What is the function of an auto-transfer switch (ATS) in modern aircraft electrical systems?

• A. It regulates voltage on the main bus
• B. It automatically transfers load from a failed power source to a standby or alternate source
• C. It provides emergency solar power
• D. It locks power to avoid switching

Answer: (B)

The main idea is that an auto-transfer switch automatically moves the electrical load from a failed power source to a standby or alternate source so essential systems stay powered. It constantly monitors the health and availability of power sources (voltage, frequency, present/absent sources) and, when a primary source drops out or falls out of acceptable range, it swiftly connects the load to a backup source using interlocked contactors. This seamless transfer helps keep critical avionics, flight instruments, and essential systems energized with minimal or no interruption. It also prevents backfeeding and unsafe cross-connections between sources through proper interlocks.

The other options don’t fit because:

• Regulating voltage on the main bus is done by voltage regulators and power conversion hardware, not by the ATS.

• Providing emergency solar power isn’t a standard function of aircraft electrical systems.

• An ATS doesn’t lock power to avoid switching; its purpose is to enable automatic switching when a source failure is detected.