1996 Charkhi Dadri mid-air collision

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 Soviet-era 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 claimed – 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 ft 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 team noted that the 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. Moreover, neither plane was equipped with a traffic collision avoidance system, which would have alerted crew on both flights.

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

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