A Russian heavy-lift military cargo plane crashed on takeoff Sunday in Siberia, killing all 11 crew members on board, officials said. The crash was the second in less than a month to involve an Il-76 , the mainstay of the Soviet and Russian airforce since the 1970s. These and a string of other accidents have raised concerns about the condition of Russia's aging fleet of Soviet-built aircraft. The cause of Sunday's crash was not yet known. The four-engine plane had just taken off from Mirny in the Sakha Republic when it tilted to the right and was unable to gain altitude, said Vasily Panchenkov, a spokesman for the Interior Troops, which were flying the aircraft.