[brandmeyer]

So long as we are being pedantic: The sensor just reads dynamic pressure. To get IAS from the pressure signal it is sufficient to solve the equation dynamic_pressure = density * indicated_airspeed^2, where density is taken to be air at a reference temperature at sea level (mumble mumble except for transonic and supersonic conditions mumble). In order to compute true airspeed, you need to measure the absolute pressure and temperature of the air you're traveling through in order to calculate the density.

The aerodynamic forces are proportional to the dynamic pressure, which is the native reading on the sensor. But when you relate dynamic pressure to indicated airspeed, the do-not-exceed airspeed doesn't change, so you can stick a single red line on the IAS gauge that's always valid.

Uh, I think they have a red line on the gauge, anyway. I just saw an unmanned prototype break up in flight partially because of the lack of said red line, so I'm kinda just assuming that passenger aircraft have one.

[mikeash]

When it comes to redline, there are some factors which relate to true airspeed, not indicated. Smaller aircraft (maybe larger too, I'm just limiting it to "smaller" due to my own ignorance) at higher altitude tend to be limited by flutter (a situation where positive feedback from movements of the wings or other surfaces shakes the plane apart) which mostly depends on true airspeed. In my plane, for example, never-exceed speed (Vne) is 143kts indicated at sea level, 121kts indicated at 10,000ft, and a mere 73kts indicated at 36,000ft.

The 143kts figure is marked on my airspeed indicator, but above 10,000ft I have to remember to refer to it elsewhere if I want to go fast.

[drewkett]

FYI

  dynamic_pressure = 0.5 * ref_density * equivalent_airspeed^2

Its not a function of indicated airspeed. Indicated airspeed needs to be corrected for the compressibility of air to obtain the equivalent airspeed. At low airspeeds/altitudes, the indicated airspeed and equivalent airspeed are very close, so your equation would hold up just fine.

As a result at higher altitudes and faster speeds, the red line on an indicated airspeed gauge would be variable to account for the effects of compressibility.