Final answer:
The prime risk for airplanes flying through volcanic ash clouds is engine damage and system failure due to the ingestion of fine ash particles, leading to possible grounding of aircraft.
Step-by-step explanation:
The prime risk for airplanes flying through volcanic ash clouds is the potential for ash to damage aircraft engines and other critical systems. Volcanic ash is made up of tiny particles of rock and glass, which can be drawn into an airplane's engines, causing them to malfunction or fail. The ash can also abrade the surfaces of the aircraft, including the windows, fuselage, and wings, impairing visibility and affecting the aerodynamics of the plane. These risks have led to the grounding of aircraft that cannot navigate safely through volcanic ash plumes.