Preventing generator failures during Auxiliary Power Unit (APU) start
Effectivity: Challenger 300/350/3500 aircraft
ATA: 24
By: Abderrahman Karra – Technical Services Reference Documents:
- Advisory Wire AW300-24-0289 Rev 1 – Generator Failures During APU Start
- Reference Instruction Letter RIL BD-0069 – Generator Load Test Procedure
Background
Despite the publication of AW300-24-0289 Rev 1 and RIL BD-0069, we continue to observe cases of generator feeder cable corrosion and generator failures during APU start and during normal dual generators operation. These issues can lead to ESS POWER ONLY configuration, in-flight generator failures, and unnecessary operational disruptions.
Key Findings from Past Events
Investigations into multiple in-flight generator failures revealed the following root causes:
Oxidized or corroded generator power feeder cable lugs, either at the generator end or the Direct Current Power Center DCPC interface.
Under-torqued hardware at generator terminal studs and/or DCPC connections refer to TASK 24−31−05−400−801.
Incorrect installation sequence of terminal hardware Refer to TASK 24−31−05−400−801.
These discrepancies led to degraded conductivity, causing current imbalances between the two electrical feeders connecting the generator and their DCPC input that could trip the Differential Protection Monitoring Sensors DPMS and fail the generator.
Recommended Actions
To mitigate the risk of generator failure, we urge all operators to:
1. Inspect and maintain generator and DCPC power feeder cable lugs:
Ensure all lugs are free of oxidation or corrosion, clean if required.
Verify correct installation sequence and torque values as per Aircraft Maintenance Manual AMM TASK 24−31−05−400−801.
2. Perform the Generator Load Test (RIL BD-0069):
Validate the robustness of your aircraft’s electrical system under increasing current load conditions.
The procedure includes APU, left, and right generator load tests, each designed to simulate abnormal configurations and identify potential weaknesses due to increased feeders’ resistance.
If any GEN FAIL CAS messages appear during testing, follow the detailed troubleshooting steps in RIL BD-0069.
For further assistance, contact Bombardier CRC or your Field Service Representative.