What must a third artificial horizon system be after a failure of the electrical generating system?

In the context of aircraft systems, a third artificial horizon, also known as an attitude indicator, is crucial for maintaining control and situational awareness, especially after a failure of the electrical generating system. The requirement for the artificial horizon to be operational without selection is vital. This means that it should automatically function without the need for the crew to engage or activate it, ensuring that pilots can rely on it in critical situations without any additional input or delay.

After the failure of the electrical generating system, it is essential that this backup system still provides accurate information regarding the aircraft's orientation relative to the horizon. This is particularly important in adverse conditions where visual cues may be limited, such as in clouds or during night operations. Having a third artificial horizon that operates automatically enhances safety and reduces workload for the crew during emergency situations, allowing them to focus on troubleshooting the electrical failure instead of managing multiple systems.

Thus, the requirement for the third artificial horizon to be operational without any need for selection ensures a fail-safe mechanism that adds redundancy to the aircraft's critical flight information systems.