World's deadliest "mid-air" collision

On the 12th of November 1996, Saudia Flight 763, a Boeing 747 en route from Delhi-Indira Gandhi International Airport (DEL/VIDP), India to Dhahran International Airport (DHA/OEDR), Saudi Arabia, and a Kazakhstan Airlines Flight 1907, an Ilyushin Il-76 en route from Shymkent Airport (CIT/UAII), Kazakhstan to Delhi-Indira Gandhi International Airport (DEL/VIDP), India collided over the village of Charkhi Dadri, around 100 km (62 miles) 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 to occur in India. There were 312 POB the Saudi flight and 37 POB the Kazakhstan flight. The crash was caused by failure of the Kazakhstani crew to maintain the correct altitude, because of confused dialogue with the tower, communicated via the radio operator, while also apparently fighting turbulence.

Investigations showed the Kazakh aircraft had descended below its assigned altitude of 15,000 feet and was flying at 14,500 feet. A few seconds later the same aircraft had descended another 310 feet, giving an altitude of 14,190 feet, 810 feet below its assigned altitude. Shortly afterwards both aircraft collided, plummeting down in flames and crashed in an arid farming area.

The Saudia flight departed Delhi at 18:32 local time while the Kazakhstan flight was descending simultaneously to land at Delhi. Both flights were controlled by approach controller. The Kazakhstan Airlines plane was cleared to descend to 15,000 feet when it was 74 nautical miles (137 km) from the beacon of the destination airport, while the Saudia plane, travelling on the same airway but in the opposite direction, was cleared to climb to 14,000 feet. About eight minutes later, around 18:40, the Kazakhstan flight reported having reached its assigned altitude of 15,000 feet, but it was actually lower, at 14,500 feet, and still descending. At this time, ATC advised the flight, "Identified traffic 12 o'clock, reciprocal Saudia Boeing 747, 10 nautical miles (19 km). 

Report in sight."

The controller called Flight 1907 again and again, but he received no reply. He tried to warn of the other flight's distance but was too late. The two aircraft collided, with the tail of the Kazakhstan flight slicing through left wing of the Saudia 747 and horizontal stabiliser.

The factors contributing to the unauthorised descent of Kazak aircraft to FL-140, departing from the assigned FL-150, were:
1) inadequate knowledge of English language of Kazak pilot, resulting in wrong interpretations of ATC instructions.

2) poor airmanship and lack of proper CRM (Crew Resource Management) skill on the part of PIC (Pilot-in-Command) compounded by leadership quality lacking in him.

Aircraft Information No 1

Airline: Saudi Arabian Airlines

Code: SV/SVA

Aircraft: Boeing 747-168B

Registration: HZ-AIH

Serial Number: 22748

Engines: 4 Rolls-Royce RB211-524C2

First Flew: 03/02/1982

Age at accident: 14 Years 10 Months

Aircraft Information No 2

Airline: Kazakhstan Airlines

Code: K4/KZA

Aircraft: Ilyushin Il-76TD

Registration: UN-76435

Serial Number: 1023413428

Engines: 4 x Aviadvigatel PS-90 turbofan engines