What action is not allowed regarding deviations from ATC clearance in response to TCAS alerts?

Returning to clearance after the RA (Resolution Advisory) resolution is optional because TCAS (Traffic Collision Avoidance System) alerts prioritize safety over adherence to ATC (Air Traffic Control) instructions in critical situations. When a TCAS alert is issued, the pilot must take immediate action to resolve a potential conflict with another aircraft, even if it means deviating from ATC clearance. Once the immediate threat is resolved, it is up to the pilot's discretion to return to the original ATC clearance based on the current traffic situation and ATC communication.

This aligns with the fundamental principle of TCAS, which is designed to prevent mid-air collisions by providing timely advisories. Therefore, while it is generally expected for pilots to communicate with ATC about their actions after resolving an RA, the decision to return to the original clearance is made based on safety considerations at that moment.