What are the primary electrical power sources on a typical narrowbody, and what provides power if the primary sources fail?

• A. Engine-driven generators (IDGs) are the primary power sources; if they fail, the APU generator and external power provide power, with a battery supplying essential DC power and a RAT providing emergency electrical power.
• B. APU generator is the primary source; engine-driven generators are used only as backups.
• C. External power is primary; IDGs provide emergency power only.
• D. Battery provides primary power; all others are backups.

Answer: (A)

Engine-driven generators supply the primary electrical power on a typical narrowbody, handling the main AC power needs during flight and engine starts. When those primary sources aren’t available or can’t meet demand, the APU generator can provide power in the air or on the ground, and external power units can supply power while the aircraft is on the ground. The aircraft also uses a dedicated essential DC power system backed by a battery to keep critical loads alive when AC power isn’t present. For true emergencies, a Ram Air Turbine deploys to drive a generator, providing emergency electrical power to the essential/emergency buses until normal power is restored. This setup matches the described arrangement: IDGs as the primary source, with the APU generator and external power as backups, a battery for essential DC power, and the RAT for emergency power.