B737-800 flight crew reported experiencing electrical power issues during taxi resulting in the aircraft needing to be towed back to the gate.
Synopsis
B737-800 flight crew reported experiencing electrical power issues during taxi resulting in the aircraft needing to be towed back to the gate.
Narrative
During taxi out (single engine with APU on) EQUIP COOLING exhaust OFF light illuminated. Light extinguished after following QRH procedure to switch to Alternate. We called Maintenance Control and they confirmed that we needed to return to the gate. While coordinating the return to gate we received a master caution and TR UNIT light followed by rapid AC power cycling and then loss of all AC power and some DC power. We were able to reinstate the #2 generator but was unable to restore AC power to the left bus via APU generator or Bus Transfer switch. All three TR's (Transformer Rectifier) failed with subsequent loss of DC power. Emergency lighting was illuminated for an extended period but cabin and PA communications remained normal and CA kept the crew and passengers informed of our situation. With limited external lighting and no A system hydraulic pumps we elected to call for a tow to the gate. Maintenance tow crew took over an hour to arrive to our position on taxiway 1. We kept ZZZ ground control updated on our status but spent considerable time off frequency due to having only one VHF radio available.We were successfully towed to the gate and aircraft was removed from service. All policies and procedures were followed correctly.
Second reporter narrative
During taxi out (single engine with APU on) EQUIP COOLING exhaust OFF light illuminated. Light extinguished after following QRH procedure to switch to Alternate. I called Maintenance Control and they confirmed that we needed to return to the gate. While coordinating the return to gate we received a master caution and TR UNIT light followed by rapid AC power cycling and then loss of all AC power and some DC power. I was able to reinstate the #2 generator but was unable to restore AC power to the left bus via APU generator or Bus Transfer switch. All three TR's (Transformer Rectifier) failed with subsequent loss of DC power. Emergency lighting was illuminated for an extended period but cabin and PA communications remained normal and I kept the crew and passengers informed of our situation. With limited external lighting and no A system hydraulic pumps we elected to call for a tow to the gate. Maintenance tow crew took over an hour to arrive to our position on taxiway 1. We kept ZZZ ground control updated on our status but spent considerable time off frequency due to having only one VHF radio available.Total gate to gate time was approximately 2:04.Causal factor was an electrical fault of unknown origin. Aircraft X remains out of service the following day. I am unsure of exactly what failed electrically until I can see the corrective actions taken by maintenance. Mitigation would have been to taxi out on two engines. I elected to single engine taxi due to :40 minute ATC flow delay to ZZZ1. After the electrical failure we could not start the number one engine due to loss of engine instrumentation.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.