FADECs power source?

• A. The DC electric system below 50% N2 and PMA above 50% N2
• B. The DC electric system only
• C. The AC power bus
• D. The emergency power unit

Answer: (A)

FADEC needs a reliable power source across the full range of engine speeds. At low speeds (below about 50% N2), the FADEC is powered from the aircraft’s DC electrical system, which provides a stable, readily available DC supply when the engine isn’t turning fast enough to generate enough power on its own. Once the engine speed climbs above 50% N2, the engine-driven permanent magnet alternator (PMA) takes over to power the FADEC, leveraging the increasing generator output from the engine to keep the FADEC voltage stable without overloading the DC bus.

This arrangement supports the FADEC’s role in precisely controlling fuel flow and engine parameters from start through high-thrust operation. The AC power bus isn’t used for FADEC power in normal operation, and the emergency power unit isn’t the required source for routine FADEC power, which is why the described two-source arrangement is the correct choice.