After seven years of test flying and more than fifteen years of commercial operations Concorde remains unrivalled and unique among civil transport aircraft. Many problems, technical, political and financial have been overcome and the aircraft, flagship of both Air France and British Airways, is now proven, safe and reliable. The aircraft's unmistakable low aspect ratio delta wing, optimized for supersonic cruise, brings with it the penalty of high drag at low speed; a factor impressed on flight crews during initial conversion to type. Concorde's large flight envelope is severely restricted by the need to balance the movement of the aerodynamic centre of pressure by a shift of aircraft Centre of Gravity. The dual-compressor axial flow Rolls-Royce Olympus engines, augmented by afterburning for take-off and transonic acceleration, provide the high jet velocity and high thrust needed for Mach 2 flight.