B737-800 Flight Crew reported while climbing thru FL340 with the autopilot and autothrottle engaged; they experienced a significant loss of electrical power. The autopilot and autothrottle disconnected; Master Caution illuminated with ELEC on the six pack along with numerous amber warning lights. Flight crew returned to departure airport.
Synopsis
B737-800 Flight Crew reported while climbing thru FL340 with the autopilot and autothrottle engaged; they experienced a significant loss of electrical power. The autopilot and autothrottle disconnected; Master Caution illuminated with ELEC on the six pack along with numerous amber warning lights. Flight crew returned to departure airport.
Narrative
This event occurred on Aircraft X (Day; departed XA13) while en route from ZZZ to ZZZ1. The First Officer (FO) was the pilot flying (PF). While climbing thru FL340 with the Autopilot and autothrottle engaged; we experienced a significant loss of electrical power. The autopilot and autothrottle disconnected; Master Caution illuminated with ELEC on my six pack along with numerous amber warning lights. We also lost most background lighting at this time. I stated 'your aircraft;' canceled the Master Caution; the FO stated 'my aircraft;' and took manual control of the aircraft. I contacted ATC to request a level off at FL350 and stated that we had a problem we needed to work on. My initial thought was that we had just lost a generator; but that it would be a quick fix. After focusing my attention on the overhead panel and seeing the many amber lights along with the loss of background lighting; I realized it was more serious. I positioned the APU switch to start and told ATC that we would need to return to ZZZ. I'm not one hundred percent positive that I [requested priority handling] with ATC at that point. I know I had hit the mic switch to talk to ATC with the intent to [request priority handling] when I heard the FO say 'I have nothing on my side.' I diverted my attention to the FO's side and could see that he had no flight instruments and was hand flying while looking cross cockpit at my displays. I will add at this point that for some reason; my audio panel kept switching from VHF1 to FLT on its own making communication with ATC more difficult. At this time I felt that it was better for me to fly and have the FO assume the role of pilot monitoring (PM). The FO suggested we descend; being concerned that the APU may not start and we may ultimately have a problem with the pressurization system. I wasn't too concerned that the APU would not start as we had not been airborne for long enough for it to be cold soaked; but I was concerned about overall fuel pressure to the engines as we had numerous fuel pump low pressure lights illuminated. I agreed and he began to coordinate for a lower altitude with ATC as I took the aircraft and directed him to check the QRC and open the QRH for the proper electrical checklist. After taking control of the aircraft; I found that I could not actuate the elevator trim using the switches on my yoke and asked the FO if his worked. He tried his and stated that his did not work either. We were both a bit unsure which checklist was appropriate as the Bus Switching (electrical) panel looked normal initially; and no lights were illuminated on the Generator Drive and Standby Power panel. However; from all appearances; we had suffered a major loss of AC power and were operating on standby power. We both agreed that; even though no Transfer Bus Off lights or Source Off lights were illuminated; the most appropriate checklist to start with was the Loss of Both Engine Driven Generators checklist. The APU started normally and indicated that the APU generator was available. After completing the initial steps of the checklist with no restoration of power; I had the FO hold the checklist and contact the flight attendants to let them know we were working an electrical problem; that we were returning to ZZZ; and to find out if they had enough lighting in the back and ask if everyone was okay. After taking control of the radios so the FO could run the checklists; I continued to coordinate with ATC for lower altitude; giving the requested info; and requesting Airport Rescue and Firefighting (ARFF) equipment meet us at ZZZ. Once level at 16;000 ft. and approximately 45 miles from ZZZ; I requested delay vectors for time to work the problem and had the FO take control of the aircraft. Just as I was relinquishing control; I felt the trim wheel move slightly. I thought the FO may had begun to trim; so I did not say anything. The FO then observed the same thing. My thought was that maybe the Speed Trim system was doing that; but unsure how it could have done that given the failure of AC power that we had. I knew that neither one of us had done it and that neither one of us had use of electric trim; so I elected to move both stab trim cutout switches to cutout; and remove it completely from the equation. I informed the Flight Attendants that we were still working our electrical problem and informed the passengers that we would be returning to ZZZ. I then elected to review the Loss of Both Engine Driven Generators checklist and determined we had both arrived at the same conclusion. Since it appeared as though we were on Standby Power; or some state in between; I decided to review the Additional Information at the end to the checklist; regarding the 'unlikely event that a landing must be made on standby power.' I'm glad I did that; as it would come into play later. At this point; I went thru the overhead panel to review all of the various indications we were presented with. The Window Heat panel indicated overheat on two of the windows; as expected with a loss of power. The Low Pressure light for Electric Pump 2 was illuminated on the Hydraulic panel. The pneumatic duct pressure needles were both pointing at the (approximately) 12 o'clock position; very unusually high. We were also noticing that the temperature in the aircraft was getting very warm. The FO attempted to adjust the temperature using the temperature control knobs; but that seemed to have no effect. I directed him to turn all three temp control knobs to the off position in hopes that the packs would regulate to 75 and 65 degrees respectively. That had no effect so I later directed him to turn the trim air switch off and leave it at that. We had no Pack or Zone Temp lights illuminated. We checked the pressurization system and it seemed to be functioning properly the entire time with no lights illuminated. We had fuel in the center tank; so all six fuel pump switches were in the on position; but at this point; four of the six had low pressure lights illuminated. The only ones providing pressure were the left center and forward pump in the left wing. The FO then brought to my attention that we were beginning to get out of balance on the fuel. I reconfigured the fuel panel to correct for that and continued to evaluate the problem. The AC and DC Metering panel had only the BAT DISCHARGE light illuminated. The TR UNIT light was not illuminated. I was confident that the BAT DISCHARGE light was due mostly to the APU start. The panel indicated -5 amps and 25 volts for the battery at this point. Gen 2 showed no frequency and no volts; but the GEN OFF BUS light for number 2 was not illuminated. Gen1 indicated good frequency and volts; but did not appear to pick up the load with the initial step of the checklist and the GEN OFF BUS light remained illuminated. At this point; we had the APU generator connected to Bus 1. It appeared to me that the majority (but not all) of the problem may have been with Transfer Bus 2. I did a quick review of the Transfer Bus Off checklist in hopes that it may be of some help; but ultimately we were stuck with what we had. I then decided to take control of the aircraft and directed the FO to sit back and methodically look thru all of our indications and verify that we were not overlooking anything.While the FO was performing that task; I communicated our request for the ILS XXC at ZZZ. We had just departed ZZZ and we knew that ZZZ was VFR. With the BAT DISCHARGE light illuminated; and the significant power loss we had experienced; I did not want to delay any further; so we informed ATC that we were ready to continue to ZZZ. I then transferred control of the aircraft and radios to the FO and requested that; with the next ATC controller; he confirm that they were aware we were a [priority] aircraft and we wanted the ARFF equipment to meet us. I accomplished the Non-Routine Landing Considerations checklist; coordinated with the Flight Attendants; and advised the passengers that we would be landing shortly and that they would see airport safety vehicles around the aircraft after we had stopped on the runway. I then took control of the aircraft and the FO resumed the duties of PM. As we were approaching ZZZ we were advised that the ILS was out of service on XXC and were asked if we would like XXL. Given our current circumstances; I wanted the longest runway available. The FO then asked for XXR and was told that the ILS XXR was out of service as well. Apparently the FAA was performing some flight checks of the equipment that night. I asked the FO to request the RNAV XXC; mostly just for initial lateral and vertical guidance; as we wanted a long final due to the hand flying and manual trim necessary for the approach. We discussed threats; I briefed the approach; and accomplished all normal checklists. Before beginning the approach; I reviewed with the FO what was in the Additional Information on the Loss of Both Engine Driven Generators checklist and that; in particular; the flap position indicator may be inoperative for us. We also discussed that the Speed Brake Do Not Arm light was illuminated; so I would not arm the speed brakes for landing and that I would need to manually deploy them on landing. I began to slow the aircraft and configure for landing on Runway XXC. The FO pointed out that we had proper indications on the leading edge devices; but no indication on the flap gauge for the trailing edge flaps. I let him know that with each flap selection; I could feel the pitch change anticipated with normal flap movement and there was no rolling tendency. Everything felt as though it was operating normally. Due to time constraints; I had decided not to run the Stab Inoperative Checklist; but I am aware that the Stab Trim Inoperative checklist states to plan a flaps 15 landing. Given that the weather and visibility at ZZZ was very good; unsure of anti-skid operation; and in consideration of everything else that was going on; I wanted the approach to be as normal as possible and elected to land with flaps 30. I was comfortable hand flying and the manual trim inputs required were very manageable. Much more so than in the sim. I informed the FO that if we needed to go-around for any reason; I anticipated not using full go-around power and that we would do things at a slower pace to account for the manual trim required. We landed safely on XXC and brought the aircraft to a stop. I retracted the speed brakes and directed the FO to move the flaps to 40. The FO communicated with ARFF while I quickly made a PA to the passengers and spoke briefly with the Flight Attendants. As we were waiting for ARFF to inspect the aircraft; I noticed that we had a TR UNIT and ELEC light illuminated now that we were on the ground. We got the 'all clear' from ARFF and had them follow us as we taxied back to the terminal.Cause: Electrical system malfunction.Suggestions: The electrical non-normals should have some notes regarding fuel balance. I know that is part of normal flying duties; but it is often difficult to analyze various electrical problems; and with everything going on; that can easily fall out of your crosscheck and be missed until the fuel is out of balance.
Second reporter narrative
The incident occurred on Day; around XA30. I was the pilot flying (PF); and the Captain was the pilot monitoring (PM). We were climbing through flight level 340 when we heard a loud clunk with the loss of all electrical power. The power quickly came back quickly but my side of the aircraft was very degraded in the instrumentation I had. The Captain requested a level off at flight level 350 and I began to fly and level off the aircraft using the Integrated Standby Instrument System as I had a Primary Flight Display (PFD) and a Multifunction Flight Display (MFD) but no instrumentation or navigation on them. They appeared to look like they were stuck in the aircraft startup mode. The Captain immediately started the APU and reported to ATC we had an issue. Once we realized the aircraft was not going to recover itself through the backup systems; he [requested priority handling] and he took control of the aircraft as it was easier for him to fly then me at this time. The systems that I had on my side of the aircraft seemed to be hit or miss in what worked as I had a com 2 panel; but no nav 2 panel. We lost the transponder and altitude reporting system. Auto Pressurization seemed to be working as it was supposed too but a lot of warning and caution lights that were supposed to be working did not. In my limited experience with the airplane; it was very confusing in what exactly was happening to the electrical system.The Captain told me to reference the QRC and then the QRH. I asked him if he thought the dual loss of generator's was appropriate. He responded with it was and I began to run through the procedure. As I was running through the procedure; he requested a return to ZZZ and a lower altitude as I brought to his attention the APU may not start and if we lost the auto pressurization system it would be easier to control at a lower altitude. I worked through the procedure; and it did nothing to fix or alleviate the problem. I then glanced over the loss of a single engine driven generator and that procedure was almost a duplicate to the loss of all generator's procedure. The APU would not connect or at least showed no indication of connecting to the right side of the aircraft. We leveled off at 16;000 ft. and I told the Captain that none of the procedures were working and that I was comfortable flying the aircraft if he wanted to look over everything and see if I had missed something. He agreed and I took over the flight controls and radios so he could do the checklists without being distracted. We had ZZZ Center give us vectors around as it was easier than trying to get the aircraft into a defined holding pattern. The Captain ran through the procedures and got the same results as I did. We then evaluated what we had and decided the best place to land was ZZZ since the weather was VFR there. The Captain then took back over the flying duties and comms and had me look over the entire cockpit to see if either of us missed anything. Every switch and button was in its desired place per the checklist and normal operating procedures. There were no visible circuit breakers popped or any true indication of something out of place. After that I took over the radios and began to get ready to land at ZZZ. We requested ILS XXC but found out the ILS was decommissioned on that runway and XXR. We did not want to land on XXL as it was the shortest runway; and we did not know for sure what systems were actually going to operate normally. We then requested RNAV Z XXC and setup for that approach. The Captain flew the approach to landing perfectly and we stopped on the runway to have Airport Rescue and Firefighting (ARFF) check the airplane over. They found nothing visible and followed us to the gate. Cause: The aircraft is one of the oldest in the fleet.
NASA callback
Reporter stated the root cause was not determined; but the issue was resolved by ground maintenance replacement of both generator control units and main bus tie breaker. Reporter commented that the checklists are not well designed to deal with this kind of electrical issue and the flight crew was unsure of the exact nature of the failure.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.