The aircraft has a maximum speed rating, beyond which the structure may fail. That speed rating is based on the aerodynamic forces on the aircraft, which are proportional to the indicated airspeed, not the true airspeed. That's why the instrument panel almost universally shows IAS. That way the pilot doesn't need to perform an altitude-based nonlinear conversion in his head during an emergency to see that he's about to kill everyone on board.

Just so you know the forces are actually proportional to the square of equivalent airspeed which is indicated airspeed adjusted for the compressibility of the air at the sensor. Indicated airspeed is still a better number than true airspeed for approximating the aero forces

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.

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.

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.

Why? Do you take issue to the fact that scales show you a measurement in kg rather than N despite it varying as you move around the world (and potentially with changes in temperature, depending on the type of scales)? Velocity/mass is far more intuitive than pressure/force even if they don't apply exactly the same in all cases.

It's about what works easiest for the users of the system.

kg/N varies, but by less than your normal margin of error. That's vastly different from deciding to use newtons to weigh "kilograms indicated" on your trip to mars when they give a vastly different number.

My point was that we don't weigh ourself in 'newtons' despite that being what we are actually measuring. We make some reference conversion to a much more intuitive unit, kilograms. We rely on the fact that it's a pretty constant conversion between the two.

Is this really much different than that? I don't think so. The aerodynamics of the plane (stresses on the structure, performance of the aircraft w.r.t. stall, etc.) all remain ~constant for a given IAS which is why it's used. We're just making a dynamic pressure measurement and converting it to a speed in such a way that it factors in all the environmental conditions that influence what we actually care about - how the plane responds in the air.

It depends on the scales one uses. You're right for most bathroom scales, but not for balance-beam scales as one tends to find in physicians' offices; those measure mass and are accurate in a wide variety of accelerated reference frames (and in particular are insensitive to anisotropies in the Earth's gravitational field).

Accurate weight-measuring scales rely on m = F_w/g, where m is mass, F_w is the normal force delivered by the scale's measuring surface to the measured object, and g is almost always g_n := 9.806 m/s^2 per ISO 80000-3:2006 (item 3-9.2) or similar standards. (Tbh the ISO document does not do much more than offer up a defined value.) This relation only holds (and only approximately) when the scale is used as intended by its designer, and that condition picks out a set of preferred (and accelerated) reference frames. A "contact camp" supporter (like Baez[1]) would say that the measurement of weight is accurate in a much wider variety of frames, crucially including in local free fall.

Modern weight-measuring scales will almost always use SI units internally, but most can supply a variety of output units (the one in my bathroom can show kg, stone, lb, Newtons, and the value read from the piezo element package. It sadly has no internal bubble level or equivalent, and no accelerometer, and its software doesn't make use of data from devices within the mobile device it runs on (which could be placed on the scale between my feet). Finally, it was expensive enough to do any or all of the above, so I won't recommend it).

More colloquially:

http://www.livestrong.com/article/343183-the-differences-bet...

[1] http://math.ucr.edu/home/baez/physics/General/Weight/whatIsW... -- and revisiting this wonderful page is the real reason for this reply. :-)

The N/Kg conversion might be off by a factor of what, 1% in extreme cases (Denver)? Whereas IAS can easily be a factor of two or more different from the plain English sense of "airspeed".

My point is that airspeed is a vector measured relative to something. In this case, it's set to a specific situation (25C/101.3kPa/0% humidity) and then measured relative to the stagnation pressure at that point because then no matter those variables, the plane reacts more or less the same way for the pilot, who is the one who needs to understand what's happening. Your 'plain English' sense of 'airspeed' is pretty irrelevant to the most critical parts of flight - takeoff, landing and not exceeding the plane's design limits.

Moreover, it also means that if you're flying a very basic plane, you can use external stimuli to understand what the other variables are doing and account for them (also noting that IAS converges to the other 'air speeds', within reason, as you approach the ground). You have markers on runways, wind socks, etc. to all get a feel for what your IAS translates to on the ground and thus how much speed you're going to have when you hit the runway. That seems far more useful to me than having some kPa display on the dash.

> Your 'plain English' sense of 'airspeed' is pretty irrelevant to the most critical parts of flight - takeoff, landing and not exceeding the plane's design limits.

I know the pilot needs a measure that behaves like IAS. I just think it's misleading to call the thing we measure "airspeed" when it can be so radically different from, well, airspeed.

> You have markers on runways, wind socks, etc. to all get a feel for what your IAS translates to on the ground and thus how much speed you're going to have when you hit the runway. That seems far more useful to me than having some kPa display on the dash.

I don't understand this argument. Surely if you were flying in a plane that displayed kPa those same markers etc. would give you exactly the same sense for how a given kPa reading translates into speed on the ground. All that would change would be that the conversion factor would be slightly different. Is "110 knots" any more intuitive (to a non-pilot) as a measure of everyday speed on the ground than "20 pascals"?

I'm not familiar with that one. Does that mean to take 300mg?

I hope so. It sure as hell can't mean 300g, and anything other than mg would be crazy, but I'm not feeling very confident here.