Avro RJ (and BAe 146) – Electrical Bus Assignment Testing (Channel 1 vs Channel 2)
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Following up on my earlier report about the electrical bus assignment logic, I have now done a more detailed test series on the Avro RJ, specifically comparing Channel 1 loss (GEN 1 failure) against Channel 2 loss (GEN 4 failure) with both Bus-Tie switches set to OPEN and the APU shut down.
The purpose of the two-channel electrical system is redundancy: a single channel failure should never leave the crew without instruments or communications. Each side of the cockpit should be independently powered so that either failure is survivable with roughly equal levels of degradation.
TEST CONFIGURATION (both tests):
- Engines 1 and 4 running
- APU OFF
- Bus-Tie AC: OPEN
- Bus-Tie DC: OPEN
TEST 1 – GEN 4 OFF/RESET (Channel 2 lost)
Overhead panel annunciators:
- ANTI SKID INOP
- L INNER LO PRESS / R OUTER LO PRESS
- DC BUS 2 OFF / AC BUS 2 OFF
- GEN 4 OFF LINE
- R PITOT HTR FAIL
MWP captions:
- ANTI SKID
- FUEL
- ICE PROT
- ELECT
Flight deck effects:
- Right EADI: OFF
- Right EHSI: still running
- Right altimeter: OFF
- Right clock: OFF
- Right VHF NAV: OFF
- Right FMC: OFF
- DME 2: OFF
- Left side: no failures, fully operational
This result is largely correct. Channel 2 loss affects the right side of the cockpit, while the left side remains fully operational. The captain retains full instrumentation to continue flying.
TEST 2 – GEN 1 OFF/RESET (Channel 1 lost)
Overhead panel annunciators:
- L OUTER LO PRESS / R INNER LO PRESS
- DC BUS 1 OFF / AC BUS 1 OFF
- ESS AC OFF / ESS DC OFF
- INTAKE 1-4 LO PRESS (with engine anti-ice selected)
- AUX PITOT HTR FAIL
- L PITOT HTR FAIL
- R VANE HTR FAIL
- RECIRC VALVE
MWP captions:
- ICE PROT
- FUEL
- ELECT
- SCRN HEAT SEL OFF
- FLT REC OFF
- ENG A-ICE ON (anti-ice valves fail-safe open due to loss of solenoid power)
- AIR CON
Flight deck effects:
- Left EADI: OFF
- Left EHSI: OFF
- Left altimeter: OFF
- Left clock: OFF
- Left FMC: OFF
- Left TCAS: OFF
- Right EADI: OFF
- Right EHSI: OFF
- Right TCAS: OFF
- Thrust computer: OFF
- Upper and lower engine indicator rows: OFF
- YD: red light
- EL TRIM: red light
With GEN 1 loss, both sides of the cockpit are effectively dead. The crew loses all primary flight instruments, important engine indications, and the thrust computer. This is not survivable in the same way as a Channel 2 failure.
Expected Behaviour (per FCOM)
The whole point of having two independent channels is that a single channel failure should never leave the crew without instruments or communications. Per the FCOM:
- Left ADI is powered by 26V ESS AC (Channel 1)
- Right ADI is powered by 26V AC 2 (Channel 2)
- DME 1 is powered by ESS AC / ESS DC (Channel 1)
- DME 2 is powered by AC 2 / DC 2 (Channel 2)
- Avionics Master 1 controls: left instruments, VOR 1, ILS 1, DME 1, FMS 1, weather radar, GPWS, CVR, FDR, left TCAS, DFGS 1
- Avionics Master 2 controls: right instruments, VOR 2, ILS 2, DME 2, FMS 2, right TCAS, DFGS 2
With a Channel 1 failure, the RIGHT side instruments (right EADI, right EHSI, right TCAS) should all remain operational on Channel 2. In the sim, they all fail, indicating that most equipment is wired to Channel 1.
Additional item – BATT NO CHARGE 7-second delay
Per the FCOM (Avro Electrical, Chapter 8, Topic 11), the BATT NO CHARGE annunciator has a built-in 7-second delay from fault detection to illumination. In the sim, the annunciator appears to illuminate immediately when the charging condition is lost. This is a minor point but worth noting for completeness.
Since the failure logic itself is already working correctly (systems do fail when their assigned bus is lost), the fix is most likely a matter of reassigning individual services from Channel 1 to Channel 2 in the configuration, rather than requiring new logic or code.