In current airline systems TAS is an input, but not compass heading. In fact most modern jets don't have a magnetic compass system, only the standby compass is magnetic. Magnetic Heading is an output from the Inertial System based on True Heading corrected by magnetic variation derived internally from a database of the earth's magnetic field; the database being updated at regular intervals as the earth's magnetic field changes. True heading is the direction the aircraft is pointing, with the INS strapped down on the airframe. The accelerations along each of the axes derives the direction and velocity of motion. Any difference between the direction the aircraft is moving and the direction it is pointing gives the drift angle. The difference between track and heading provides the basis for deriving wind and drift. Although CAS, TAS and Altitude are measured by barometric means and computed for display on electrical ASI/Machmeters and Altimeters by the DADC, Vertical Speed is usually inertially derived and presented on an electrically driven IVSI.
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Through difficulties to the cinema