Which component is NOT part of the essential bus?

The essential bus in an aircraft refers to the electrical power distribution system that supplies power to critical flight instruments and systems needed for safe operations. This typically includes the primary flight display (PFD), the air data computer (ADC), and other key components that contribute to the operation and management of the aircraft during flight, particularly in instrument conditions.

The primary flight display (PFD) is crucial because it provides the pilot with vital flight information, such as attitude, altitude, airspeed, and navigation data. The ADC processes this data and delivers it to the PFD, making both components instrumental in managing the flight effectively.

Flight instruments are a broader category that may include both essential and non-essential instruments. The mentioned options like the PFD and ADC are explicitly part of the essential bus because they are critical for maintaining control and situational awareness in flight.

The standby battery, which is also part of the aircraft's electrical system, is intended to provide backup power in case of a failure in the main power supply. While crucial for overall safety, the standby battery itself does not directly provide power to flight-critical systems within the essential bus; rather, it serves as an emergency backup.

Therefore, flight instruments as a general term can encompass both essential and non-essential instruments