When is the ram air inlet lip heated?

The ram air inlet lip is heated when the Wing Anti-Ice (WAI) is selected on. This heating is an important process for preventing ice accumulation, which can adversely affect aircraft performance and safety. When WAI is activated, it triggers the heating elements in the ram air inlet, ensuring that the airflow is kept clear of ice even in conditions where icing could occur, such as during flight in winter weather or in clouds where moisture is present.

In contrast, the other conditions listed are either not applicable or do not trigger the heating of the ram air inlet lip. For example, with the engines off, there is no engine bleed air to supply the heating element, which effectively means the inlet would not be heated. High altitudes alone do not necessitate heating unless WAI is specifically activated, as altitude is not the determining factor for ice accumulation. During takeoff, the ram air inlet heating is not limited to that phase alone; it is tied to the WAI system's operation, which can be activated or deactivated based on conditions rather than the flight phase alone. Thus, the correct understanding aligns with the operation of WAI to ensure safe flight conditions through effective ice prevention measures.