Strange Vmo limits.

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hello bacalau,

i do not know the baeAtp, but vmo, basically, is a structural limitation: pitot pressure & expressed in eas, equivalent airspeed. rember the acronym: ICETea for IAS, indicated airspeed + position error( assumed mostly positive sign)= CAS, calibrated airspeed + compressibility error( always negative sign)=EAS, equivalent airspeed + density correction=TAS, true airspeed.

for info, these different speeds & correction must be added/substracted algebraicly.

so assume position error as constant & positive, then
ias + pos.error=cas & thus ias vs cas change in a similar, symmetrical way when altitude increases.

cas + compressibility error(-)=eas & compress.error becomes more & more negative when altitude increases.

now back to your aircraft & using the formula: ias/cas + compr.err(-) =eas
from sl to 15000ft : if you keep eas(the limiting structural speed), constant, then because the compressibility error increases in size & is negative, the ias/cas increases. hope this makes sense.

now, at a certain altitude,called the crossover altitude,here 15000ft, indeed a low figure,as you mentioned Mcrit & a thick wing, due to the compressibility error becoming to important, ias/cas are not representative enough anymore & we change to Mmo. keeping limiting M constant, the vmo in ias/cas/eas now decreases with altitude cfr the formula which i will not elaborate: eas= 661xMxsquare root of p/p°.
661kts=speed of sound at sl.
M=mach number=tas/a ( a= local speed of sound, only function of temperature).
p=local air pressure, p°=std air pressure, 1013Hpa.


keeping mach constant with increasing altitude & decreasing air pressure, eas & ias/cas now decreases. this is evidenced on the airspeed indicator by the "barberpole"(vmo/mmo pointer) decreasing.

as, why at fl220 vmo increases again by only 1 kt, i don't know for sure, maybe a very small change in position error after all?