Brian C Friesner
28th Feb 2005
Answer: Helicopter performance is dependent on a number of factors, including air density. As altitude increases, air density decreases and the performance of the helicopter falls as well. At the height of Mt Everest, the air density has fallen to a sufficient degree that most helicopters would be unable to function (few helicopters are capable of functioning above 20,000 feet, nine thousand feet lower than the summit of Everest).