Sounding Off In “Return of High-Speed Air Travel Faces Environmental Headwinds” (Feb. 25-March 10, p. 58), the phenomenon of Mach cutoff is described as occurring when aircraft fly at a speed that is supersonic at its cruise altitude, but not on the ground, because the local speed of soundreduces as air density decreases with altitude. Since sound velocity in an ideal gas is a function of temperature only, it would be more accurate to say: The local speed of ...


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