Incident Overview
On 12 July 2017, at approximately 22:20 UTC, a Boeing 737-400 (type B734) operating a flight from Warsaw (EPWA) to Tbilisi (UGTB) encountered an electrical malfunction about one hour into the flight. The first indication was the failure of the CPT MAP LIGHT. Within minutes, the MC (Master Caution) illuminated and the left IRS (Inertial Reference System) displayed a DC FAIL warning. Shortly thereafter, the right IRS DC FAIL lamp began to glow and flicker before becoming steady.
Crew Actions
The flight crew checked the circuit breakers and found that the BAT CHARGER breaker had tripped. They then performed the procedures outlined in the Quick Reference Handbook (QRH) for the IRS DC FAIL checklist. The decision was made to return to the departure airport. The crew monitored the system voltage, which indicated a gradual depletion of the battery. After dumping excess fuel, the aircraft landed safely on runway 15 at 01:40 UTC. The aircraft was handed over to maintenance personnel, and the flight was subsequently canceled.
Previous Maintenance
An analysis of the aircraft's maintenance records revealed that similar malfunctions had occurred on 3 and 5 July 2017. In both instances, the MAIN BATTERY and BATTERY CHARGER were replaced. Following the incident, the operator's engineer contacted the aircraft manufacturer with a description of the malfunction and a request for technical assistance. Based on the manufacturer's response, a job order (JO/LLE/006/17/R00) was issued to the MRO organization to perform the recommended corrective actions, with a completion deadline of 16 July 2017.
Probable Cause
The investigation determined the cause to be damage or improper operation of relay R89 in the battery charging electrical system.