KEAS = KTAS * sqrt(sigma)

KEAS = knots equivalent airspeed
KTAS = knots true airspeed
sigma = density of the air that the plane is flying through, relative
to standard sea level density.

For example, if you're flying through air half as dense as sea level
air, sigma = .5. The square root of sigma = .71 (approx.), so KEAS is
about 71% of KTAS at that altitude.

Sigma can be gotten from an online calculator or table. Enter
"standard atmosphere" in a search engine and several of these will
show up.

The traditional E-6B pilot's circular slide rule ("whiz wheel")
automatically takes care of the square root of sigma. You simply set
altitude on an auxiliary scale. That makes EAS on the inner main scale
correspond to TAS on the outer scale.

--

Paul Hirose
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