How long will standby instrument power support the ISI?

Standby instrument power is designed to provide a reliable source of power to essential instruments in the event of a primary power failure. The correct answer emphasizes that standby power will support the Instrumentation System Interface (ISI) for a duration of 3.5 hours. This timeframe is significant because it ensures that essential navigation and flight data remain available to the pilots, enhancing safety and situational awareness during critical phases of flight when main power sources may be compromised.

The duration of 3.5 hours is derived from specific design specifications, ensuring that the standby system can sustain operations long enough for the crew to respond to emergencies, manage transitions to backup systems, or complete safe landings. In contrast, other options provide shorter or longer durations that do not align with the engineered duration set by the aircraft's standby instrument power system. Thus, emphasizing the 3.5-hour duration highlights the aircraft's commitment to safety and reliability during critical operations.