What action should aircrews take immediately after identifying ice formation on the aircraft?

When aircrews identify ice formation on the aircraft, the immediate action they should take is to turn on the PROBES ANTI-ICE switch. This switch is designed to activate heating elements that prevent ice from accumulating on critical sensors and instruments, thereby ensuring accurate readings and safe operation. Ice on probes can lead to erroneous airspeed or altitude readings, which can be dangerous during flight.

Turning on the PROBES ANTI-ICE switch helps maintain the reliability of these instruments and is a crucial step in managing icing conditions. Other actions like increasing altitude or performing a rapid descent may not provide immediate relief from the icing situation and could potentially introduce further risks. Turning off all engines is not a practical or safe response to icing; maintaining engine operation is essential for flight control and safety. Therefore, activating the PROBES ANTI-ICE switch is the most appropriate and effective action to manage ice formation on the aircraft.