Standby instruments are generally powered by which source?

Standby instruments are generally powered by the battery bus. This is crucial because in the event of primary electrical system failure, standby instruments need to remain operational to provide essential flight information to the pilot. The battery bus ensures a reliable power source since it is designed to maintain power to critical instruments when the main power supplies might be compromised.

Using the battery bus allows for continuous operation of essential instruments without depending on the main power system, which could fail due to various reasons like generator failure or electrical system malfunctions. This redundant power source is a safety feature, ensuring that pilots have access to backup instruments that are vital for flight safety and decision-making in emergencies.

Other sources like the AC power supply and alternator are typically involved in the primary systems and may not be available during failure scenarios, while the main battery could be used in certain situations, it is the battery bus that is specifically designated for standby instruments to guarantee they remain powered independently of the primary system.