ELEC AC BUS L, R

📚 Sources: QRH 6.1-6.2 (R53), FCOM 6.30.1

1. QRH Reference & Condition

• QRH Section: 6.1 – 6.2
• EICAS Level: ⚠️ Caution (Beeper)
• Condition: The AC bus is not energized.
• Objective: To restore power or use the APU to supply another source of power.

2. Non-Normal Checklist

CAUTION

Attempt only one reset per flight.

Step 1 — Generator Reset

Step	Action
1	GEN CTRL switch (affected side) … … Off, then ON

graph TD
    A["ELEC AC BUS L or R<br/>⚠️ Caution"] --> B{"GEN CTRL sw<br/>Off → ON"}
    B -->|"Message blanks"| DONE["■ ■ ■ ■<br/>Checklist Complete"]
    B -->|"Message stays"| C["APU selector<br/>START → ON<br/>(if APU available)"]
    C --> D{"APU running?"}
    D -->|"Yes"| D2["Do NOT accomplish<br/>ELEC GEN OFF checklist"]
    D2 --> E{"ELEC AC BUS<br/>message?"}
    E -->|"Blanks"| DONE2["■ ■ ■ ■<br/>Checklist Complete"]
    E -->|"Stays"| F["Go to Step 4"]
    D -->|"No / APU unavailable"| F

Step 4 — Bus Tie Reset

Step	Action
4	BUS TIE switch (affected side) … … Off, then AUTO

• If message blanks: ■ ■ ■ ■
• If message stays: Do NOT accomplish the following checklists:
  • WINDOW HEAT
  • HYD PRESS PRI

3. System Explanation

When an AC bus loses all power sources, the ELMS attempts automatic bus tie transfer. If that fails, the crew must manually attempt generator reset, then APU start, then bus tie reset. (FCOM 6.20.4)

One reset attempt per flight — repeated resets risk electrical damage to generators and bus protection equipment.

Impact of Unpowered AC Bus

The affected side loses:

• Transfer bus → TRUs (DC power loss on that side)
• Utility bus loads
• Galley power
• Associated electric hydraulic pump

4. FCTM Recommended Techniques

• Monitor remaining generators and electrical synoptic carefully
• Consider reducing electrical load (IFE/PASS SEATS OFF, CABIN/UTILITY OFF) if on single generator
• Plan for possible further degradation

5. Cross-References

• Electrical System — System architecture
• ELEC GEN OFF L, R — If generator breaker remains open
• ELEC BACKUP GEN L, R — Backup generator failure
• RAM Air Turbine (RAT) — If both transfer busses lost