FAA Ground School Practice Test

When is a magnetic compass particularly inaccurate?

1. During unaccelerated flight

2. During descents

3. During turns

4. During climbs

The correct answer is: During turns

A magnetic compass is particularly inaccurate during turns due to the phenomenon known as magnetic dip and the effects of the airplane's banking. When an aircraft turns, the magnetic compass can exhibit what is called "compass turning errors." This occurs because the compass is aligned with the Earth's magnetic field, and as the plane banks, the fluid inside the compass can cause it to lag behind the actual heading of the aircraft. This results in an inaccurate reading, often showing a turn in the opposite direction initially or an exaggerated turn. Additionally, compass errors are influenced by the horizontal component of the Earth's magnetic field, which changes as the aircraft changes its orientation. In a banked turn, the compass may also show a momentary deviation from the true heading due to these factors and any magnetic interference present. Consequently, while navigating during turns, pilots must rely on other instruments in conjunction with the compass to ensure accurate heading information.