Final answer:
The Pt2/Tt2 probe anti-ice is turned on manually by the pilot to prevent icing on critical airspeed and altitude measurement equipment in an aircraft.
Step-by-step explanation:
The Pt2/Tt2 probe anti-ice system in an aircraft is turned on manually by the pilot. This system is crucial for safety as it helps prevent the formation of ice on the Pitot-Static system which is essential for accurate airspeed and altitude readings. When an aircraft enters conditions conducive to icing, the pilot is responsible for activating the anti-ice system to ensure the functionality of these critical probes. Automatic anti-icing systems do exist for certain components on some aircraft, but the Pt2/Tt2 probe typically requires manual activation.