Which instrument is typically not powered by the battery bus?

The primary electronic display is typically not powered by the battery bus because it is usually connected to the essential bus instead. The essential bus is designed to provide power to critical systems that must remain operational in the event of a battery failure or when powered solely by the battery. The primary electronic display is integral for providing vital flight information to the pilot and is prioritized in terms of power supply to ensure functionality in critical flight situations.

In contrast, the standby altimeter, standby airspeed indicator, and standby turn coordinator all serve as backup instruments. They rely on the battery bus, as their function is primarily in redundancy for failure scenarios. Thus, while the primary electronic display is essential, it is strategically powered through a different system—highlighting the importance of ensuring that critical displays have a reliable power source separate from that which powers non-essential or backup systems.