Tonic Please,
You asked " What is EAS ?. Mad (Flt) Scientist addressed it correctly but rather fleetingly. One of my pet "hobbyhorses" is that most pilots don't have a real appreciation of the importance of EAS, due to no fault of their own.
EAS = Equivalent Air Speed, Ve. It is the speed that pilots want to see, and need to see, but cannot because CAS, which pilots do see, is affected by compressibility. CAS is corrected for compressibility at ISA Sea Level, where CAS = EAS. At higher levels, relative to the increasing Mach Number, Indicated Airspeed lies to the pilot indicating more than he/she really has, and airspeed is "money in the bank".The higher you fly, the greater becomes the error.
EAS, as Mad (Flt) Scientist stated, is the kinetic value of the airstream expressed in knots. That's what we want to see in flight, but don't. At low altitudes, it really doesn't matter a great deal, e.g. even at 400 Kt CAS at 10,000 feet, the EAS is 393 Kt. If, however, minimum manoeuvre for an aircraft at a particular weight is 250 KIAS at low level, the aircraft would need to be flown at 266 KIAS at 37000 feet to have the same stall protection, as 250 KIAS equates to only 236 Kt EAS at that level - stick shaker time!
For a given weight, the CAS for all of the following situations increases with increasing altitude, whilst EAS remains constant - Stall speed, Holding speed, Best Angle of Climb speed, Maximum Range Cruise Speed, Vmo, Minimum manoeuvre speed, and even V1, Vr, and V2 at relatively low altitudes. All of this, of course, applies in the regimes of flight where airspeed is the primary performance management parameter, above Mcrit, at high altitudes where the effects of Reynolds number and kinematic viscosity come into play, Mach No. or a mix of EAS and Mach No. become more important.
Private Pilots please note - It's been EAS you've been assuming that you have when you calculated TAS using CAS and Density Height, actually this is DAS but carries little error at low speeds and altitudes - horrendous errors at jet type speeds and altitudes prevail, but they could do it with EAS also.
If EAS were introduced as standard cockpit equipment tomorrow, the bulk of Aircraft performance manuals would be significantly reduced. It beats me why "they" didn't introduce it years ago. I'd be genuinely interested to hear if anyone can think of ANY advantage of retaining CAS with all of it's attendant errors.
Vive la revolution!
Old Smokey (aka Old Grumpey)