Charkhi Dadri, one of the worst mid-air collisions: What happened in the cockpit that day | Long Reads News

The Indian Express’s coverage of the November 12, 1996 disaster. Archive

“For any air traffic controller, it’s the worst nightmare… a midair collision,” said air traffic controller V K Dutta in the National Geographic series, Air Crash Investigation, talking about the midair collision between a Saudi Arabian Airlines Boeing 747 and a Kazakhstan Airlines Ilyushin IL-76 on November 12, 1996, over Charkhi Dadri in Haryana, about 65 km west of Delhi.

Dutta should know. On the fateful day, he was the approach controller on duty at the Indira Gandhi International Airport in New Delhi and was in contact with the pilots of both the Boeing 747 and IL-76.

At 6.40 pm, the jets collided midair. The massive jets, covered in flames, plunged into the mustard fields below, in two wreckage fields seven kilometres apart. All 349 people on board both the aircraft were killed.

In 1996, I was a second-year undergraduate student. The year before, I had logged over 150 sorties, strapped inside a glider cockpit as an NCC cadet at Bhubaneswar airport. Three IAF pilots were our instructors. The glider in which we trained had four instruments: an air speed indicator (ASI), a vertical speed indicator (VSI), an altimeter and what was known as a turn & slip indicator. I had not been near a commercial jet yet, leave alone step inside one.

But the analog altimeter with three needles (shortest for thousands of feet, middle for hundreds of feet, longest for tens of thousands of feet) in our glider was the same one you would find in any cockpit with analog instruments, including big jets. So, I felt I had something to say on the Charkhi Dadri crash. I wrote a ‘letter to the editor’, The Telegraph, Kolkata, speculating not on what could have caused the crash but all the ways in which an altimeter can show erroneous readings. After all, the two jets collided as both, inadvertently, were at the same altitude. I would be proven wrong, of course, my letter little more than a display of my youthful impetuousness.

I preserved the India Today issue on the crash for several years, flipping through it from time to time. One of the photographs carried in the issue was very haunting: three passenger seats from the Saudi jet, still joined together, found in the fields upright, a lifeless passenger occupying one seat.

The Indian Express devoted its entire Page 1 to the tragedy. Among the stories was Ashwini Sarin’s poignant account of what he saw at the site where the wreckage of the Saudi airliner lay. “There are no survivors here, nobody to tell me what happened at that fateful moment, no one to tell the story,” he wrote.

The Indian Express’s coverage of the November 12, 1996 disaster. Archive

November 12, 1996: The crash

Saudi Flight 763 was a scheduled passenger service from Delhi to Dhahran in Saudi Arabia, with 312 passengers and crew on board. In the cockpit were 45-year-old Capt Khalid al-Shubaily, first officer Nazir Khan and flight engineer Ahmed Edrees.

Kazakh Flight 1907 was a chartered flight from Chimkent in Kazakhstan to Delhi. There were 37 tourists and crew on board. In the cockpit were 44-year-old Capt Alexander Cherepanov, first officer Ermek Dzhanbaev, flight engineer Alexander Chuprov, navigator Zhahanbek Aripbaev and radio operator Egor Repp.

Dutta was the approach controller at Delhi airport at the time, guiding both aircraft.

An airspace is divided into segments, each handled by a different controller. The tower controller handles aircraft in the immediate vicinity of an airport. The next chunk of airspace is handled by approach controllers such as Dutta, guiding approaching or departing aircraft. Beyond the approach controller’s airspace, area controllers guide aircraft.

Kazakh Flight 1907 was flying at 23,000 ft, about 74 nautical miles from Delhi airport, when its crew first contacted Dutta. He cleared the flight to descend and maintain 15,000 ft.

According to all accounts – ATC transcripts and the Cockpit Voice Recorder (CVR) data from the two aircraft – after takeoff, Saudi Flight 763 was first cleared to fly at 10,000 ft and then at 14,000 ft. Dutta instructed the crew to maintain 14,000 ft and stand by for permission to climb higher.

In those days at Delhi airport, for both arriving and departing commercial jets, there was only a single corridor. The other corridors were reserved for the military. Which means, the Kazakh and Saudi jets were flying towards each other in the same air corridor from opposite directions. Which is why, Dutta instructed the Kazakh crew to maintain 15,000 ft and the Saudi crew to maintain 14,000 ft to ensure the mandatory 1,000-ft separation between the jets when they crossed paths.

Delhi airport then was equipped only with a primary radar. It did not have a Secondary Surveillance Radar (SSR). With a primary radar, controllers can only see on their screen the various aircraft present in the airspace without any other information.

An SSR works differently. It automatically engages with the transponder on board an aircraft and relays vital details like the aircraft’s identity, position, altitude, speed etc to the controller. With an SSR, a controller need not ask a pilot about the aircraft’s altitude, exact position etc.

Working with only a primary radar, Dutta depended on the pilots of the Kazakh 1907 and Saudi 763 to know their altitudes. Both crew acknowledged Dutta’s instructions to maintain 15,000 ft and 14,000 ft, respectively. “Saudi seven six three (will) maintain one four zero (14,000 ft),” the Saudi crew acknowledged. This was their last transmission to the ATC.

“Kazakh 1907, now reached one five zero (15,000 ft)…,” acknowledged the crew of the IL-76, which was now 46 nautical miles from the Delhi airport.

Dutta had to take the pilots’ word on the aircraft’s altitudes. Without an SSR, he had no way to independently verify if the altitudes he had assigned were being complied with.

As both jets flew towards each other, Dutta warned the Kazakh pilots of the Saudi jet approaching them, “Roger. Maintain 150. Identified traffic at 12 o’clock, reciprocal Saudi Boeing 747, 14 miles. Report in sight.”

Here’s what Dutta’s transmission meant:

Clock code method is widely used in aviation to identify other traffic in the vicinity visually. Imagine a clock face. Imagine your aircraft as a dot at the centre of the clock face, its nose pointing towards 12.

“Roger. Maintain 150.” (Kazakh pilots are told to maintain 15,000 ft.)

“Identified traffic at 12 o’clock.” (On a clock face, Saudi jet was at 12 o’clock position relative to Kazakh jet.)

“Reciprocal Saudi Boeing 747, 14 miles.” (Saudi jet was flying towards Kazakh jet on a reciprocal direction — opposite of Kazakh jet’s direction — and was 14 miles away from Kazakh jet.)

The Kazakh crew responded, “Now looking 1907.” That was their last transmission to the ATC.

The ATC tape transcript shows that the flight crew of the Saudi Arabian flight had let out a prayer in those final moments, seeking forgiveness from Allah: “Ashhadu an la ilaaha illallah.”

The pilot of an USAF cargo flight, which was descending into Delhi and was in the vicinity of the Saudi and Kazakh jets, first alerted Dutta about the midair collision. “We saw something to our right, looks like a big fireball, looks like a big explosion,” the pilot told Dutta.

In the aftermath, the Directorate General of Civil Aviation appointed Capt K P S Nair, deputy director, Flight Crew Standards, as Inspector of Accident.

What the investigation found

The government set up a Court of Inquiry led by Delhi High Court’s Justice R C Lahoti, who was assisted by Capt A K Verma, Director (Air Safety), Air India, and Air Cmde (retd) T Pannu, former director, operations (ATC), IAF.

The investigation did not find any fault with Dutta and said he had given correct instructions to both flight crew. The inquiry did not find any fault with the equipment or the altimeters either.

It held that the midair collision happened because the Kazakh pilots did not maintain their assigned altitude of 15,000 ft and descended to 14,000 ft.

In the course of the inquiry, the investigators discovered something unusual. The Kazakh jet, though assigned to fly at 15,000 ft, had sliced the Boeing 747, assigned to fly at 14,000 ft, not from above but from below.

The inquiry found that at one point when the Kazakh crew acknowledged that it was at 15,000 ft, the aircraft’s actual altitude was 16,439 ft. Prior to impact, the Kazakh jet had descended to 14,000 ft.

As recorded in the CVR, four seconds before the jets collided, Repp, the radio operator on the Kazakh aircraft, saw the Saudi jet coming and shouted, “Get to 150 (15,000 ft), because on the 140th, uh that one uh…!”

Why would such an experienced crew commit such a fatal error?

There was no mechanical failure or defect in either of the aircraft. There was no fault on the part of Delhi ATC. Investigators focused their attention on the Kazakh crew and what went on in the cockpit in those final seconds.

It found that it was Repp, the radio operator of the Kazakh flight, who was communicating with Dutta. Repp sat behind the pilots, facing sideways. He did not have an altimeter in front of him to check the aircraft’s altitude. He needed to glance over the shoulder of the captain or co-pilot to see the readings on their altimeters.

Inside the cockpit, there was no verbal acknowledgement from the pilots that they had heard Dutta’s communication with Repp and understood it. Investigators found shortcomings in Capt Cherepanov’s leadership, and the communication and teamwork among the crew of the Kazakh aircraft.

Another possible reason for the Kazakh jet deviating from its assigned altitude, investigators felt, could be the pilot’s poor proficiency in English, who may have misunderstood the altitude assigned to the Saudi jet as his own.

Kazaki authorities, defending the pilots, argued that the jet was unable to maintain 15,000 ft and lost height because of turbulence. It was ruled out by the inquiry.

Recommendations for the future

The inquiry made several safety recommendations to prevent such an accident in future, among them, equipping major airports with SSRs and separate air corridors for arriving and departing aircraft.

Neither the Saudi jet nor the Kazakh jet were equipped with Traffic Alert and Collision Avoidance System (TCAS). The onboard equipment automatically monitors other aircraft in the vicinity and resolves potential conflicts without waiting for ATC guidance. For example, if two planes are on a crash course, TCAS on one aircraft will ask the crew to climb, while TCAS on the other aircraft will ask the crew to descend. The inquiry recommended the mandatory fitting of TCAS on aircraft.

The writer is Deputy Associate Editor, The Indian Express, and an aviation enthusiast