At what indicated airspeed does a spiral occur when the motion is increasing?

In analyzing the conditions for a spiral maneuver, particularly when the motion is increasing, it is essential to consider the dynamics of the aircraft during flight. A spiral is a condition characterized by a continuous decrease in altitude while maintaining a bank, which can be influenced by indicated airspeed.

The correct answer indicates that at 160 knots indicated airspeed (KIAS), the increasing motion in a spiral becomes pronounced. At this airspeed, the aerodynamic forces acting on the aircraft are such that the aircraft's control surfaces and weight create a situation where the pilot may have difficulty maintaining level flight due to the increased load factor associated with the bank angle. The inertia and forces at this particular speed lead to an escalating descent if proper corrective measures are not taken.

Lower airspeeds may not generate the same aerodynamic characteristics needed to develop a pronounced spiral, while higher speeds can result in more stable flight conditions or lead to an increase in the aircraft's ability to sustain level flight without descending. Thus, 160 KIAS represents a critical threshold at which the increasing spiral motion becomes evident and challenging to control, making it the correct answer to this question.