What action should be avoided regarding trim air while on the ground?

Turning on trim air while taxiing should be avoided because it can affect the air conditioning system's performance and may lead to a situation where cabin air flow is inconsistent. Trim air is designed to regulate cabin temperature, and during taxi, the aircraft is not in its standard operating conditions. Activating trim air in this phase can lead to discomfort for passengers and crew, as it may not provide the desired effect while the aircraft is still on the ground, potentially causing fluctuations in cabin temperature.

In contrast, using trim air during pre-flight checks or when parked is typically acceptable since these actions are performed in a controlled manner and ensure that systems are functioning correctly without impacting the operational phase. Utilizing trim air for cooling can be essential in maintaining a comfortable environment, but it is best done when in flight, where airflow and pressure are optimized. This highlights why it's crucial to manage trim air usage appropriately to enhance the overall performance and comfort within the aircraft.