Final answer:
The EICAS status message AC ESS ALTN notifies that the aircraft's AC essential bus is powered by an alternate source due to a contingency such as a primary power source failure.
Step-by-step explanation:
The EICAS status message AC ESS ALTN indicates that the AC essential bus in an aircraft is currently being supplied by an alternate source of power. This could be due to a failure or disconnection of the primary source. In aviation, the electrical system is critical for the operation of vital instruments and systems. The essential bus is configured in such a way that it is always powered during flight to ensure the continuous operation of these critical systems. The alternate source could be another AC bus, a battery, or an emergency generator, depending on the aircraft's design and the nature of the failure.