Due to precession, what must the pilot periodically adjust to agree with the heading indicator?

Precession refers to the gradual change in orientation of a spinning body, which can affect various instruments in an aircraft, particularly the magnetic compass. As an aircraft moves, the Earth's magnetic field can cause the compass to deviate or indicate a heading that doesn't align with the actual magnetic north due to this phenomenon.

To maintain accurate navigation and ensure that the heading indicated on the compass aligns with the aircraft's actual heading, pilots must periodically adjust for these discrepancies. This adjustment is essential because the magnetic compass can be affected by factors such as acceleration, deceleration, and turning, in addition to precession.

The other instruments mentioned do not require adjustments based on precession. The vertical speed indicator measures the rate of climb or descent, the airspeed indicator reflects the aircraft's speed, and the altimeter shows the altitude based on air pressure. None of these instruments are influenced by the magnetic orientation of the aircraft in the same way that the magnetic compass is. Therefore, the periodic adjustment must be made to the magnetic compass to maintain accuracy in heading information for effective navigation.