What N2 threshold changes the FADEC power source from AC Essential Bus to FADEC Alternator?

What N2 threshold changes the FADEC power source from AC Essential Bus to FADEC Alternator?

• A. 50%
• B. 40%
• C. 60%
• D. 70%

Answer: (A)

The key idea is how FADEC stays powered as the engine spools up. At low speeds FADEC gets power from the AC Essential Bus, but once the engine reaches a level where the engine-driven FADEC alternator can reliably supply power, FADEC switches to that alternator. This happens when the engine reaches roughly halfway to its normal operating speed, a point where the alternator can consistently meet FADEC’s power needs without overloading the bus. Choosing this mid-spool point keeps FADEC powered continuously through start and takeoff, without relying on the electrical bus that can be more vulnerable during a ramp-up. If the switch occurred earlier than this, the FADEC might lose power during the critical start phase; if it happened later, the system would be unnecessarily tied to the AC Essential Bus longer than needed. So the best answer reflects the transition to the engine-driven FADEC power source as the engine speed reaches the mid-spool region, ensuring reliable FADEC operation.

The key idea is how FADEC stays powered as the engine spools up. At low speeds FADEC gets power from the AC Essential Bus, but once the engine reaches a level where the engine-driven FADEC alternator can reliably supply power, FADEC switches to that alternator. This happens when the engine reaches roughly halfway to its normal operating speed, a point where the alternator can consistently meet FADEC’s power needs without overloading the bus. Choosing this mid-spool point keeps FADEC powered continuously through start and takeoff, without relying on the electrical bus that can be more vulnerable during a ramp-up. If the switch occurred earlier than this, the FADEC might lose power during the critical start phase; if it happened later, the system would be unnecessarily tied to the AC Essential Bus longer than needed. So the best answer reflects the transition to the engine-driven FADEC power source as the engine speed reaches the mid-spool region, ensuring reliable FADEC operation.