What happened
On July 12, 2017, at approximately 22:20 UTC, a Boeing 737-400 was operating a flight from Warsaw (EPWA) to UGTB. Approximately one hour into the flight, the crew observed the failure of the captain's map light. Shortly thereafter, several critical electrical warnings appeared on the flight deck, including the MC signal and an IRS DC FAIL indication for the left Inertial Reference System (IRS). This was followed by a flickering and eventually a steady IRS DC FAIL indication for the right IRS.
Upon inspecting the circuit breakers, the crew identified that the BAT CHARGER breaker had tripped. The crew performed the necessary procedures according to the Quick Reference Handbook (QRH) for an IRS DC FAIL situation. As the crew monitored the aircraft's electrical system, they observed a progressive depletion of the battery voltage. Due to the deteriorating electrical state, the crew decided to divert the aircraft back to the departure airport. After burning off excess fuel, the aircraft landed safely on runway 15 at approximately 01:40 UTC. The aircraft was subsequently handed over to maintenance services.
The investigation
The investigation examined the aircraft's maintenance logs, which revealed that similar electrical issues had occurred on July 3 and July 5, 2017. In those previous instances, both the main battery and the battery charger had been replaced. Following this incident, the operator's maintenance engineer contacted the aircraft manufacturer to report the recurring malfunctions and request technical assistance. Based on the manufacturer's guidance, a specific job order was issued to an MRO organization to address the ongoing battery discharging issue.
Findings
- The primary cause of the electrical failure was the malfunction of the R89 relay within the battery charging electrical system.