FAA Ground School Practice Test

What will the magnetic compass indicate if the aircraft is decelerated while on a west heading in the northern hemisphere?

1. A turn toward the north

2. A turn toward the south

3. No change in indication

4. A climb indication

The correct answer is: A turn toward the south

When an aircraft is decelerating while on a west heading in the northern hemisphere, the magnetic compass will indicate a turn toward the north. This phenomenon is known as "compass turn error," which occurs due to the physics of the magnetic compass. In the northern hemisphere, when the aircraft slows down, the compass tends to lag behind the actual heading. As the airplane decelerates on a west heading, the magnetic compass will momentarily show a turn toward the north before settling back to the correct heading. This happens because the magnetic compass relies on the balance of the magnetic field and does not instantly adjust to changes in velocity. Essentially, the change in motion influences the compass card, causing it to momentarily reflect a heading that is not the actual heading of the aircraft. Understanding this behavior is crucial for pilots, as it can lead to misinterpretation of headings during maneuvers, especially in scenarios involving acceleration and deceleration.