The AC REF BUS DEGRAD advisory, with no loss of power indicates which condition?

• A. AC bus is overloaded
• B. REF bus is offline
• C. One of the two transformers has failed
• D. Both transformers have failed

Answer: (C)

Two transformer-rectifier units feed the AC REF bus in parallel, providing redundancy for avionics power. When one TRU fails, the REF bus is still powered by the remaining unit, but the loss of one source triggers an AC REF BUS DEGRAD advisory because the system is no longer fully redundant. If both TRUs failed, you’d lose the REF bus power entirely, which isn’t what this advisory indicates. Likewise, an overload on the AC bus or the REF bus going offline would present different indications. So, the advisory with no loss of power points to one of the two transformers failing, leaving the other to supply the REF bus.