(...)No, the real problem is not mechanical. The real danger is exceeding the Never Exceed Speed, noted as Vne.

Many pilots assume that operating at high altitude (greater than 12,500 ft, say), even with the increased power supplied by a turbocharger, will not be a problem if the mechanical problems are solved. Sure, they can go faster, but not so much faster that they exceed the limitations marked in living color on the airspeed indicator. How, they ask with apparently perfect logic, can the airplane be exceeding Vne if the needle is in the green arc?

Because the airspeed indicator is The Gauge That Lies. Despite its name, an airspeed indicator does not measure speed. It measures “q” – dynamic pressure caused by packing air molecules into a tube. Now, several limiting speeds like stall speed (bottom of the green and white arcs), gust loads (top of the green arc), and maneuvering speed (blue line) are also functions
of q, so they may be read directly off the dial. In these cases, the logic is true.

This logic is NOT true for the very important red line at the top of the yellow arc. Here’s why:

Consider an aircraft flying in smooth air at cruise speed. The aircraft structure is then slightly disturbed (such as by turbulence). In response, the aircraft structure will oscillate with amplitude decreasing until the oscillation stops altogether. This dynamically stable response is due to damping acting on the system, either from the aircraft structure and/or air. If the cruise speed is incrementally increased there will be a particular speed at which the amplitude of structural oscillation will remain constant. The speed at which constant amplitude oscillation can be first maintained is
defined as the “critical flutter speed”, or more generically “flutter speed”. Flutter is almost a pretty word. You’d associate it with butterflies and silk handkerchiefs. But in the engineering sense, it can be highly destructive. Once flutter has started, the amplitude may quickly become so large that a structure will disintegrate, literally shaken to pieces.

Remember, as the airplane climbs, there are fewer air molecules and less air pressure, so the needle on The Gauge That Lies reads a lower speed, even though the airplane is actually going just as fast. That’s why True airspeed is faster than Indicated. But flutter does not depend on Indicated Air Speed/dynamic pressure. It is directly related to True
Air Speed — the velocity of the air passing by the airframe. The velocity of the excitation force is the prime concern, not the magnitude. It is very possible to exceed this critical “flutter speed” without encountering flutter if there is no initial disturbance. But if the critical flutter speed is exceeded and then a disturbance is encountered, the aircraft structure will begin to oscillate in response to the velocity of the passing air. This is not a typical resonance, where either increasing or decreasing the speed will move the aircraft away from the critical frequency and the vibration will
stop on its own. Going faster merely pumps more energy into the system, increasing the amplitude of the flutter. Go faster, flutter harder. Only going slower and lowering the velocity of the air over the airframe will solve the problem.

You’ve probably seen film of the collapse of the Tacoma Narrows Bridge. Built before the aerodynamics of bridges was fully understood, this bridge could probably have withstood the dynamic pressures of a hurricane. But one day, the wind speed
was just right – about 42 mph -- to match the natural flutter frequency of the bridge. The bridge started moving, undulating more and more until the whole structure collapsed. There’s still hundreds of tons of concrete and twisted steel out there at the bottom of the bay. The slow writhing of the bridge makes good video, but when flutter occurs in an airplane, destructive
failure can be reached at a speed that human senses would detect as a sudden explosion, rather than a vibration. There is
no warning, no time to react, certainly no time to slow down.