What must occur when ice accretion is observed on the flight compartment side windows?

When ice accretion is observed on the flight compartment side windows, activating the wing and cowl anti-ice systems is the appropriate action to take because it addresses the potential for ice buildup on critical aerodynamic surfaces. The presence of ice on the windows indicates the aircraft is in conditions conducive to ice formation, which can affect both visibility and performance.

By activating the anti-ice systems, the aircraft can prevent further ice accumulation, ensuring that both the wings and engine inlets are heated adequately to maintain safe operating conditions. This is crucial not only for visibility for the pilots but also for maintaining the aerodynamic efficiency and engine performance of the aircraft, reducing the risk of ice-related complications during flight.

Other options such as descending immediately are not necessary unless specified by other operational considerations, while switching to manual bleed mode does not directly relate to managing ice accretion on the windows. Maintaining cruising altitude without addressing the ice can lead to hazardous situations. Thus, activating the anti-ice systems is the most appropriate and effective response.