What effect does ice buildup have on the stall IAS and the stall angle?

Ice buildup on an aircraft's wings and control surfaces significantly alters aerodynamic characteristics, impacting both stall speed (indicated airspeed, IAS) and stall angle of attack.

When ice accumulates on the wings, it essentially modifies the airfoil shape, leading to increased drag and modified lift characteristics. The buildup of ice disrupts the smooth airflow over the wing, which can raise the stall IAS, meaning the aircraft must achieve a higher speed to generate enough lift to prevent stalling. This increase in stall IAS is primarily due to the degradation of the wing's lift performance caused by the ice, which requires more airspeed to maintain the same lift.

At the same time, the presence of ice also influences the stall angle of attack. The distorted airflow over the iced wing causes the stall angle to decrease. This means that the aircraft can reach a stall condition at a lower angle of attack than it would under normal conditions, exacerbating the stall characteristics.

Together, these changes illustrate why the correct choice is that ice buildup increases stall IAS (requiring a higher speed to stall) and decreases stall angle (allowing for stall at lower angles of attack). Thus, the effects of ice on stall characteristics are crucial for pilots to understand for safe aircraft operation.