1996 Charkhi Dadri mid-air collision

On 12 November 1996, Saudia Flight 763, a Boeing 747 en route from Delhi, India, to Dhahran, Saudi Arabia, and Kazakhstan Airlines Flight 1907, an Ilyushin Il-76 en route from Chimkent, Kazakhstan, to Delhi, collided over the city of Charkhi Dadri, around 100 km west of Delhi. The crash killed all 349 people on board both planes, making it the world's deadliest mid-air collision and the deadliest aviation accident ever in India. The final report from the investigation found that "[T]he root and approximate cause of the collision" was the failure of the Kazakh crew to maintain the correct altitude. Contributing factors included the poor English language skills in the Kazakh cockpit resulting in inadequate interpretation of directions provided by air traffic control, and three specific incidents of failures in crew resource management (CRM) by the Kazakh crew. The report also suggested technical enhancements (including ACAS and SSR) that would provide assistance in preventing a future crew's mistakes being allowed to go unchecked in real time.

Saudi Arabian Airlines Flight 763
Saudi Arabian Airlines Flight 763 was a Boeing 747-168B, registration HZ-AIH, departing from Delhi as part of a scheduled international Delhi–Dhahran–Jeddah passenger service with 312 people on board. The crew on this flight consisted of Captain Khaled al-Shubaily (aged 44), First Officer Nazir Khan (aged 37), and Flight Engineer Ahmed Edrees (aged 33). Al-Shubaily was a veteran pilot with 9,837 flying hours.

There is some dispute as to the nationalities of the passengers of Flight 763. According to an article published on 14 November 1996, 215 Indians, many of whom worked in Saudi Arabia as blue-collar workers, boarded the flight, along with 40 Nepalis and three Americans. However, according to a different article, the passenger manifest included 17 people of other nationalities, including nine Nepalis, three Pakistanis, two Americans, one Bangladeshi, one British, and one Saudi Arabian. Twelve of the crew members, including five anti-terrorism officials, were Saudi Arabian citizens.

Kazakhstan Airlines Flight 1907
Kazakhstan Airlines Flight 1907, an Ilyushin Il-76TD with registration UN-76435 was on a charter service from Chimkent Airport to Delhi. The crew consisted of Captain Alexander Robertovich Cherepanov (aged 44), First Officer Ermek Kozhahmetovich Dzhangirov (aged 37), Flight Engineer Alexander Alexanderovich Chuprov (aged 50), Navigator Zhahanbek Duisenovich Aripbaev (aged 51), and Radio Operator Egor Alekseevich Repp (aged 41). Cherepanov was highly experienced, with 9,229 flight hours.

A company from Kyrgyzstan chartered the flight, and the passenger manifest mostly included ethnic Russian Kyrgyz citizens planning to go shopping in India. Thirteen Kyrgyz traders boarded the flight.

Accident
SVA Flight 763 departed Delhi at 18:32 local time (13:02 UTC). KZA Flight 1907 was descending simultaneously, to land at Delhi. Both flights were controlled by approach controller V.K. Dutta. Immediately after take-off, the SVA flight was cleared to an initial altitude of 10000 ft. At 18:34, Dutta cleared the KZA plane to descend to 15000 ft when it was 74 nmi from the beacon of the destination airport. Two minutes later, at 18:36, Dutta cleared the SVA plane, travelling on the same airway but in the opposite direction, to climb to 14000 ft. At 18:38, the SVA crew reported reaching 14,000 feet, and requested a higher level. Dutta told them to hold their altitude and standby, to which First Officer Khan replied, "Saudi 763 will maintain one four zero."

At 18:39, the KZA flight reported having reached its assigned altitude of 15,000 feet, but it was actually higher, at 16348 ft, and still descending. At this time, Dutta advised the flight, "Identified traffic 12 O'Clock, reciprocal, Saudia Boeing 747 at ten miles, likely to cross in another five miles. Report, if in sight." Radio Operator Repp requested clarification, to which Dutta replied, "Traffic...is at eight miles, level 140." Repp acknowledged the update, and signed off with, "Now looking 1907."

Less than a minute later, at 18:40, the crew of a United States Air Force cargo flight made a radio call saying they had seen "a big explosion" at their two o'clock position. Dutta attempted to contact the SVA and KZA flights, but received no response. The two aircraft had collided, with the left wing of the KZA flight slicing through the left wing of the SVA 747 while the left horizontal stabilizer of the 747 sliced off the vertical stabilizer (including the horizontal stabilizer) of the KZA flight. The crippled Saudi Boeing quickly lost control and went into a rapidly descending spiral with fire trailing from the wing, and broke up in mid air before crashing into the ground at a nearly supersonic speed of 1135 km/h. With most of its left wing and vertical stabilizer gone, the Ilyushin went into a flat spin and crashed into a field at a flat attitude near the wreckage of the Saudia plane. All 312 people on board SVA763 and all 37 people on KZA1907 were killed.

The recorder of the Saudi Arabian plane revealed the pilots said the Islamic Istighfar (forgiveness prayer) and recited the Shahada before impact with the ground.

The collision took place about 60 mi west of Delhi. The wreckage of the Saudi Arabian aircraft landed near Dhani village, Bhiwani District, Haryana. The wreckage of the Kazakh aircraft hit the ground near Birohar village, Rohtak District, Haryana.

Investigation and final report
The crash was investigated by the Lahoti Commission, headed by then-Delhi High Court judge Ramesh Chandra Lahoti. Depositions were taken from the Air Traffic Controllers Guild and the two airlines. The flight data recorders (FDR) were decoded by Kazakhstan Airlines and Saudia under the supervision of air crash investigators in Moscow and Farnborough, England, respectively. The ultimate cause was held to be the failure of Kazakhstan Airlines Flight 1907's pilot to follow ATC instructions, whether due to cloud turbulence or due to communication problems.

The commission determined that the accident had been the fault of the Kazakhstani Il-76 crew, who (according to FDR evidence) had descended from the assigned altitude of 15000 to 14500 ft and subsequently 14000 ft and even lower. The report ascribed the cause of this serious breach in operating procedure to the lack of English language skills on the part of the Kazakhstan aircraft pilots; they were relying entirely on Radio Operator Repp for communications with the ATC. As part of this, the report suggested that First Officer Dzhangirov (and possibly Captain Cherepanov) might have misunderstood Dutta's final radio call, and assumed that the Saudi 747's altitude (14,000 feet) was their own assigned altitude. Indian air controllers also complained that the Kazakhstani pilots sometimes confused their calculations because they are accustomed to using metre altitudes and kilometre distances, while most other countries use feet and nautical miles respectively for aerial navigation. Although the crew in this particular collision did not appear to have made a computational error, Kazakhstan Airlines did not have enough foot-marked altimeters for all crewmembers. Repp did not have his own flight instrumentation and had to look over the pilots' shoulders for a reading, which likely limited his own situational awareness.

Kazakh officials found such a misunderstanding implausible, because their records showed Dzhangirov sufficiently fluent to teach English to other Kazakh pilots at the time. Instead, they concluded – from the Kazakhstani plane's misleading flight data records – that the aircraft had descended while their pilots were battling turbulence inside a bank of cumulus clouds. Meteorological reports, the affidavits of a Lockheed C-141B Starlifter crew that observed the crash, and conclusive analysis by the investigators – showing that the specific flight data record the Kazakh officials focused on proved only that the record was based on faulty recording equipment on the plane  – disproved the Kazakh theory that turbulence was to blame.

However, whether or not particular crewmembers fully understood the radio transmissions or were distracted by atmospheric conditions, the five-person crew was sufficiently large that it likely struggled to maintain common situational awareness and coordinate crew actions.

Just a few seconds before impact, Repp had realised that the pilots were flying below the required 15,000 feet and brought it to their attention. Cherepanov gave orders for full throttle, and the plane climbed, only to hit the oncoming Saudi Arabian plane. The tail of the Kazakhstani plane clipped the left wing of the Saudi Arabian jet, severing both parts from their respective planes.

Furthermore, the investigation noted that Indira Gandhi International Airport did not have secondary surveillance radar, which provides extra information, such as the aircraft's identity and altitude, by reading transponder signals; instead the airport had a primary radar, which produces readings of distance and bearing, but not altitude. In addition, departures and arrivals both shared a single corridor within the civilian airspace around New Delhi. While most areas have separate corridors: one for departures and another one for arrivals, the airspace of Delhi in 1996 had only one civilian corridor for both departures and arrivals, because much of the airspace was taken by the Indian Air Force.

Due to the crash, the air-crash investigation report recommended changes to air-traffic procedures and infrastructure in New Delhi's air-space:


 * Separation of inbound and outbound aircraft through the creation of 'air corridors'
 * Installation of a secondary air-traffic control radar for aircraft altitude data
 * Mandatory collision avoidance equipment on commercial aircraft operating in Indian airspace
 * Reduction of the airspace over New Delhi that was formerly under exclusive control of the Indian Air Force

Aftermath
The Directorate General of Civil Aviation subsequently made it mandatory for all aircraft flying in and out of India to be equipped with an airborne collision avoidance system. This set a worldwide precedent for mandatory use of traffic collision avoidance system.

, there is an ongoing effort by the Charkhi Dadri district administration to develop a memorial honoring the victims of the mid-air collision. The proposed memorial, which would consist of names and other information of the victims, would be located at a to-be built memorial-park in the district. However, the district administration is waiting to get an approval for the project from the Haryana government, and plans to include the participation of Ministry of Civil Aviation of India, Airport Authority of India, and embassies of Saudi Arabia and Kazakhstan in the development of the memorial.

Documentaries
Miditech, a company based in Gurugram, Haryana, produced a documentary about the disaster called Head On!, which aired on the National Geographic Channel.

The collision was also the subject of the 2009 episode "Sight Unseen", of the Canadian-made, internationally distributed documentary series Mayday.