In what situation must aircraft approach speed be increased by 10 KIAS?

Increasing the aircraft's approach speed by 10 knots indicated airspeed (KIAS) is essential when there is ice accumulation during landing. Ice on the wings and control surfaces reduces aerodynamic efficiency, affecting lift and control response. To compensate for the degraded handling characteristics and enhance margins of safety, pilots must adjust the approach speed upward. This allows for a better response in case of potential stall or control issues while also providing a greater margin for error, ensuring the aircraft remains controllable throughout the landing phase when ice is present.

In summary, the correct increase in approach speed in the presence of ice is a preventive measure to maintain safe aircraft handling and performance, especially during critical phases of flight like landing.