Our geoid, roughly an oblate spheroid, is an equipotential surface which cancels out the effect of the centripetal centrifugal force . ( See equatorial bulge . ) If we measured altitude from the center of the earth instead of the surface of the geoid, then it would be easier flying at a constant elevation south, but the equatorial sea-level landing field would be at a higher altitude than the boreal sea-level departure field, canceling things out.
