What failsafe must exist for the third gyroscopic bank-and-pitch indicator in an aircraft?

The requirement that a power source must provide reliable operation for 30 minutes after total failure of the electrical generating system ensures that the third gyroscopic bank-and-pitch indicator remains operational during critical phases of flight, even in the event of an electrical failure. This is vital for maintaining situational awareness for the flight crew, particularly in scenarios where visual or other primary sources of information are lost.

This fail-safe is designed to prevent situations where pilots might be deprived of essential flight data during emergencies when the main electrical systems cannot function. By providing a backup power source for at least 30 minutes, the aircraft remains in a controllable state, and the crews can respond more effectively to recovery procedures.

Other options may focus on periodic maintenance or alternative systems but do not provide the immediate backup required for effective operation in critical conditions. Therefore, ensuring that there is a robust, independent power source that activates automatically on failure of the main systems is paramount for flight safety.