Helicopter rotor of gravity fore-aft. However, it requires the expense of two large rotors rather than the more common one large main rotor and a much smaller tail rotor. The Boeing CH-47 Chinook is the most common tandem rotor helicopter. Coaxial rotors are a pair of rotors mounted one above the other on the same shaft and turning in opposite directions. The advantage of the coaxial rotor is that, in forward flight, the lift provided by the advancing halves of each rotor compensates for the retreating half of the other, eliminating one of the key effects of dissymmetry of lift: retreating blade
1994 Scotland RAF Chinook crash data greatly reduced the amount and quality of data available to subsequent investigations. Information on speed and height were derived from the position of cockpit dials in the wreckage, and the wreckage's condition. During 1994, the RAF had begun to fit these recording devices across the Chinook HC.2 fleet prior to the accident; this process was completed in 2002. The aircraft had not been classified as being airworthy. It was mandated upon the RAF that it was "not to be relied upon in any way". The documentary series "Seconds From Disaster" featured the accident in the episode "Chinook Helicopter Crash".