What should be done at the first indication of icing?

When icing is first detected during flight, activating the PROBES ANTI-ICE switch is the appropriate initial response. This action is crucial because it helps to prevent ice accumulation on critical aircraft sensors and probes, which can affect flight instruments and systems that rely on accurate air data. By keeping these components clear of ice, pilots maintain better control and awareness of aircraft performance, ensuring safety during operations in icing conditions.

Timely activation of anti-ice systems is essential because once ice begins to form, it can change the aerodynamic properties of the aircraft and lead to dangerous situations such as reduced lift, increased drag, and potential stall conditions. Thus, addressing icing conditions immediately helps mitigate these risks effectively.

The other options do not address the immediate need to manage icing. For example, descending to ground level may not always be feasible or safe, aborting the flight plan may not be necessary if icing can be managed, and significantly increasing speed could exacerbate the situation by promoting ice buildup or making control more difficult. Therefore, activating the PROBES ANTI-ICE switch is clearly the most prudent first step in response to icing indications.