When is a magnetic compass reading considered accurate during flight?

A magnetic compass reading is considered accurate during straight-and-level unaccelerated flight due to the nature of how magnetic compasses function. In this state, the aircraft maintains a constant altitude and heading, allowing the compass to align correctly with the Earth's magnetic field without the influence of bank angles or acceleration forces.

When an aircraft is in straight-and-level flight without any acceleration or deceleration, the airplane's attitude does not change, which means that the compass’s readings remain stable and true to the magnetic heading. Changes in bank angle, acceleration, or deceleration can lead to compass errors, such as magnetic dip or deviation caused by the aircraft's systems and construction.

In contrast, other situations presented—such as having constant airspeed, turning with excessive bank angles, or nighttime flights—do not inherently provide the conditions necessary for accurate compass readings, since they either introduce the potential for errors or do not relate to the compass's primary functionality in relation to the Earth's magnetic force. Therefore, the accurate reading of a magnetic compass is best ensured during straight-and-level unaccelerated flight.