Originally posted by JohnMartin
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The entire aircraft is level, i.e. perpendicular to the gravity vector. If you believe in Calculus, consider the plane (the geometrical abstraction, not the aircraft) tangent to the sphere of the Earth at the point directly under the aircraft, the aircraft is level with that plane - and continues to be level with the tangent plane as the aircraft proceeds.
Thus, the aircraft is always level while at cruising altitude.
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