Under what condition should a pilot notify ATC of a variation in speed while on an IFR flight plan?

Notifying ATC of a variation in speed is essential for maintaining safe separation and ensuring effective air traffic management during an IFR flight. The correct condition for when a pilot should communicate a change in speed is when the average True Airspeed (TAS) changes by 10 knots or 5 percent. This guideline is in place to ensure that any significant variation in speed—whether due to weather, aircraft configuration, or operational decisions—is communicated to ATC. Such notifications allow air traffic controllers to adjust their traffic management strategies, maintaining safe distances between aircraft and optimizing their operating routes.

The significance of this condition lies in the fact that variations in speed can have considerable effects on the planning of air traffic control operations. A deviation of 10 knots or 5 percent is generally considered a threshold that could affect arrival times and separation from other aircraft. By following this protocol, pilots help maintain the efficiency and safety of air traffic operations, which is essential for all IFR flights.

Other choices reflect different conditions or metrics, but they do not align with the standard guidance provided for pilots operating under IFR. For instance, changes in ground speed or altitude are important, but they fall outside the specific context of speed variations that warrant ATC notification.