One instrument almost all aircraft have is an airspeed indicator. Of course, it’s important to know how fast we’re going, if for no other reason than bragging rights, and that’s true for airplanes, rotorcraft, gliders and even dirigibles (hot air balloons need not apply). Piston-powered personal airplanes, meanwhile, often have a bunch of color-coded markings on their airspeed indicator (ASI), denoting things like best single-engine rate of climb (VYSE) or minimum-control airspeed (VMC), when considering twins, and flap operating range (VFO) and normal operating range (VNO) for everything else, including the twins. There also are white, green and yellow arcs, plus another red line at the high end of the ASI when flying a twin. Single-engine airplanes just have the single red line at the top end, the never-exceed speed (VNE).
One thing piston-powered airplanes have that turbine-powered airplanes don’t? A yellow arc, which is the airspeed range between VNO and VNE. That’s for certification reasons: if you replace a piston engine and bolt on a turboprop, your new red line is at the top of the green arc, and the yellow arc is eliminated. For that matter, the green arc may be eliminated, also, leaving only the white one.
