When must changes to a flight plan's True Airspeed (TAS) be communicated to ATC?

The requirement to communicate changes to a flight plan's True Airspeed (TAS) to Air Traffic Control (ATC) is based on maintaining accurate and safe flight operations within controlled airspace. A change of 5% or greater is significant enough to impact separation and traffic flow, making it essential to inform ATC for safety reasons.

When a pilot experiences a change in TAS of this magnitude, the existing flight plan may no longer accurately represent the aircraft's performance and its expected flight profile. Therefore, by communicating such changes, ATC can adjust their traffic management strategies, ensuring that appropriate separation between aircraft is maintained. This not only enhances safety but also ensures that air traffic controllers can provide accurate guidance based on an aircraft's updated capabilities.

Changes below this threshold, such as 3% or less, may not significantly impact the overall air traffic management, explaining why they do not require notification. A change exceeding 10% would certainly be noteworthy, but the standard protocol is clearly set at the 5% threshold to allow for effective and timely communication with ATC.