The Mi-8 helicopter went down during a training mission late Wednesday, killing Roman Frolov, the chief of the Zhukovsky aviation rescue centre, and two other men. It crashed into a forest about 10 kilometers (6 miles) southeast of the capital. The cause of the crash wasn't immediately clear, but some Russian reports mentioned engine failure as a possible cause. The Emergencies Ministry on Thursday praised Frolov as a seasoned professional who has proven his skills in putting down forest fires and other missions. The twin-engine Mi-8 has been a mainstay of Soviet and then Russian aviation since it entered service in the late 1960s and also has been widely used worldwide.