How is standby power typically supplied for essential flight instruments if main power is lost?

• A. Through the essential bus and standby power source such as a standby battery or dedicated generator.
• B. Via the engine's primary generators only.
• C. Via the environmental control system.
• D. No standby power is available.

Answer: (A)

Standby power for essential flight instruments comes from a dedicated source that feeds the essential bus, so even if the main electrical system fails, critical displays and flight references stay powered. This is typically either a standby battery or a dedicated standby generator. The battery provides immediate, short-term power to keep the attitude indicator, airspeed, altitude, heading, and basic flight instruments alive, while a standby generator (often tied to an APU or powered by a ram air turbine) can supply power for a longer period to maintain those core systems. Relying on the engine-driven primary generators wouldn’t guarantee power when the main electrical system is lost, and the environmental control system isn’t a power source for the instruments.