The black box is especially critical to determine the cause of the crash as the aircraft did not send out any distress messages or signals before slamming into the mountains of Arunachal. One of the few clues investigators have is eyewitness accounts by villagers that they saw the aircraft on fire before it went down.

Defence Ministry officials said that the upgrade deal has been signed with the Foreign Trade Enterprise of Ukraine and the life-extension program will start this year itself. While the first batch of aircraft will be sent to Ukraine between 2009 and 2013 for refurbishment, the remaining aircraft will be upgraded at the Kanpur-based Base Repair Depot of the Air Force.

The upgrade will include life-extension programs for the engines, new avionics and installation of modern communication equipment. The IAF hopes to extend the life of the aircraft by close to 15 years while the refurbished engines will also improve the range and cargo-carrying capacity of the aircraft.