Narrative:

We departed ZZZ1 on time, headed for ZZZ2 with ZZZ3 as our alternate. Aircraft was carrying a previous MEL for an inoperative #1 engine driven generator. We powered the #1 busses using the APU generator per the MEL. The takeoff, departure, climb, cruise, and descent portions of our flight were uneventful. The ZZZ2 ATIS said the weather was 1 mi visibility, -4 temperature, -3 dew point, light wind from the east, plan on the ILS runway 10R. We briefed the ILS runway 10R approach and planned on performing a company procedures approach. I was about to read the before landing checklist when the first officer said that we didn't have any gear indications. I did a quick press-to-test on all six gear indicators and all of the bulbs (red and green) worked normally. Next, I did a quick scan of all of the circuit breaker panels and nothing obvious was popped. As we passed through 800 ft AGL, I directed the first officer to execute a go around. He called, 'go around thrust, flaps 15, landing gear-up.' we got the power up, the flaps tracking to 15, and the airplane climbing. Because we thought we had a gear problem, we elected to leave the gear down. I called the tower and told him that we were on the go for an unsafe gear indication. The first item we tested was the light test switch to see if we could get the gear indicators to light up. When the switch was placed into test, none of the indicators in the entire cockpit lit up. We quickly decided that our gear problem was actually an electrical problem. A scan of the flight instruments showed that everything was working normally except the stand-by ADI, both N1 indicators, and both egt indicators. With the first officer continuing to fly, I did a more thorough scan of the circuit breaker panels. I found one circuit breaker directly behind the captain's seat that was popped. It was labeled #2 compass xfmr. I reset the breaker and nothing noticeable changed. Next I shot a quick call off to operations and asked them to call dispatch and have dispatch and maintenance control meet us on company frequency. Operations gave the appropriate response. I then checked the electrical panel. No lights were illuminated. Volts and frequencys for the APU generator and the #2 engine were normal. While I was checking overhead panels, everything else looked normal except the four special 'lights on the overhead flight control panel. Those illuminated lights were automatic slat fail', feel diff press, mach trim fail, and speed trim fail.' we attributed these lights to our electrical problem. At this point, we were in day VMC conditions with the ground, the terrain, and the southwestern portion of the field in sight. I elected to remove APU generator power from the number one busses and then put the APU generator back on line. No relief. I did the same thing with the number two engine driven generator and the entire cockpit returned to normal. A quick scan showed that we had no flags or fault lights. The light test switch worked as well as the recall system. Everything was normal. We had three green gear indicators and told approach that our problem was solved and that we were ready for another approach. The subsequent approach, landing and rollout were uneventful. We taxied to the gate and shut down. All indications were normal and remained that way. I called dispatch and he connected me with maintenance control. I relayed our event as stated above. Maintenance was confused by the indications we had because none of the system indications we had were related electrically. He also stated that the circuit breakers that were pulled for our MEL (#1 engine gen) should not have had any effect on those systems. They asked me if I was comfortable with taking the jet to the next city, which was our next stop. I said that it was because, except for the MEL, everything was working normally and I had no reason to believe that it wouldn't continue to do so. The weather in ZZZ2 was improving and the forecast for our next stop was clear. Maintenance said that he would do some research to see if he could make any connections with our problem and that he would call us in our next city if he had any news. I made no write up in our logbook, nor was it suggested.the flight to ZZZ4 was uneventful. While en route to ZZZ4 I was contemplating the timing of our electrical problem. I wondered if it had anything to do with the coupled approach and the opening of the TR3 interconnect relay at glide slope intercept. That was about the time everything happened. So, on approach to ZZZ4 we decided to couple the autoplt in an attempt to duplicate our problem. We flew a coupled approach to glide slope intercept and everything worked normally. Landing, rollout and shutdown were also normal. Callback conversation with reporter revealed the following information: the reporter said it happened again at a later date and was removed from service and it was out of service for several days. The maintenance findings were two failed contacts on the R-1 transfer bus relay.

Google
 

Original NASA ASRS Text

Title: A B737-300 ON APCH AT 4200 FT LOST PARTIAL ELECTRICAL PWR WITH LOSS OF LNDG GEAR POS INDICATION. EXECUTED 'GAR.' SWITCHED PWR SOURCES AND ALL NORMAL.

Narrative: WE DEPARTED ZZZ1 ON TIME, HEADED FOR ZZZ2 WITH ZZZ3 AS OUR ALTERNATE. ACFT WAS CARRYING A PREVIOUS MEL FOR AN INOPERATIVE #1 ENG DRIVEN GENERATOR. WE POWERED THE #1 BUSSES USING THE APU GENERATOR PER THE MEL. THE TKOF, DEP, CLIMB, CRUISE, AND DSCNT PORTIONS OF OUR FLT WERE UNEVENTFUL. THE ZZZ2 ATIS SAID THE WEATHER WAS 1 MI VIS, -4 TEMP, -3 DEW POINT, LIGHT WIND FROM THE E, PLAN ON THE ILS RWY 10R. WE BRIEFED THE ILS RWY 10R APCH AND PLANNED ON PERFORMING A COMPANY PROCS APCH. I WAS ABOUT TO READ THE BEFORE LANDING CHKLIST WHEN THE FO SAID THAT WE DIDN'T HAVE ANY GEAR INDICATIONS. I DID A QUICK PRESS-TO-TEST ON ALL SIX GEAR INDICATORS AND ALL OF THE BULBS (RED AND GREEN) WORKED NORMALLY. NEXT, I DID A QUICK SCAN OF ALL OF THE CIRCUIT BREAKER PANELS AND NOTHING OBVIOUS WAS POPPED. AS WE PASSED THROUGH 800 FT AGL, I DIRECTED THE FO TO EXECUTE A GAR. HE CALLED, 'GAR THRUST, FLAPS 15, LNDG GEAR-UP.' WE GOT THE PWR UP, THE FLAPS TRACKING TO 15, AND THE AIRPLANE CLBING. BECAUSE WE THOUGHT WE HAD A GEAR PROB, WE ELECTED TO LEAVE THE GEAR DOWN. I CALLED THE TWR AND TOLD HIM THAT WE WERE ON THE GO FOR AN UNSAFE GEAR INDICATION. THE FIRST ITEM WE TESTED WAS THE LIGHT TEST SWITCH TO SEE IF WE COULD GET THE GEAR INDICATORS TO LIGHT UP. WHEN THE SWITCH WAS PLACED INTO TEST, NONE OF THE INDICATORS IN THE ENTIRE COCKPIT LIT UP. WE QUICKLY DECIDED THAT OUR GEAR PROB WAS ACTUALLY AN ELECTRICAL PROB. A SCAN OF THE FLT INSTRUMENTS SHOWED THAT EVERYTHING WAS WORKING NORMALLY EXCEPT THE STAND-BY ADI, BOTH N1 INDICATORS, AND BOTH EGT INDICATORS. WITH THE FO CONTINUING TO FLY, I DID A MORE THOROUGH SCAN OF THE CIRCUIT BREAKER PANELS. I FOUND ONE CIRCUIT BREAKER DIRECTLY BEHIND THE CAPT'S SEAT THAT WAS POPPED. IT WAS LABELED #2 COMPASS XFMR. I RESET THE BREAKER AND NOTHING NOTICEABLE CHANGED. NEXT I SHOT A QUICK CALL OFF TO OPS AND ASKED THEM TO CALL DISPATCH AND HAVE DISPATCH AND MAINT CTL MEET US ON COMPANY FREQ. OPS GAVE THE APPROPRIATE RESPONSE. I THEN CHKED THE ELECTRICAL PANEL. NO LIGHTS WERE ILLUMINATED. VOLTS AND FREQS FOR THE APU GENERATOR AND THE #2 ENG WERE NORMAL. WHILE I WAS CHKING OVERHEAD PANELS, EVERYTHING ELSE LOOKED NORMAL EXCEPT THE FOUR SPECIAL 'LIGHTS ON THE OVERHEAD FLT CTL PANEL. THOSE ILLUMINATED LIGHTS WERE AUTO SLAT FAIL', FEEL DIFF PRESS, MACH TRIM FAIL, AND SPD TRIM FAIL.' WE ATTRIBUTED THESE LIGHTS TO OUR ELECTRICAL PROB. AT THIS POINT, WE WERE IN DAY VMC CONDITIONS WITH THE GND, THE TERRAIN, AND THE SOUTHWESTERN PORTION OF THE FIELD IN SIGHT. I ELECTED TO REMOVE APU GENERATOR PWR FROM THE NUMBER ONE BUSSES AND THEN PUT THE APU GENERATOR BACK ON LINE. NO RELIEF. I DID THE SAME THING WITH THE NUMBER TWO ENGINE DRIVEN GENERATOR AND THE ENTIRE COCKPIT RETURNED TO NORMAL. A QUICK SCAN SHOWED THAT WE HAD NO FLAGS OR FAULT LIGHTS. THE LIGHT TEST SWITCH WORKED AS WELL AS THE RECALL SYSTEM. EVERYTHING WAS NORMAL. WE HAD THREE GREEN GEAR INDICATORS AND TOLD APCH THAT OUR PROB WAS SOLVED AND THAT WE WERE READY FOR ANOTHER APCH. THE SUBSEQUENT APCH, LNDG AND ROLLOUT WERE UNEVENTFUL. WE TAXIED TO THE GATE AND SHUT DOWN. ALL INDICATIONS WERE NORMAL AND REMAINED THAT WAY. I CALLED DISPATCH AND HE CONNECTED ME WITH MAINT CTL. I RELAYED OUR EVENT AS STATED ABOVE. MAINT WAS CONFUSED BY THE INDICATIONS WE HAD BECAUSE NONE OF THE SYSTEM INDICATIONS WE HAD WERE RELATED ELECTRICALLY. HE ALSO STATED THAT THE CIRCUIT BREAKERS THAT WERE PULLED FOR OUR MEL (#1 ENG GEN) SHOULD NOT HAVE HAD ANY EFFECT ON THOSE SYSTEMS. THEY ASKED ME IF I WAS COMFORTABLE WITH TAKING THE JET TO THE NEXT CITY, WHICH WAS OUR NEXT STOP. I SAID THAT IT WAS BECAUSE, EXCEPT FOR THE MEL, EVERYTHING WAS WORKING NORMALLY AND I HAD NO REASON TO BELIEVE THAT IT WOULDN'T CONTINUE TO DO SO. THE WEATHER IN ZZZ2 WAS IMPROVING AND THE FORECAST FOR OUR NEXT STOP WAS CLR. MAINT SAID THAT HE WOULD DO SOME RESEARCH TO SEE IF HE COULD MAKE ANY CONNECTIONS WITH OUR PROB AND THAT HE WOULD CALL US IN OUR NEXT CITY IF HE HAD ANY NEWS. I MADE NO WRITE UP IN OUR LOGBOOK, NOR WAS IT SUGGESTED.THE FLT TO ZZZ4 WAS UNEVENTFUL. WHILE ENRTE TO ZZZ4 I WAS CONTEMPLATING THE TIMING OF OUR ELECTRICAL PROB. I WONDERED IF IT HAD ANYTHING TO DO WITH THE COUPLED APCH AND THE OPENING OF THE TR3 INTERCONNECT RELAY AT GLIDE SLOPE INTERCEPT. THAT WAS ABOUT THE TIME EVERYTHING HAPPENED. SO, ON APCH TO ZZZ4 WE DECIDED TO COUPLE THE AUTOPLT IN AN ATTEMPT TO DUPLICATE OUR PROB. WE FLEW A COUPLED APCH TO GLIDE SLOPE INTERCEPT AND EVERYTHING WORKED NORMALLY. LNDG, ROLLOUT AND SHUTDOWN WERE ALSO NORMAL. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR SAID IT HAPPENED AGAIN AT A LATER DATE AND WAS REMOVED FROM SVC AND IT WAS OUT OF SVC FOR SEVERAL DAYS. THE MAINT FINDINGS WERE TWO FAILED CONTACTS ON THE R-1 TRANSFER BUS RELAY.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.