What initial action should be taken if a BIU #1 DEGRADED (A) ACAWS message appears?

The appearance of a BIU #1 DEGRADED (A) ACAWS message indicates that there is an issue with the Bus Interface Unit (BIU) number one, which is critical for the operation of various systems on the aircraft. The correct initial action in this scenario is to pull the appropriate electrical control breaker, specifically ecb 216 BIU 1-AC.

This action is crucial because it effectively disconnects the problematic unit from the electrical system, preventing any potential further issues or malfunctions from occurring. By isolating the unit, it allows the crew to assess the situation without the risk of exacerbating the degradation or causing additional complications within the aircraft systems.

In contrast, checking all circuit breakers, performing a full system reset, or reporting the issue to maintenance may be considered supplemental actions but are not the immediate step you should take when the message first appears. These could lead to unnecessary delays in addressing the core issue at hand. The focus should always be on isolating the fault first, and pulling ecb 216 is the most direct and appropriate response to a degraded message on BIU #1.