Which precaution should be taken when using mobile ground de-ice/anti-ice equipment?

Applying fluid to the lower door sills and door bottoms prior to closing for flight is crucial to ensure protection against ice formation. This practice helps seal areas that are prone to ice accumulation, thereby minimizing the risk of ice entering the aircraft cabin or interfering with door operation. Ice can form in the areas around the doors, and if not addressed, it could cause safety concerns during the flight or affect the airworthiness of the aircraft.

By treating the lower door sills and bottoms, you are effectively preventing the potential for ice to build up and create issues when the doors are operated. This precaution is especially important in colder temperatures or in conditions conducive to ice formation, as it ensures that the aircraft is prepared for safe operation after de-icing procedures are completed.

In contrast, the other options do not directly relate to effective de-icing practices and could pose safety risks. For example, opening air-conditioning pack valves does not directly affect the efficacy of de-icing and may not be relevant to ground operations. Operating the airplane engines above idle during de-icing could lead to safety hazards or other complications, and ensuring all windows are closed, while generally a good operational practice, does not contribute to the effectiveness of de-icing procedures.