What can be a sign of ineffective de-icing?

Ice accumulation on the aircraft surfaces during flight is a clear indicator of ineffective de-icing. Effective de-icing procedures are essential for ensuring that an aircraft's performance is not hindered by ice, which can lead to significant aerodynamic issues and safety hazards. When ice starts to accumulate, it can alter the shape of the aircraft surfaces like wings and propellers, affecting lift, control, and overall flight dynamics. This scenario points directly to the de-icing process failing to adequately remove or prevent ice formation.

The other options, while they may indicate a well-functioning aircraft operation, do not relate directly to the effectiveness of de-icing. Smooth flight operations and increased passenger comfort suggest that the flight is running as planned, and normal fuel consumption typically indicates that the aircraft is functioning efficiently. However, these factors do not provide insight into the state of ice accumulation on the aircraft surfaces.