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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.
| Attributes | |
| ACN | 1040480 |
| Time | |
| Date | 201209 |
| Local Time Of Day | 0601-1200 |
| Place | |
| Locale Reference | ZZZ.Airport |
| State Reference | US |
| Environment | |
| Light | Dawn |
| Aircraft 1 | |
| Make Model Name | B757-200 |
| Operating Under FAR Part | Part 121 |
| Flight Phase | Parked |
| Component | |
| Aircraft Component | Inverter |
| Person 1 | |
| Function | Pilot Not Flying First Officer |
| Experience | Flight Crew Last 90 Days 175 Flight Crew Total 14335 Flight Crew Type 3198 |
| Events | |
| Anomaly | Aircraft Equipment Problem Critical |
Narrative:
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 air data computer 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.
Original NASA ASRS Text
Title: 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.
Narrative: 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.
Data retrieved from NASA's ASRS site as of July 2013 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.