Which essential bus is always powered, indicating the first level of integrity?

The essential bus, avionics 1, is designed to always remain powered, thereby indicating the first level of integrity. This bus is crucial because it provides power to the critical systems that are necessary for safe operation of the aircraft, particularly in emergency situations. The integrity of the essential bus ensures that the avionics and other vital systems remain operational when other sources of power may be lost or compromised.

The other options pertain to power distribution within the aircraft but do not meet the criteria of always being powered. For instance, the main bus typically powers a wider range of systems and could be impacted by various failures. Avionics 2 is often designated for less critical systems that might not need to remain powered at all times. Non-essential buses serve systems that are not critical for the immediate operation or safety of the aircraft, so they do not require constant power. Thus, understanding the role of the essential bus and its priority in maintaining aircraft integrity is fundamental in aviation operations.