ias vs Mach
hello every one,
there is an easy formula that says it all. but first an assumption : forget the "position error" between ias(indicated airspeed) & eas(equivalent airspeed resulting from pitot pressure). so ias = +/- eas & assume atmosphere as roughly std.
eas = 661 x M x Vdelta.( i could elaborate how to find this formula but kiss, keep it simple st...d). 661kts = speed of sound at sea level. M = Mach n°, greek delta = ratio p/p0 ; p0 = 1013hpa. now the crossover altitude/flight level we are looking for depends on which climb schedule we want to fly or which one we programm in the fms. e.g :280kias/M.76. so 280= 661 x .76 x Vdelta . delta is the only unknown factor. so first at lower level we climb at constant eas, delta(pressure ratio) decreases with altitude so Mach(much lower than .76) must increase to keep eas cte. at a certain altitude/fl M= .76 & eas = 280: we are at the crossover alt/fl . we calculate delta & look in a std atmosphere table(which we all carry in our kitbag, don't we?) for the corresponding fl.
in our example delta = p/p0= 0.31 = fl290.
i totally agree, in normal ops, you let the automatics do this math.above the crossover alt/fl we climb now at M cte, delta still decreases with altitude so, now ias/eas decreases. in desent the whole process is reversed.
an analog formula is availabla for true airspeed: TAS =661xMxVtheta. theta=T/T0. T0=273+15=288Kelvin at sea level std.
i need a cup of coffee now.
bm.