Aircraft Electrical Systems Practice Test

TRU failure on the DC bus shows up as loss or unstable DC voltage feeding the bus. The Transformer Rectifier Unit is what converts AC to the DC power that the bus uses, so when it fails the DC output goes away or becomes erratic. The most common root causes are problems inside the TRU’s DC-side components: rectifier diodes that have failed, transformer winding faults that disrupt the rectified output, or fuses that have blown and opened the circuit. Each of these directly cuts or destabilizes the DC supply, which is why loss of DC voltage or erratic DC voltage is the telling symptom.

Lubrication issues don’t affect electrical power delivery, so they aren’t related. Overvoltage or reversed polarity concerns would point to different faults in the power system or battery setup, not a typical TRU failure presenting as a DC bus loss. Excessive hum could indicate transformer issues in some situations, but it isn’t the specific symptom associated with a TRU failing to feed the DC bus.