When all generators are lost in flight, the aircraft relies on its emergency power source to keep critical systems alive. In the ERJ-145, the essential buses are powered by the batteries. External power is not available in flight, and the APU’s generator is not contributing in this scenario, so the only remaining energy path to the essential buses is the battery. The “in-flight only” note reminds you that on the ground there may be different power arrangements, but during flight the batteries provide the essential power to keep the critical systems operational for a safe approach and landing.

The APU generator serves as an extra AC power source for the aircraft’s electrical distribution. It can feed the essential and main AC buses when the engine-driven generators aren’t available or during engine start, ensuring vital systems, avionics, lighting, and other electrical loads stay powered. This power source is not limited to cabin lighting and it isn’t restricted to ground use; it can be used in flight as well if required.

When cockpit displays lose their AC power, the signs you’ll see are centered on the displays themselves failing to refresh data and dimming, with warning or advisory messages appearing. Primary indicators such as EICAS or ECAM pages may show no data or a degraded presentation, and the backlighting or brightness typically decreases. These cues collectively signal a power fault to the display system without implying immediate engine action or other unrelated faults. Immediate engine shutdown isn’t caused by a loss of display power, and external power lights staying on doesn’t guarantee full display operation since the buses feeding the displays may be affected. If power is restored or standby power is available, you might see some relief of these symptoms, but the characteristic response to AC power loss is degraded or blank data with caution/alert messages.

When a generator drops offline, the electrical system reconfigures to isolate the failed source and rely on the remaining healthy source. The bus transfer contactors (BTCs) control cross-connection between the DC buses. With the #1 generator failed and the APU available, the system opens BTC2 to prevent backfeeding into the failed generator and to keep the two buses separated so the APU can power the load without fighting the failed source. This ensures proper source priority and avoids reverse power flow. If BTC2 stayed closed, there could be unwanted cross-connection that risks backfeeding the failed generator and destabilizing the power distribution.