A difficult subject!
Not an easy subject, and one that often creates confusion. I trained at ETPS and have subsequently worked on a number of projects concerning air data systems and one concerning flutter, so hopefully I can add some clarity!
Firstly, consider dynamic pressure (Pd). This is calculated from either 0.5 x actual density x TAS, or 0.5 x ISA sea level density x EAS. Either version gives you the same dynamic pressure. This is why EAS is an Equivalent Air Speed; for a given dynamic pressure it is the air speed equivalent to TAS if the density is assumed to be ISA sea level density instead of the actual air density the aircraft is experiencing.
The reason for using EAS is that TAS is hard to determine because it needs a knowledge of the actual air density, which is impossible to measure directly in flight. By fixing density at the ISA sea level value EAS thus becomes a simple and very useful proxy for dynamic pressure, which pilots need to know for all sorts of reasons including takeoff performance, stall avoidance, minimum control speed, minimum drag speed, gear and flap limits, etc.
A key benefit of EAS is that because it links a speed value to a dynamic pressure value regardless of actual density, important speeds such as stall, flap and gear limits, etc, don't change with height or OAT until Mach effects start to come in to play. If your airspeed limits were expressed in TAS you would need to recalculate them whenever the air density changed, which would not be particularly fun, helpful or safe!
Unfortunately in flight there is no easy way to measure EAS accurately because a pitot-static probe measures total pressure (Pt) and static pressure (Ps) in order to determine impact pressure (qc = Pt - Ps), which is not quite the same as dynamic pressure. Dynamic pressure assumes the air does not compress and change density in the pitot tube as it comes to a halt, whereas in reality at higher speeds the air does compress, meaning the measured impact pressure is greater than the free-stream dynamic pressure. In short, measuring EAS is not easy!
So, before the days of air data computers your ASI gave you an indicated air speed (IAS). If your ASI was accurate and the pitot static probe feeding it happened to measure an accurate representation of free-stream static and total pressure (i.e. the position error or pressure errors were very small) then at speeds below about 0.3M (onset of compressibility) your IAS would closely approximate EAS. Sadly it is not at all easy to position pitot-static probes in such a way that they measure free-stream Pt and Ps accurately, and errors in Ps in particular are hard to eliminate entirely, unless one uses a very long nose boom or trailing static in order to move the static pressure port outside the influence of the aircraft's static pressure field. Any error in measured Ps causes an error in qc and thus errors in IAS. During flight test these errors are measured and then compensated for (by means of static source error correction data in the air data computer), thereby converting IAS to CAS. CAS will be a much better approximation to EAS than IAS would be, which is why CAS is so important. If an aircraft has an air data computer the PFD will almost certainly display CAS, not IAS, although some aircraft display EAS.
So, to summarise, IAS is an approximation of EAS but includes pressure errors and does not correct for compressibility effects either, so it is only of use on slow speed, low altitude aircraft that fly well below Mach 0.3. CAS is a better approximation to EAS because the pressure errors are virtually eliminated over most of the flight envelope but compressibility effects are still present, so at higher speeds CAS starts to over-read significantly. However, as a tool for avoiding unsafe air speeds (high or low) CAS is perfectly adequate for most aircraft, particularly for subsonic flight.
This brings us back to TAS. In an Air Data Computer this is often back-calculated from Mach Number and Total Air Temperature, which means there is no need to calculate EAS as an intermediate step. Mach number is calculated directly from Ps and qc, OAT is calculated from TAT and Mach, then OAT is used to calculate the free stream speed of sound, from which TAS is calculated using Mach. TAS is needed for navigation and also for derivation of the current wind vector and drift angle, provided one has a means of measuring GS and TRK (eg GPS, IRS, Doppler radar, etc).
For design engineers and flight test engineers TAS is needed to calculate climb and turn performance, because TAS is a geometric speed, i.e. it represents an actual distance travelled in an actual period of time. In contrast, IAS, CAS and EAS are just proxies for dynamic pressure and are not actual speeds as such.
Lastly, flutter. EAS is used for defining and assessing flutter because it is a useful proxy for dynamic pressure regardless of altitude and OAT, provided Mach effects are not an issue. Once Mach effects start to influence flutter behaviour then flutter limits are usually given as both an EAS and Mach limit, so at lower altitudes the EAS limit takes precedence and at higher altitudes the Mach limit takes precedence. The cross-over altitude depends on the actual EAS and Mach values the flutter data is based on. However, the structural damping of flutter is influenced by air density and hence altitude. Therefore TAS can also be used to define speeds where flutter can be a problem. But for flight test purposes EAS is easier to use.
I hope this step-by-step but perhaps somewhat long-winded explanation is helpful!
Regards, WF
Last edited by WeekendFlyer; 27th Jun 2015 at 21:39.