In case of failure of the horizontal stabilizer anti-ice or deice system, with both switches off and ice suspected, the maximum flap deflection is limited to:

• A. 5 degrees
• B. 10 degrees
• C. 15 degrees
• D. 20 degrees

Answer: (B)

When the horizontal stabilizer anti-ice/deice system is inoperative and you suspect ice, the tail surfaces are more vulnerable to ice buildup, which can reduce elevator effectiveness and alter trim in unexpected ways. Deploying flaps changes the aerodynamic loads on the tail and can shift the airflow in a way that exacerbates icing or reduces pitch control margins. To keep the aircraft controllable and maintain safe behavior in this icing risk, the recommended action is to limit flap deflection to a modest value.

Ten degrees provides a balance: it helps maintain safe approach and climb performance without pushing the aerodynamic load on the tail or increasing the chance of ice accumulation on the stabilizer to a point where elevator authority could be compromised. Using larger flap deflections (such as 15 or 20 degrees) would significantly increase tailplane loading and the potential for ice-related instability, while a very small deflection (like 5 degrees) might be too restrictive for acceptable handling.