per hour, and the only place that we could get such

a restriction occurring was on the face

of the fuel oil heat exchanger.

NARRATOR: Investigators have to prove three things.

First, that substantial amounts of ice

can accumulate inside the fuel pipes.

Second, that the ice can suddenly be released.

And finally prove that it can block a device specifically

designed to melt it.

BRIAN MCDERMID: We use different methods of controlling

the environment around the pipes that range from dry ice

to cold fuel and to hot air to simulate the environment

around the pipes that the aircraft

experienced during the flight.

NARRATOR: After each fuel cooling test,

investigators look inside the pipes for ice,

but they can never get much ice to form,

let alone block the fuel lines.

And the fuel temperature was -30 degrees centigrade.

Very little ice would stick to the inside of the fuel pipes.

NARRATOR: Investigators know the temperature

on the accident flight didn't get much colder,

so how had the ice formed?

PHILIP SLEIGHT: We still hadn't answered the question of where

does the ice come from in the first place and indeed

was it ice or was there something

else that we've missed.

NARRATOR: Surprisingly, when the fuel temperatures are warmer,

the breakthrough arrives.

If you have water in the fuel, then that water will

freeze and form ice crystals.

And then when the temperature gets to about -20 degrees

centigrade, then those ice crystals

will start to stick together--

would also stick to the inside of the pipes.

NARRATOR: Below -20 degrees Celsius, minus 4 Fahrenheit,

any ice crystals in the fuel are too cool to attach

to the pipes, but in the sticky range

20 to -8 degrees Celsius, minus 4 to plus 17 Fahrenheit,

slushy ice forms and sticks to the sides of the fuel pipes.

So when we demonstrated that ice could actually

build up and grow onto the walls of the fuel pipes,

there was a certain amount of surprise

as to how much could actually grow.

And now we're getting somewhere.

Well, the question for us at this stage

was how could the ice come off the pipes.

NARRATOR: Investigators think they have the smoking gun.

All right.

NARRATOR: But they still can't prove how a small bit of ice

could bring down a $200 million plane.

They continue analyzing thousands

of comparative flights to see what made Flight 38 unique,

but after seven months, they still can't crack the case.

MARK FORD: From 35,000 Rolls Royce powered flights,

it was less than 1% who had the same features

as the accident flight.

NARRATOR: Then on November 26, 2008, more

than 10 months after the crash.

The importance of finding an answer

is underscored when the elusive culprit strikes again.

Another 777 runs into trouble 39,000 feet

above the United States when one of its engines

simply stops working.

Fortunately in that case, the engine recovered,

and the aircraft landed safely at Atlanta.

NARRATOR: Even though it didn't result in an accident,

it reinforces concerns that there's

a potentially dangerous flaw on every 777 around the world.

The incident captures the attention

of the British Airways crash investigators.

DAVID LEARMOUNT: The Delta Shanghai

flight really was investigated with huge interest.

The engines were Rolls Royce engines of the same type.

So they immediately went looking for evidence of same problem.

NARRATOR: And, in fact, they do find

important similarities between the Delta airliner

and British Airways Flight 38.

PHILIP SLEIGHT: The aircraft had operated

a long sector from Shanghai in China to Atlanta.

The fuel temperatures were within the sticky range

that we are defined during the research.

We were able to see that the reaction of the engine

was very similar to that of the 777 at Heathrow.

NARRATOR: Later examination of the Delta flight

shows no evidence of any electronic, mechanical, or fuel

system problems, strongly suggesting that the culprit was

indeed ice.

When we heard about the Delta flight,

we were more convinced than ever that we

were on the right track.

Seattle continue their testing.

It's been a year since the accident,

and the problem hasn't been found or fixed.

They've run hundreds of simulations

and spent millions of dollars.

They still can't get the ice in the fuel lines

to clog the fuel oil heat exchanger

and cause the kind of blockage that brought down Flight 38.

PHILIP SLEIGHT: We came to a stage

where we had not actually come up with what

had caused this accident.

This time, the media and also the general public,

there was a bit of unrest as to why we had

not yet come up with an answer.

NARRATOR: Industry pressure to resolve

this case is intensifying.

Investigators still need to know what made Flight 38 different

from thousands of others that had

flown under similar conditions.

To find out, they retrace every moment of Flight 38's journey.

The voyage from Beijing involved a gradual climb, a steady

cruise, and a gradual descent.

The fuel temperatures fell and rose

accordingly, causing ice to accumulate in the fuel lines.

That ice posed no danger unless it was released.

Now investigators study the fuel flow

and find that it was kept steady for most of the flight.

The autopilot maintained constant speed and low power

for hours, never demanding an abrupt increase in engine power

until just before they reached the runway.

PHILIP SLEIGHT: The approach into Heathrow

was quite a turbulent approach.

And the engines were demanding various levels of thrust power.

It's not me, that's the auto throttles doing their thing.

PHILIP SLEIGHT: And there were four changes in fuel flow, one

of which was a very high fuel flow

of around about 12,000 to 13000 pounds per hour demanded.

NARRATOR: A closer review of the American, Delta flight

reveals that its engine rolled back

under the same circumstances, a sudden demand for power

after a long period of consistent speed.

Investigators are now set to duplicate

these precise conditions.

They hope that by reproducing the moments just

before landing, which was the only part of the flight where

the crew suddenly required more power,

they may finally get the answer they're looking for.

The problem that we were looking at

is very, very difficult to replicate,

and during what was one of the last tests,

we actually managed to get all the elements together.

NARRATOR: After simulating running engines

at constant speed, investigators increased the power.

BRIAN MCDERMID: We allowed the ice

to accumulate for three hours, and then

the flow rate was increased.

NARRATOR: What happens next breaks the case wide open.

In Boeing's Seattle lab, investigators sudden demand

for more engine power causes the fuel pressure

downstream of the fuel oil heat exchanger to drop dramatically.

And when investigators examined this vital component--

BRIAN MCDERMID: We then saw that ice had

formed across the face of it.

We did it.

NARRATOR: Soft ice that had formed deep

inside the fuel lines broke free when the pressure

was abruptly increased.

It restricted the flow of fuel to the engines.

Investigators have finally found the Achilles heel

hidden deep inside the 777.

The tubes that bring the fuel through the FOAG

jut out just above the container of hot oil.

It's just a few millimeters but is enough to prevent

the ice from coming in contact with a hot surface

below and melting.

Now the ice doesn't totally restrict

the fuel flowing through it.

It has some porosity.

So the fuel will continue to flow through,

but the flow rate is much lower than it should be.

NARRATOR: In all of the tests, only one gave investigators

the result they were looking for,

the one that exactly matched the journey of Flight 38.

While the plane flew over Russia, water in the fuel

turned to ice.

At -20 Celsius minus 4 Fahrenheit,

it began to build up along the inside of the pipes.

The steady speed of the aircraft ensured this accumulation

was never interrupted.

Then as the plane approached Heathrow,

turbulence resulted in the first demand for power

since much earlier in the flight.

It's not me.

That's the ultra throttles doing their thing.

You might want to keep the autopilot on a little longer.

Then things cascaded into a serious problem.

The gushing fuel washed the ice through the fuel system

until it built up against the face of the FOAG

with disastrous results.

Pete, I can't get power on the engines.

It's not giving me power.

NARRATOR: Facing a threat no one knew existed--

Mayday, speed bird, speed bird.

NARRATOR: The pilots didn't have a chance to solve the problem.

But why had the American Delta crew

been able to clear the blockage while the British Airways

flight ended in near catastrophe?

It's discovered that after just a few seconds

of reducing engine power to idle, the blockage in the FOAG

clears.

But this maneuver was not available to the British

Airways pilots because they were so close to the ground.

Soon after the Seattle tests, Rolls Royce redesigned

the fuel oil heat exchanger.

So what Rolls Royce did was to actually

remove those protruding tubes, and you now have a flush face.

And they found that if they flattened this surface, even

if ice crystals did form in the fuel,

they'd gold straight down the holes in the block.

Very, very easy fix.

NARRATOR: Other aircraft manufacturers

are ordered to ensure that their systems aren't

vulnerable to the same problem.

Peter Burkill, John Howard, and the rest of the crew of Flight

38 received the British Airways safety medal

for their performance during the accident.

It's the company's highest honor.

Crew have been presented with an unprecedented failure,

and they did the best they could in the time

that they had available.

The crew did as good a job as they could,

and since they really only had about 30 seconds to think

about what they were going to do about this,

they couldn't have done any better than they did.

NARRATOR: The ice that brought down one of the world's most

sophisticated airplanes was gone by the time investigators

showed up.

DAVID LEARMOUNT: Weird thing about this investigation

was that the culprit had fled the scene.

NARRATOR: The inquiry into what happened

consumed thousands of hours of manpower

and cost millions of dollars.

These people really pulled out the stops to find out,

and the reason, we have to know.

NARRATOR: With unrelenting diligence

and a little inspiration from Sherlock Holmes,

the mystery was finally solved.

PHILIP SLEIGHT: Whatever is left, however improbable,

must be the cause.