Under which conditions must the cowl anti-ice be on?

• A. Ground: OAT 10 C and below with visible moisture on contaminated surfaces; Air: SAT 10 C and below with visible moisture, icing conditions signaled by ice detection system
• B. Ground: OAT above 10 C; Air: SAT above 10 C; only during landing
• C. Ground: any temperature; Air: any condition
• D. Only when ice is visible on the engine nacelles

Answer: (A)

Engine nacelle anti-ice must be on whenever the nacelle leading edges are at risk of icing, because ice can alter the airflow into the engine and lead to performance problems or stalls. The conditions that trigger it are: on the ground, when outside air temperature is 10 C or colder and there is visible moisture on surfaces that could be contaminated (think wet runways, mist, drizzle, or fog); in the air, when the sensed air temperature is 10 C or colder with visible moisture, or when the ice detection system indicates icing conditions.

This setup targets the real icing risk: subfreezing temperatures combined with moisture. Ice forming on the nacelle can disrupt airflow into the engine and reduce efficiency, so the anti-ice system is activated to prevent buildup.

Other scenarios listed—such as using anti-ice at any temperature, or only during landing, or only when ice is visibly present—don’t reflect when icing actually threatens the nacelle. The correct approach ties activation to the combination of temperature and moisture, plus any ice-detection signaling, rather than a blanket or purely visual rule.