During flight, the magnetic compass indications are accurate only in which condition?

• A. Only in straight and level unaccelerated flight
• B. Only during climbs
• C. Only during turns
• D. In any flight condition

Answer: (A)

The magnetic compass is a simple device whose needle points toward magnetic north, but its accuracy depends on the forces acting on it. When the aircraft is flying straight and level and not accelerating, there are no extra inertial forces distorting the needle, so the compass reading reflects the earth’s magnetic field reliably.

As soon as you enter a climb, descent, or any change in speed (accelerating or decelerating), or start a turn, those inertial forces and the changing gravity vector disturb the needle. In a turn, lateral/centrifugal forces cause the compass to tilt and lag as it tries to follow the changing heading. In climbs or descents, vertical acceleration shifts the internal balance, producing a bias in the reading. Because of these dynamic effects, the compass is not dependable outside straight, level, unaccelerated flight.

So the best reliability occurs only in straight and level unaccelerated flight. During other phases, cross-check with other heading references or instruments.