B757-200
Flight Crew Last 90 Days 175; Flight Crew Total 14335; Flight Crew Type 3198
Situational Awareness; Troubleshooting; Confusion; Communication Breakdown
Aircraft Equipment Problem Critical
General Maintenance Action
Human Factors; Manuals; Aircraft; Chart Or Publication
The preflight Standby Power Test passed (no Bus 'Off' light) but generated a Standby Inverter Status message. The Status message could be erased; but it returned when the Standby Power Test was repeated. A check of the electrical EICAS Maintenance page showed no volts or frequencies on the Standby AC Bus during the test. In addition; components listed in the Flight Manual as being powered by the Standby AC Bus were unpowered during the test (e.g. the Captain's flight instruments all had flags because the left ADC was not being powered). Clearly we had a bad Standby Inverter. It was replaced and everything worked normally. Here's the problem. Why did we not get a Bus 'Off' light or a Standby Bus 'Off' EICAS advisory during the test? The Flight Manual lists the components that are powered by the Standby AC Bus. One of these components is the Standby AC Bus 'Off' indication. This implies that the sensor to tell you the Standby AC Bus has no power is actually powered by that very same bus. If true; this means the warning will never work because it has no power. How ironic. The Flight Manual lists the same thing for the Standby DC Bus. If the plane is really wired this way; then looking for a bus off light or EICAS advisory during the Standby Power Test has no validity whatsoever. Our procedures regarding status messages during preflight allow us to press on if we can erase the status message. In this case; that would have led to us departing with an inoperative Standby Inverter. Perhaps some status messages need a little more scrutiny; but unfortunately pilots are given very little information about the meanings of status messages. A table in the flight manual listing all status messages and their meanings would be quite helpful.
Reporter stated their current preflight checks require a Standby Power Test prior to each flight on their B757-200 aircraft; compared to their previous procedure of once a day. Since the Standby Power Bus 'Off' light did not illuminate during their Test; a pilot might assume the Standby Power from the Inverter for the AC Standby electrical Bus was functioning 'OK'.  Except; in their situation; they kept getting an EICAS Status message each time they initiated the Test. And; as he mentioned in his report; if the EICAS Status message can be erased; then the flight crew is good to go. Reporter stated they were on the ground at the gate; when he performed the Standby Power Test for the Inverter which allowed the Flight crew to have access to the EICAS Maintenance page.  After seeing that all of the Captain's flight instruments had flags due to the ADC was not being powered; he decided to access the EICAS Electrical Maintenance page and that's where he saw no volts or freqs on the Standby AC Bus during the Test.  Had they been in flight and needed Standby Power; they would not have known the Inverter had already failed prior to departure; because the Standby Power Bus 'Off' light had not illuminated. The same 'Off light is used for indicating a Standby DC or AC Bus Power failure. He was recently informed their Flight Manual references for the list of components powered by the Standby DC and AC Bus were found to be incorrect; even after all these years of B757s being in service.Reporter stated Maintenance told him the reason the Standby Power Bus 'OFF' light did not illuminate was due to the Inverter had a partial failure and apparently would not power the Standby AC Bus.  But the purpose of the Standby Power Bus 'Off' light 'IS' to indicate a Bus failure of the Inverter. If Maintenance is correct; then there may be other partial Inverter failures that have occurred and are unknown to the Flight crews flying B757s.
A First Officer questions why a partial failure of the Standby Power Inverter on their B757-200 would not illuminate the Standby Power Bus 'Off' light during a preflight Standby Power Test. Pilot notes that without the Bus light illuminating and with the ability to erase EICAS Status messages; flight crew could unknowingly depart with an inoperative Standby Inverter.