Original subtitles

Outside one of India's busiest airports,

the early evening sky ignites into a fireball.

This cloud just lit up,

it felt like you could feel the heat.

Flaming wreckage falls from the sky.

It's a horrifying crash.

I realized that the clumps that I saw all around were

either the remnants of the aircraft, or dead bodies.

When investigators arrive,

they find the twisted remains of two passenger jets.

Three hundred and forty-nine people are dead.

Vital clues are buried deep underground.

Investigators must piece together the day's events.

Saudi 763 will maintain 140.

Kazakh 1907 now reach 150.

How had two crews that should have known

each other's positions...

ended up in the worlds deadliest mid-air collision.

Mayday! Mayday!

Indira Gandhi International Airport, New Delhi.

A Gateway to India.

November the 12th, 1996.

Busy?

Air Traffic Controller VK Dutta arrives

for the late afternoon shift.

There was one supervisory officer.

There was one person who was sitting beside me

and he was assisting me.

And... the things were all normal.

But normal at Indira Gandhi doesn't mean calm.

This is becoming one of the most

congested airports in the world.

In 1990, the Government of India signed

open skies agreements with several different countries.

Those pacts have made it easier

for foreign airlines to land at Indian airports.

Many airlines are taking advantage of the new policy.

Air traffic controllers are now handling more traffic than ever.

The open skies policy acted as a catalyst in the air traffic growth.

The volume of traffic grew from, I believe, 175 movements a day

to around 225 movements a day.

We have to perform...

we have to perform, uh, during that time.

So whether 10 aircraft come on that time, or 20 comes,

so, you have to handle them.

Just past six o'clock,

Saudi Arabian Airlines Flight 763 takes off into the sunset.

Gear up.

Gear up.

Clear on left.

Captain Khalid Al-Shubaily

powers the Boeing 747 away from the runway.

His Co-Pilot Nazir Khan handles all radio communication.

There are 289 passengers on board.

Many are Indian workers returning to their jobs in the Middle East.

Saudi 763. Airborne 03,

contact radar 127 decimal 9.

While Flight 763 was on the runway,

its movements were tracked and directed

by an Air Traffic Controller in the airport's tower.

279. Good day.

But shortly after the plane takes off,

the Tower passes the pilots over to

Approach Controller VK Dutta,

who's in another room at the airport.

Approach Controllers at Indira Gandhi

guide planes through the airspace beyond the runways.

They're in charge of all arriving and departing flights within 110 kilometers.

The Approach Controller normally

is responsible for arrival and departures of the aircraft

which are coming in.

They are safely and expeditiously, you know,

they are brought to the final approach.

Tonight, Dutta is controlling five flights.

Some of these are leaving the airport,

while others are coming in.

His task is to keep those planes spaced safely apart,

but not so far apart as to cause any delays.

There is continuous coordination with the tower

who handles the ground traffic.

So he would tell approach controller

that this fellow is ready, and, you know, uh...

he is to be accommodated between these two flights.

So that kind of dynamics we do...

we were doing at that time.

Saudi 763, approaching flight level 100.

Roger, climb flight level 140.

Clear to climb 140, Saudi 763.

This evening, a United States Air Force cargo plane

is coming in for a landing.

Even though it's an Air Force jet,

this flight is Dutta's responsibility.

I think we probably had three or four

back and forths with the controller,

where he was verifying our altitude,

or checking on our altitude,

telling us about other traffic in the vicinity.

Saudi 763, approaching 1-4-0 for higher.

Saudi Flight 763 reaches its last assigned altitude of 14,000 feet.

The pilots request permission to fly higher.

Roger, maintain flight level 140.

Stand-by for higher.

In order to coordinate incoming and outgoing traffic,

Dutta wants the Saudi flight to stop climbing.

Saudi 763 will maintain 140.

Dutta is concerned with another plane,

now flying east for a landing at Indira Gandhi Airport.

It's a passenger jet operated by Kazakhstan Airlines.

The Ilyushin-76 is a massive Russian plane.

Originally built for the military,

a modified version is now widely used as a commercial airliner.

Dutta wants it to pass over top of the Saudi flight before landing.

before landing.

After that, he'll let the Saudis continue their climb.

They were supposed to cross,

uh, maintaining a 1000 feet of separation.

Kazakh 1907, now reaching 150.

46 miles from Delta Papa November.

Roger, maintain flight level 150.

Identified traffic 12 o'clock reciprocal, Saudi Boeing 747.

Dutta doesn't want the Kazakh flight to be surprised to see the Saudi jet.

Since planes don't have radar to track other aircraft,

they rely on controllers to warn them about other planes nearby.

Maintaining 150,

Kazakh 1907. Will report.

How many miles?

All pilots tuned to the approach control

radio frequency are able to hear one another

as they communicate with the ground.

How many miles?

Eight miles now.

It was strictly visual,

and trying to, trying to pick up on the radios

where everyone else is at, and what they're up to.

Traffic is at eight miles level 140.

Suddenly, a massive explosion shatters the evening's calm.

All at once, out of the right side of the window,

right side of the cockpit, um, this cloud just lit up.

Are those missiles?

To my mind's eye they were missiles

kind of cork screwing picking up speed was what they looked like.

And they looked like they were coming right at us.

We actually started veering the aircraft away

before it became apparent that they weren't coming at us.

The Saudi Airlines jet spirals towards the ground.

Delhi, this is 1815.

1815, say again.

Got on the radio right away called the controller.

We saw something that looks like a big explosion.

Then it struck me that something has gone wrong.

On Dutta's radar screen, both the Saudi jet

and the Kazakh plane have simply vanished.

When I saw the blips are coming,

and so I watched the sweep again.

But they were not there at all.

We then heard the controller call for...

for the Saudi Air jet and the Ilyushin.

Saudi 763?

With no response.

That was obviously very bone chilling.

Something tragic has happened in the skies near New Delhi's airport.

VK Dutta is about to become the target of an investigation

into one of the deadliest plane crashes of all time.

Kazakh 1907, report your position.

At New Delhi's airport,

a sense of dread is growing.

Two planes have vanished from Approach Control's radar.

The worst nightmare

an air traffic controller

has is the mid-air collision.

Saudi 763?

Together, the two missing planes

were carrying 349 people.

Kazakh 1907, report position.

I gave one or two calls to Kazakh and Saudi.

Saudi 763?

I forgot how many kids I have,

where my wife is, whom I'm related to.

US Air Force Pilot Captain Tim Place

soon confirms the worst.

Delhi, this is 1815.

1815, what did you see?

Two distinct fires on the ground.

Two fires on the ground.

Confirm?

Confirmed.

Confirmed.

Roger.

My senior came to me,

so I told him, uh...

an accident had happened.

I told our people who were to do the search and rescue

that the aircraft have crashed

at 40 nautical miles west of Delhi.

Two planes have gone down over Charkhi Dadri.

Charkhi Dadri is a town 65 kilometres west of New Delhi.

Its mustard and grain fields are now burning

with the twisted wreckage of two ruined airplanes.

All of a sudden, the sky turned a bright red color,

and I saw fire and smoke.

One plane started to come towards us.

There was fire and parts of the plane were flying everywhere.

People were running. The plane's engine broke off

and the plane started to spin out of control and fell into the field.

I was told that there's been a mid-air collision.

Apparently there's a jumbo jet which may be involved,

and why don't you just head out?

There was this adrenaline rush in that sense, I was...

I was very new in the business,

and I knew I had to get that story.

Vishnu Som is assigned to report

on the crash for New Delhi Television.

He arrives in the darkness of night, a few hours after the crash.

We parked our vehicle on the side of the road,

and it was a...

I mean, there was a fair bit of moonlight.

He and his cameraman

are one of the first media crews to arrive on the scene.

Turn off the light.

We don't want to attract attention.

The police have likely cordoned off the area.

I stepped onto the field,

and, I mean, I thought it was a fallow field,

nothing was growing over there, because it was just sand.

Um, and you know, because it was dark

and our only visual reference was, was the moonlight,

we just sort of kept going.

And then, I remember seeing clumps around me.

You know, just these, these little piles on, on all sides.

And because there wasn't enough light, I didn't know what exactly it was.

And then I stopped, because I said, you know,

something's terribly wrong over here.

Turn on your lights.

Turn on your light.

I realized that the clumps that I saw all around me were

either the remnants of the aircraft, or dead bodies.

There was a tree just maybe

fifteen feet away from me to my right.

There was this burnt corpse on that tree,

and that's an image which, which, you know, comes back

and continues to haunt me even after so many years.

As night turns to day,

hope of finding any survivors fades.

None of the passengers aboard either plane has survived.

Three hundred and forty-nine people are dead.

It's the worst mid-air collision of all time.

News of the crash spreads around the world.

What remains clear is that there was more than

one set of factors that might have resulted in the collision.

We have two very separate wreckage fields.

Captain KPS Nair

is one of the first investigators on the scene.

I was aghast,

horrified.

I can't explain it, because I had never seen anything like that before.

It is something...

which I can't explain.

The Kazakh flight and the Saudi Airlines jet

have fallen seven kilometres from each other.

Investigators have two separate crash sites to examine.

But they know one cause will explain both accidents.

It's unlike a single aircraft falling somewhere.

Two large body aircraft

colliding with each other

in the sky... There are different causes.

How had two planes that were supposed to be 1,000 feet apart collided?

Investigators consider three possibilities.

An error on the part of the air traffic controller,

an error by one of the crews,

or the failure of an instrument on one of the planes.

They hope the mangled wreckage will hold the clues they need.

Their first priority is to find the black boxes from both planes.

They record cockpit conversations

along with critical data about the flight,

like its altitude, speed and heading.

Although fire has ravaged the wreck sites,

the black boxes from both planes

are found on the first day of the investigation.

But for the moment, they hold their secrets.

It will take several months for experts to extract data from the recorders.

In the meantime, investigators focus on conversations

between the two planes and the air traffic controller.

In this particular case, we...

had the evidence of the ATC transcript.

Without onboard radar,

the planes around Delhi's airport

rely on air traffic controllers to guide them.

VK Dutta comes under enormous pressure.

Did he make mistakes that caused the worst mid-air collision ever?

Those times were bad.

These media people, they cried foul.

So I told them that, you know, there's no fault of mine.

Hi, I'm VK Dutta.

I'm sorry to keep you waiting.

Have a seat.

Investigators want to know everything

that Dutta did on the night of the crash.

Was traffic heavy?

Evenings are always busy.

Dutta's radar doesn't track a plane's altitude.

Instead, controllers in New Delhi

write a plane's last reported position on a strip of paper.

The strips are continually updated.

It's the only way for controllers to keep track

of the altitude of the planes under their control.

Level information is not there.

So level has to be confirmed by the pilot.

Kazakh 1907 now reaching 150.

Air traffic controllers can only know a plane's altitude

when the pilots report it.

Are these your notes?

Yes, sir. This is the Kazakh one,

and this is the Saudi one.

Investigators learn that Dutta was in charge

of five flights at the time of the accident.

And that the Saudi and Kazakh flights were flying

in opposite directions of the same aerial pathway.

The area that Dutta manages is divided into a network of air corridors.

Controllers use them to channel flights in and out of the airport.

But many of the corridors are used for military flights.

In fact, even at such a busy airport,

there's only one main corridor for commercial planes.

In cases like this, there are strict rules he has to follow.

Between two aircraft the requirements specify

a vertical minimum separation of 1,000 feet.

When aircraft fly through the sky,

they leave turbulence in their wake, like boats on the water.

It can affect the planes around them.

Air Traffic Controllers deal with this

by keeping planes at different altitudes.

Roger, maintain flight level 140. Standby for higher.

Dutta was supposed to ensure that the two planes

were 1,000 feet apart as they approached each other.

He planned to do what he had done many times before.

Have the incoming plane pass 1,000 feet

over the top of the outbound plane.

It was a routine procedure, but somehow,

the two planes had ended up on a collision course.

One of the two planes was not where it should have been.

Investigators wonder if Dutta made a mistake

that led to the horrific crash.

Pouring through the ATC transcripts,

investigators quickly learn that Dutta had given

the two planes the proper directions.

But since his radar doesn't display altitude,

there was no way for him to tell if the planes followed those instructions.

Investigators are so convinced that Dutta did nothing wrong

that three days after the crash he's back on duty.

When my family and other people heard this,

there was a little sigh of relief,

that, you know, our guy is safe.

Investigators return to the crash site.

They are hoping to recover certain instruments from the two aircraft.

Perhaps a mechanical failure had led

one of the planes off course.

Normally, the altimeter would have stopped at the point

where the accident took place.

And we wanted to know what altitude

they were registering at the time of the accident.

But the Saudi cockpit has plunged deep into the ground.

The accident investigation becomes an excavation.

The Saudis' nose portion was well below the earth.

It was about 10 to 12 meters, um, below.

The end of the nose had gone that far below.

Machines had to be brought in.

Excavators to dig up and lift the cockpit of the Saudi aircraft.

The altimeters of the Kazakh flight are easier to find.

That plane didn't crash nose first.

There's some chance there are clues about

its exact altitude at the time of the accident.

When they're pulled from the wreckage,

investigators discover that there is indeed

something strange about the altimeters.

Both the captain and the co-pilot

have an altimeter in front of them.

But in this case, they do not have same reading.

There was a difference between the two

by about 300 feet.

Strange.

Perhaps the conflicting altimeters

had led the crew off course.

But the difference in readings

may simply have been caused by the force of the crash.

At this point, investigators don't know.

Mostly there, and there.

As close as you can get.

Investigators need to discover

which of the two planes was at the wrong altitude.

They explore other evidence.

The pattern of the damage on the two planes

may help answer a critical question.

What was the relative angle between the two aircraft

at the time of the... their contact?

Most of the pieces from the two planes are kilometres apart.

But a large section of the tail

from the Saudi aircraft is found near the beginning of the debris field.

This suggests it was one of the first pieces to come off that plane.

The significant point of contact

in this particular accident in the air, was between

the tail end of the IL-76,

and the wing portion of

the Boeing 747 of Saudi Air.

The tail of the Kazakh plane appears to have pierced

through the left wing of the Saudi jet.

If that's the case, the Kazakh flight

hadn't been above the Saudi jet when they collided

as air traffic controllers thought.

It must have been below it.

It's a puzzling discovery that adds to the mystery.

Investigators still don't know

which plane was in the wrong airspace.

What they do know is that once the two planes hit,

there was no hope for either crew.

After hitting the other plane's wing,

the Kazakh plane's tail tore through

the horizontal stabilizer at the rear of the Saudi 747.

Five-and-a-half meters of it is torn off.

Without it, the Saudi crew can't control their plane.

The result is, the aircraft goes into uncontrollable spiral.

And in this particular case,

I would say, both aircraft had gone into that situation.

What they've found in the wreckage of the two jets

frustrates the investigators.

They now understand how the planes collided,

but they still don't know why they were at the same altitude,

or in fact, what altitude they collided at.

How had the Kazakh jet, which was supposed to be above the Saudi flight,

ended up below it?

Almost two weeks after the accident,

Captain Ashok Verma joins investigators at the scene.

By that time, the operations for digging

the Saudi Air cockpit had been completed.

Whatever equipment was recoverable had been recovered.

Like the rest of the evidence uncovered so far,

the instruments from the Saudi plane

don't help explain what happened.

The force of the crash has completely destroyed them.

I can't find anything. Too much damage.

I agree. Look at this.

We could hardly find any useful material.

With the disappointing discovery,

the investigators are forced to move on.

They've learned all they can at the crash site.

The case now depends on what they can find out from the black boxes.

Investigators hope that somewhere on the cockpit recordings,

or buried in the flight data, are the clues they need.

Where were the planes when they collided?

And how had they ended up on a deadly collision course.

Get to the 150, because at the 140...

It's been three months since

the mid-air collision of two passenger jets near New Delhi.

Investigators have been frustrated by a lack of conclusive evidence.

They hope the planes' black boxes will help them solve the case.

In order to avoid any suggestion of bias,

the boxes from the Saudi flight are being analyzed in England.

Technicians from the Air Accidents Investigation Branch

try to extract valuable data.

The neutrality of state where the recorders are handled

was an essential thing.

Each plane was equipped with two black boxes,

a flight data recorder, and a cockpit voice recorder.

The flight data recorder contains information

on dozens of aspects of a plane's performance.

Among them, the altitude, the airspeed,

and the changes pilots make to the flight controls.

The cockpit voice recorder picks up all the conversations in the cockpit.

279, Good day.

As investigators try to learn more about the collision near New Delhi,

they use the information stored on the black boxes

to make a chronology of events.

Peter Sheppard is head of the recorder section

of the Air Accidents Investigation Branch.

He'll work backwards from the moment of impact.

The point of the collision is reasonably well defined

by rapid changes in parameters on each, each aircraft.

If we make that our time zero point,

we can then relate that to individual times,

and then build up, you know, a common time base.

First, Sheppard and his team concentrate on the Saudi 747.

Saudi 763 approaching 140 for higher.

Roger, maintain flight level 140.

Standby for higher.

Saudi 763, will maintain 140.

The CVR reveals that the Saudi pilots received

clear instructions about their altitude,

and seem to have understood them.

After being told to hold at 14,000 feet,

there was no discussion of climbing to a higher altitude,

which would have taken them into the path of the Kazakh flight.

Next, Sheppard looks at the flight data recorder

to confirm that the Saudi flight actually

followed the instructions it was given.

The recorder from the Saudi flight told us that

the altitude had been normal during climb out

The Saudi pilots leveled off at 14,108 feet.

Well within their safe corridor.

It had leveled at its assigned altitude,

of, um, 14,000 feet,

and continued to fly level.

The Saudi pilots followed

the ATC instructions meticulously,

which is borne out by their confirmatory calls back to the ATC.

Air traffic controllers wanted the two planes separated

by 1,000 feet,

and wanted the Saudi plane to fly below the Kazakh jet.

If the Saudi pilots did nothing unusual,

suspicion is growing that somehow,

it was the Kazakh plane that was in the wrong place.

Alarmingly, when investigators examine the information

from the Kazakh jet's flight data recorder,

they discover that it descended far below

the 15,000 feet it was supposed to stay at.

Moments before the collision,

the Kazakh plane is at 14,100 feet.

Almost 1,000 feet lower than its assigned altitude,

and less than ten feet below the Saudi flight.

It then powered directly into the Saudi jet.

The Kazakh crew had not stopped descending

at the altitude they were cleared to.

That was 15,000 feet.

But why had the Kazakh flight dropped so badly off course?

Kazakh 1907, report position.

Kazakhstan Airlines presents one theory

for their plane's dramatic loss of altitude.

The Kazakhstan defense mainly relied on presence of turbulence.

Perhaps a sudden burst of turbulence had forced the plane lower.

The data recorder of the Kazakh flight

does seem to indicate that the crew had a bumpy ride.

It shows two distinct and sudden drops of more than 400 feet.

The airline claims both of these drops were caused by turbulence.

But Peter Sheppard isn't so sure.

When we saw the jumps of 250-500 feet,

our initial response was that these can't be right.

I mean, you know, it... it was...

the rate at which they changed was beyond that,

that an aircraft can actually perform.

What else could cause the plane to drop so quickly?

Or at least seem to drop so quickly?

Sheppard searches for an explanation.

We tried to resolve the inaccuracies

in the Kazakh recording by looking at the other

parameters that were recorded

and trying to derive altitude, um...

using these other recordings.

It's intricate math. By using information

including the speed and rate of descent from the flight data recorder,

Sheppard determines that the plane was on a steady downward approach.

The reason for the apparent sharp drops is simple.

Sheppard learns that the sensor that sends altitude information

to the flight data recorder was faulty.

It would stick, and temporarily stop sending information.

When it got unstuck, it would wrongly appear

as though the plane had lost considerable altitude.

It's as if there was bit of glue on one area and it stuck in that.

And eventually...

the altitude changed so much

that the force on it made it jump...

again and it stuck there for a little while.

Investigators can now conclusively dismiss

turbulence as a factor in the collision.

So there's no sudden descent?

Scanning maintenance records,

investigators also discover that there was no problem

with the Kazakh plane's altimeters.

They conclude that the difference found

in the cockpit instruments was a result of the crash,

it didn't cause it.

To understand why the Kazakh plane kept descending

after it was told to hold,

investigators turn to the cockpit voice recorder.

It begins long before the crash itself.

Initially, there are no hints that anything is wrong.

Kazakh 1907, report level passing.

Passing 240,

Kazakh 1907.

Because the Ilyushin 76 is a modified military plane,

it has another unusual feature.

A position for a radio operator in the cockpit.

Egor Repp mans that position,

and handles all communications for the Kazakh flight.

Roger, over to Delhi Approach 127 decimal 9.

127 decimal 9. Bye bye, Kazakh 1907.

As the aircraft nears the airport,

Repp gets in touch with Approach Controller Dutta.

Delhi Approach. Good Evening. Kazakh 1907,

passing 230 to 180.

I told Kazakh to descend,

and maintain flight level 150,

that is 15,000 feet.

Roger Kazakh 1907,

descend flight level 150, report reaching.

One thousand feet vertical separation is sufficient,

and that was granted to the two aircraft.

Kazakh 1907 now reached 150.

The Kazakhstan radio operator had...

at one time stated reaching flight level 150.

It's just one minute before the impact.

And at this point the Kazakh plane appears

to be right where it's supposed to be,

one thousand feet higher than the Saudi plane.

But investigators know that instead of levelling off,

the plane continued to descend.

This is where the trouble starts.

As investigators compare the flight data information

to the cockpit voice, they notice something disturbing.

Kazakh 1907 now reached 150.

When Repp calls out that they've descended to 15,000 feet,

he's actually over 1,000 feet higher than he thinks he is.

Investigators wonder how he could make such an enormous mistake.

Why would he say he was at 15,000 feet

when in fact he was at 16,000?

They consider the layout of the cockpit.

A radio operator does not have his independent altimeter.

There are two metric altimeters fitted in front of,

uh, one each in front of the pilots,

with some effort

the radio operator could also see this.

Whatever the reason,

Repp is mistaken about his plane's height.

And he's the only one in contact with the ground.

Despite being told to remain at 15,000 feet,

the plane continues to descend.

As it does, Dutta issues a warning to the Kazakh pilots.

Identified traffic 12 o'clock reciprocal,

Saudi Boeing 747 and ten miles.

Likely to cross in another five miles.

Report if in sight.

He tells them to watch out for the Saudi flight.

But the Kazakh jet just keeps flying lower.

Ahead?

Ahead!

Just before the crash, Egor Repp seems to have

recognized that the plane is now flying dangerously low.

But his warning doesn't come in time.

Keep the 150! Do not descend!

Accelerate Sanya!

The cockpit voice recorder

proves what investigators found in the flight data.

At the time of the crash, the Kazakh jet was trying

desperately to get back on course.

Get to the 150,

because on the 140...

But investigators are still puzzled.

Why had the Kazakh plane kept descending?

After ruling out, uh, controller's error,

ruling out mechanical failure,

you have to go further into the details of human behavior

during the operation of a flight.

Kazakhstan is one of several now independent republics

that used to be part of the Soviet Union.

The national airline has a reputation at Indira Gandhi Airport.

There was always a sense that, you know,

these were small fly by night operators

essentially doing the charter business,

and they didn't necessarily follow the conventions

of modern Western aviation.

We examined the background of their knowledge of English language.

The crew in the Soviet states...

uh, they passed English examination,

but they're not fluent in spoken English.

Verma now listens more closely to the cockpit voice recorder,

looking for an indication that the crew misunderstood their instructions.

Had a language barrier caused the crash?

How many miles?

Traffic is at eight miles level 140.

Did the Kazakh pilots confuse their own altitude

with that allocated to the Saudi aircraft?

Report eight miles.

Verma finds that members of this crew

weren't communicating clearly with each other.

Egor Repp was responsible for communication with the ground,

but there's no indication that the pilot and co-pilot

were listening to his instructions.

Switch on the engine inlet heating.

While the rest of the crew was busy discussing arrival procedures,

Repp alone seemed to be occupied with their altitude.

Now looking 1907.

It is a usual practice that...

the briefing for the arrival

is completed before the start of descent...

so that all crew can pay full attention to the radio.

While the radio operator appears to have understood

that the Saudi plane was at 14,000 feet,

investigators believe the co-pilot thought that

he was cleared to 14,000 feet,

and continued his descent.

Hold the level!

What level were we given?

When the pilot does respond, he seems confused.

I think, pilots did not pay that much attention,

and relied too much on the radio operator

to navigate the aircraft during this critical phase.

The decision to increase power

and stop descending ends in tragedy.

Keep the 150! Do not descend!

Accelerate Sanya!

Get to the 150,

because on the 140...

The final report points a finger squarely

at the crew of the Kazakh plane.

In the wrong place at the wrong time,

a simple misunderstanding led to the deaths of 349 people.

The investigation concluded

that the primary cause of this midair collision

was non-adherence to the authorized altitude

allocated to the Kazakhstan aircraft.

Investigators are confident they know what caused

the worst mid-air collision in aviation history.

But there are steps they want the industry

to take to help make Indira Gandhi airport safer.

What they find is that the technology

that could have prevented this accident was already at the airport.

Saudi 763.

Investigators have determined that poor cockpit communication

and a simple misunderstanding caused a devastating accident

which killed almost 350 people.

But technology existed that could have helped avoid this accident,

systems that would have helped both the pilots in the air,

and the controllers on the ground.

Investigators are especially critical of the radar

that was being used in New Delhi.

Air traffic controllers did rely

on fairly outdated technology at that stage.

Um, there wasn't any secondary radar available

at Delhi airport there was only an old primary radar.

Primary radar sends out radio signals

to locate airplanes in the sky.

The signal bounces off the plane and back to a dish on the ground.

It reads a plane's position, but not its altitude.

The primary radar gives you 2D picture.

Secondary radar works differently.

A transponder aboard an aircraft sends a message

to the ground with key information

about the flight, including its altitude.

Saudi 763, approaching flight level 1-0-0.

Roger. Climb to flight level 140.

On the day of the collision,

Approach Controller VK Dutta could only rely on

what the crews told him about their aircraft's altitudes.

Kazakh 1907, now reached 150.

The Kazakh Radio Operator said

that they were flying at 15,000 feet.

But Dutta had no way to confirm that.

If he had known the Kazakh flight's actual altitude,

he could have diverted the Saudi flight from its path.

That is the a 3D picture.

So you can, you have some, some time to react to it.

You can take the aircraft away.

At the time of the accident,

India had ordered a $118 million

air traffic control system with secondary radar,

and more sophisticated communications

and navigational equipment.

The system had originally been scheduled to be installed

two weeks before the accident.

But on the day of the crash,

the system hadn't even been unpacked.

Secondary radar is an advancement on the primary radar.

And in this particular case,

also by getting the altitude information

it could have been possible that the accident

of this nature could have been avoided.

In the year following the accidents,

there would be three near misses

near Indira Gandhi airport.

It would take more than two years for the secondary radar system to be installed.

Today, controllers at Indira Gandhi International Airport

see an airplane's flight number, altitude and heading.

It's far more information than controller VK Dutta

had available to him on the day of the accident.

Experts believe another piece of technology

could have helped prevent the collision, TCAS.

TCAS equipment generates cautions and warnings for the crew.

It allows them time to react,

draws their attention to the situation around

and it, uh, looks into vertical separation.

TCAS is a collision avoidance system

that can be installed on board airliners.

In many countries, the technology is mandatory.

It alerts pilots when other flights are coming too close.

The system also automatically tells them what evasive action they should take.

Neither plane was equipped with TCAS.

The Indian Airports Authority has also made the airport safer

by re-designing the air corridors coming and going from the runways.

At the time of the collision, there was one main

air corridor for commercial planes

landing and taking off from Indira Gandhi International Airport.

With the increase in air traffic,

that single corridor was becoming too crowded.

In the wake of the accident,

more air corridors were opened for commercial flights.

In general, aviation safety standards has improved.

The concern for the safety has improved.

Out of date technology and poor communication led to this crash.

But like almost every aviation accident,

it was a chain of seemingly minor events

that ended in disaster.

In retrospect...

those were some very unlucky people

to meet in the sky that night.

You know, it's a... it's a one in a million chance

for that to happen. Uh...

Certainly there were other things that could have helped.

Radar, uh, collision avoidance systems...

And I... And I think we continue to make advances there.

I... I don't worry about flying.

Um, certainly... you know, destiny,

I don't know. It was... glad we weren't five minutes

in front of where we were at, that's for sure.

VK Dutta went on to have a long career as an air traffic controller.

Today, he works at a college, training young controllers.

So I'm chief instructor there, so I'm training ATC's.

I'm addicted to my job. I love it. I really love it.

Dutta, who was initially

suspected of having caused the crash,

helped implement the changes to make New Delhi's airport safer.

His efforts have paid off.

The airport now handles 20 million passengers a year.

Since the new radar system was installed,

and the new corridors opened,

there hasn't been a fatal accident at this airport.