When is an alternate power source used in an aircraft electrical system?

• A. During normal flight operations.
• B. As a permanent replacement for the main power source.
• C. During ground testing or when primary power is unavailable, to provide essential systems temporarily.
• D. When the battery is fully charged.

Answer: (C)

Backup power sources are there to keep essential aircraft systems powered when the primary power source isn’t available. They come into play during ground testing or in flight after a generator or power system failure, providing just enough power to keep critical systems—like flight instruments, avionics, and communications—operating until a safe resolution or landing. They aren’t used during normal flight because the main power source handles regular operations, and they aren’t meant as a permanent replacement for the main power source. Their use isn’t about whether the battery is charged; it’s about maintaining essential functions when the primary electrical supply is lost or being tested.