August 17, 2022


Engineer's Line

Indian Air Plane Mystery

11 min read

12th November 1996. 03:55 PM.

The city of Shymkent in Kazakhstan.

A chartered Kazakhstan Airlines aircraft took off from this city. Flying towards New Delhi.

Several small traders from neighbouring Kyrgyzstan were on board. They were going to India to buy wool,

so that they could bring it back and sell it in Central Asian markets.

Around the same time,

in Delhi’s Indira Gandhi International Airport,

A Saudi Airlines flight was getting ready to take off.

There were 312 passengers on the flight,

of whom 231 were Indians.

They were blue-collar workers,

construction workers or domestic workers.

On their way to Saudi Arabia to look for work,

so that they could earn money for their families.

The Saudi Airlines flight took off from the Delhi airport.

That evening, Air Traffic Controller VK Dutta was on duty.

He was communicating with both Saudi and Kazakhstan planes,

and was guiding them.

He was the one to give clearance to the Saudi airplane to take off from the Delhi airport.

A few minutes later, the pilots of the Saudi plane reported to him,

that the plane had reached the height of 10,000 ft.

They request clearance to go up to 14,000 ft.

Dutta grants permission.

On the other hand, the Kazakh aeroplane wants to land.

Dutta tells the Kazakh aeroplane

to remain at the height of 15,000 ft.

And not to descend any further,

because the Saudi aeroplane was flying at 14,000 ft.

There needs to be at least a vertical distance of 1,000 ft between 2 aircraft.

On his screen, VK Dutta sees the two aircraft as blinking dots.

Over the next few minutes, he sees how these two blinking dots

keep getting close to each other.

And just like that, the two dots fused into one.

And then disappear altogether.

This was the point when Dutta realised that

something terrible had happened.

Around 100 km from Delhi,

lies the small city of Charkhi Dadri.

That evening, the residents of this city saw

a bright flash of light in the sky, akin to lighting,

and then they hear the thunder.

Before they could even think about what was happening,

iron debris started falling on them.

The two planes crashed into each other.

This was the first and only mid-air collision in India.

And the worst air disaster in India’s history.

“As the search for bodies among the wreckage continues,

several Muslim victims of the disaster were buried in the mass grave.

Nearby, 70 funeral pyres were being prepared

for Hindu cremations.

Investigators have started work on the flight recorder,

these so-called black boxes of both aircraft.

To find out more about what happened just before the crash.”

“Even as a judicial enquiry ordered by the Prime Minister Mr Deve Gowda,

begins its probe into the disaster,

experts are looking at various theories that could have caused the tragedy.

“The radar which we have today, in Delhi,

is a primary radar,

and is a fairly old one.”

To understand the depths of this story,

about why it happened,

and how it impacted the aviation sector in India,

we first need to have some basic knowledge.

We’ll have to understand the rules of the sky.

Similar to the vehicles driven on the roads,

and roads built on set paths,

with the road divided into traffic lanes.

Similarly, there are set paths in the sky for aeroplanes.

All aircraft need to follow these set paths.

These paths are known as Airways.

Similar to the roads being divided into traffic lanes,

airways are divided into air corridors.

Imagine if there was only one traffic lane on the road for all vehicles.

What would happen?

There would be a high chance of an accident occurring.

For the same reason, there are multiple corridors in the airports

for the incoming aeroplanes and the leaving aeroplanes.

So that there is a restricted scope of an accident taking place.

Apart from this, the height at which the aeroplanes fly,

is called the flight level.

According to the Internation Civil Aviation Organisation’s standards,

the flight level between 2 planes,

there should be at least 1,000 ft of separation.

It means that one aircraft should be, vertically, at least 1,000 away from another.

Apart from this, there needs to be a 14 km radius for safety.

These are the basic rules of the game.

And we come to the next question,

who ensures that all aeroplanes follow the rules at all times?

This is ensured by the Air Traffic Controllers.

The ATCs.

The ATCs monitor the flying aeroplanes from the ground,

their position, speed, and their altitude (height).

It is very important to check these,

so that the aeroplanes could be kept safe.

Air Traffic Controllers can see it on their screens as blinking dots.

The aeroplanes look like blinking dots on the radar.

So the next question is,

how do the ATCs get this information?

The details of every single plane.

They get this information from the RADAR.

Radar is a device that uses radio waves

to detect objects that are at a distance from it.

Radar can detect aeroplanes, spacecraft, and even tornadoes.

But the radar used to detect aeroplanes,

can be classified into 2 parts.

Primary radars and secondary radars.

Primary radar is a system for which

antennas have been placed at strategic locations all over the world,

those antennas emit radio waves,

and when the radio waves return after being reflected by the aeroplanes,

they calculate the distance of the plane based on the time taken by the radio wave.

Primary radars can tell you the horizontal distance of an aeroplane,

but they can tell you the altitude of the aeroplane.

Secondary radars are used to calculate the vertical distance.

This is a complex bit of technology.

A transponder is placed on top of an aeroplane,

And the radio wave signals from the primary antennas,

are caught by the transponder,

and responds with its own signal.

With this, it provides more information

about the position of a flight

much more reliable information.

The technology of the secondary radar is quite new.

During the 1990s, when the incident took place,

the secondary radar wasn’t used then.

The ATCs relied solely on the data from the primary radar.

But the data from the primary radar wasn’t as reliable.

It could tell you only the approximate distance between 2 planes.

To find out the exact altitude then,

the ATCs constantly communicated with the pilots,

and the pilots would inform them about the altitudes.

And then, the ATCS would jot down the height and speed parameters,

they would use it to manually calculate and chart the flight path.

Can you imagine this?

In a busy airport like Delhi,

where 20-30 flights land and take off every hour,

how tedious this work would have been for the ATCs.

It was a stressful job.

It is still a stressful job,

but it used to be even more stressful back then.

And it wasn’t that the ATCs weren’t trained.

They were very educated and extensively trained.

but after a point, the workload and stress become too much for any person.

Delhi Airport’s Deputy Director JS Ahuja had said that

This is why at one point in the 1990s,

ATCs were considered to be the most stressful work in the world.

More stressful than the doctors.

Even more than racecar drivers.

It was the same in India.

The job of the ATCs was extremely stressful.

What were the mistakes? And who made them?

Mistakes that led to this deadly disaster.

2 planes crashed into one another.

The thing is that friends,

there is a popular theory regarding accidents.

The Swiss Cheese Model.

As you know, this is how Swiss Cheese looks.

It has many holes in it.

If you stack the cheese slices on top of each other,

the holes don’t exactly match with each other.

You can look across the slices,

because, the possibility that all the holes would align,

is very negligible.

Even if one hole doesn’t align,

you won’t be able to look through it.

It will be opaque.

It’s said that it is the same with accidents.

The accidents that take place

aren’t because of a singular reason.

Rather, multiple reasons align at the right time.

Well, it’ll be the wrong time, I suppose.

Series of small mistakes that happened at the same time,

which led to this major disaster.

Even if one of these mistakes didn’t happen,

this disaster might have been prevented.

On the historic evening of 12th November,

it happened.

A series of small mistakes added up to this disaster.

First of all, the Indian officials thought

the Saudi aeroplane hadn’t followed the instructions properly,

that even though they were instructed to stay at 14,000 ft,

the plane might have gone up to 15,000 ft,

and crashed with the Kazakh aeroplane.

But when the black boxes of the planes were found,

flight data recorder revealed,

the exact recording of the conversations between the pilots,

and they found that the Saudi aeroplane,

was at 14,000 ft,

and the crash between the two aeroplanes,

was at 14,000 ft.

It meant that the Kazakh aeroplane,

had descended from 15,000 ft to 14,000 ft.


VK Dutta had told the pilots of the Kazakh aeroplane,

to not descend below 15,000 ft.

The audio recording from the black box showed

that the pilots in the Kazakh aeroplane,

weren’t even listening properly to what the ATC was saying.

They were complaining to each other about the traffic in Delhi,

and the traffic hangups in Delhi airport.

The second thing they found,

was that these Kazakh pilots didn’t even understand English properly.

In fact, there was a radio operator on the plane,

who was translating the messages from the ATC,

from English to Russian, so that the pilots could understand.

The report of the accident concluded that

the instruction from the ATC,

was subjected to an error of understanding.

The ATC told them to maintain the flight level at a certain altitude,

but they misunderstood it.

Or they thought that the instruction was for some other flight.

They misunderstood the instruction,

and so they descended.

Friends, it was the time when the Soviet Union had recently broken up,

and Kazakhstan was a new country,

and the Russian National Airlines,

was demerged into the airlines of several countries

when the Soviet Union broke up.

The reputation of these airlines wasn’t good at all.

It’s said that once,

a pilot gave the controls of the plane to his child.

There were stories of pilots doing vodka shots before landing.

Overall, the reputation of Russian pilots wasn’t good.

But the bigger question here, is that,

one misunderstanding could lead to a disaster of this scale,

were the designers of the system

relying on proper communication between the pilots and ATCs?

The other problem was the air corridors that I talked about in this video.

One air corridor for the departing aeroplanes,

and one for the incoming planes.

But it wasn’t so in the 1990s.

There was only one air corridor.

Third: as I told you, secondary radars weren’t used at the time,

so the ATCs had to rely on the communication with the pilots.

After this horrific air disaster happened,

our government, the Indian officials,

and aviation experts all around the world,

learnt from this.

They made many significant changes to the system,

so that such an accident wouldn’t be repeated.

And thankfully, till now, such a mishap hasn’t occurred in India again.

After this, aeroplanes haven’t collided mid-air.

The reasons?

First: there are now separated air corridors

for the incoming and departing planes.

Second, the secondary radar,

the Directorate General of Civil Aviation,

is a statutory body of the Central Government.

For years it had been recommending

that secondary radars should be installed on aeroplanes,

but the government didn’t pay any heed.

But as one official said,

The government was willing to take some action.

The government made it mandatory for every flying aircraft,

to be installed with the TCAS.

Traffic Collision Avoidance System.


5 months after the Charkhi Dadri accident,

the Civil Aviation Minister CM Ibrahim announced in the Lok Sabha,

that after 31st December 1998,

all 30+ seater aircraft in India,

have to be compulsorily fitted with the TCAS.

It is basically a system that,

rings an alarm in the aeroplane,

if any aircraft gets closer than 1,000 ft of vertical distance.

Even more important than that is that

once the alarm is deployed,

this system informs the pilot

of the direction to fly in, so that the 1,000 ft distance can be maintained.

So if another aircraft approached you from below,

this alarm will ring, and it will tell the pilot to fly upwards.

This was a major step for India.

Because at the time, the TCAS technology was very expensive.

Many countries complained that

they couldn’t fit their aeroplanes with TCAS,

because it was very expensive for them.

But India stood its ground.

And eventually, other countries had to implement this system in their aeroplanes too.

The third issue raised by this accident,

was the language and its nuances.

When someone speaks,

how well can the person listening to it understand it?

Might it be misinterpreted?

The potential misunderstanding between the ATC and pilots,

how can it be avoided?

More than half the countries have their airlines,

their pilots and ATCs,

who communicate in their local language,

people from so many backgrounds and cultures,

how can they communicate with each other,

so that there is no misunderstanding?

What can be the solution?

In 1998, India raised this issue in front of the International community,

in front of the ICAO.

It was proposed that every pilot would

have a minimum level of English proficiency,

to help them communicate.

It was debated extensively.

Because in many countries, English isn’t the first language.

What could those countries do?

But some language had to be chosen,

or else there would always be miscommunications.

And so English was eventually chosen

as the international language.

Today, if any pilot has to fly an aircraft internationally,

it is mandatory that they are well versed in English.

An interesting question is that

why was English chosen?

Why is English the most popular, the number one language in the world?

I have actually made a detailed video on this,

you can watch it after this video,

the link to it is in the description.

The rule is that to fly internationally, English is a must,

but if it’s a domestic flight, within the country,

the rule will not apply.

Pilots can use their national languages,

to communicate with each other.

Basically, if we go back to the Swiss Cheese model,

the Indian government tried

along with the aviation organisations across the world,

to fill up every possible hole in the Swiss Cheese model.

So that there is no such accident in future.

And fortunately, till now,

there has been no other mid-air collision.

This is the basic reason.

Today, when you wonder,

why do two aeroplanes flying in the sky collide with each other,

why there aren’t such accidents,

this is the reason.

It’s not that all aeroplane crashes have been prevented.

We still witness some aeroplane crashes.

Like this plane crash in China,

I made this video on it,

or the Malaysian Airlines crash.

The reasons for those crashes are different.

Those may not be solved this easily.

To understand those reasons, you can watch these videos.

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