New Delhi, Nov 20 (IANS) The defence ministry has in principle approved the purchase of US firm Boeing's C-17 heavy lift aircraft for the Indian Air Force (IAF), which is hoping to replace its ageing fleet of Russian IL-76 transport aircraft, an official said Friday.

'The IAF had shortlisted the aircraft on the basis of its capability to take off and land on short runways with heavy loads. Now the defence ministry has approved in principle the acquisition and has sent a letter to the US authorities seeking formal offer for the aircraft,' a senior IAF official told IANS.

The development comes ahead of Prime Minister Manmohan Singh's visit to the US beginning Saturday.

The Defence Acquisition Council, chaired by Defence Minister A.K. Antony gave the nod for the procurement recently.

The IAF is looking at acquiring 10 C-17 Globemaster III heavy lift aircraft initially through the US government's Foreign Military Sales (FMS) route, and a proposal in this regard was being considered by the Ministry of Defence (MoD), the official said, adding that the aircraft should come in about three years after a contract is signed.

The IAF currently has less than 20 IL-76 aircraft, acquired in 1980s. An IL-76 can carry a cargo of around 45 tonnes and has a crew of six, while a C-17 can carry 70 tonnes, and is much easier to operate with a small crew of two pilots and one loadmaster (total three), thanks to its various power-assisted systems. Two observers though can also be seated.

Despite its massive size - 174 feet in length, 55 feet in height and about a 170 feet wingspan - a pilot can fly the C-17 with a simple joystick, much like a fighter aircraft that can be lifesaving in a battle zone as the aircraft can take off quickly and at steep angles. It is powered by four Pratt & Whitney F-117-PW-100 turbofan engines.