Final answer:
A pilot must notify ATC of a change in the flight plan if the true airspeed will vary by more than 5% or 0.01 Mach from what was filed. This requirement helps maintain traffic flow and ensure safe aircraft separation.
Step-by-step explanation:
Flight Plans must be changed with Air Traffic Control (ATC) if there is a change of more than 5% or 0.01 Mach when it comes to the planned true airspeed. If a pilot has filed an Instrument Flight Rules (IFR) flight plan and finds that the indicated airspeed will vary from the filed plan by more than 5% or 0.01 Mach, they are required to notify ATC as soon as practical. The reason for this is to maintain the accuracy of the ATC's traffic flow management and to ensure safe separation between aircraft in the airspace system.