What condition is critical for an accurate magnetic compass reading?

For an accurate magnetic compass reading, it is critical to be in straight-and-level unaccelerated flight. This condition minimizes the effects of acceleration, turbulence, or banking that can cause errors in compass readings. When an aircraft is in a turn, climbing, or descending, the magnetic compass can display erroneous indications due to the forces acting on it, which affects the compass's ability to align accurately with the Earth's magnetic field.

When in straight-and-level flight, the compass can express a true reading of magnetic north since it is not influenced by these external factors. This is essential for navigation and ensures pilots can trust the compass as a reliable tool for finding their heading, particularly during critical phases of flight where clear and accurate directional data is necessary.