Under what condition can the heading indicator be set?

The heading indicator can be accurately set during unaccelerated flight with level wings. This condition is crucial because it ensures that the aircraft is in a stable state, allowing for a precise reference to the magnetic compass. When the wings are level, the aircraft is not experiencing significant forces that could cause the heading indicator to precess or display inaccurate readings.

In this stable flight condition, the pilot can safely align the heading indicator with the magnetic compass, ensuring that the instrument reflects the true heading of the aircraft. Setting the heading indicator during unaccelerated flight reduces the likelihood of errors caused by the aircraft's motion, such as those that might occur during turns or climbs and descents, where changes in the aircraft’s orientation could distort the readings on the heading indicator. Thus, it’s best practice to make these adjustments while the aircraft is in level flight.