Narrative:

On approach to runway 17R at den after landing gear extension we noticed all (green and red) landing gear indicator lights inoperative (including no 'press-to-test' and test position of light switch), thus we had no indication that the landing gear was down and locked. We did go around and advised ATC of situation and requested vectors to provide time to assess our problem. I made cabin announcement of problem and initial plan. We checked circuit breakers and found none popped, but did notice that in test position of light switch other lights were inoperative: 2 window heat, left side of pitot heat/static, and 3 'B' system hydraulic low pressure/overheat (guessed we had an electric buss failure). We did initial part of irregular checklist 'landing gear unsafe indication during extension' xyz which directed checking view-port windows to confirm gear -- nose was down and locked, right side main was especially too dirty to confirm anything, and I was not certain of left main gear (although what I could see looked mechanically okay). Procedure xyz next directed a tower fly-by. I called flight attendant purser to cockpit and advised of situation and made announcement to passenger. First officer attempted radio call to company dispatch, but was unable. I then directed first officer to call maintenance to get further information on electric buss/circuit breakers related to landing gear lights. After discussion we were given radio patch with maintenance with whom I further discussed situation and we agreed that with tower observation of landing gear was down, we would plan normal landing. We did fly by and they confirmed our gear was down. I advised flight attendants (cabin advisory -- preparation for evacuate/evacuation not necessary, but be on alert) and passenger of planned normal landing. ATC informed us that company dispatcher had requested emergency vehicles for our landing. We completed procedure xyz and the other regular checklists and landed normally on runway 35R at den. After smooth touchdown and rollout I stopped airplane near emergency vehicle, they noticed no obvious problems so we taxied to our gate for deplaning. Callback conversation with reporter revealed the following information: the reporter stated the exact component failure is unknown as maintenance made no report to the crew. The reporter said no previous history of landing gear indication problems were noted in a logbook review.

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Original NASA ASRS Text

Title: A B737-300 ON APCH AT 7000 FT HAD ALL LNDG GEAR POS INDICATION LIGHTS FAIL. MADE TWR FLY-BY AND LANDED UNEVENTFULLY. CAUSE UNKNOWN.

Narrative: ON APCH TO RWY 17R AT DEN AFTER LNDG GEAR EXTENSION WE NOTICED ALL (GREEN AND RED) LNDG GEAR INDICATOR LIGHTS INOP (INCLUDING NO 'PRESS-TO-TEST' AND TEST POS OF LIGHT SWITCH), THUS WE HAD NO INDICATION THAT THE LNDG GEAR WAS DOWN AND LOCKED. WE DID GAR AND ADVISED ATC OF SIT AND REQUESTED VECTORS TO PROVIDE TIME TO ASSESS OUR PROB. I MADE CABIN ANNOUNCEMENT OF PROB AND INITIAL PLAN. WE CHKED CIRCUIT BREAKERS AND FOUND NONE POPPED, BUT DID NOTICE THAT IN TEST POS OF LIGHT SWITCH OTHER LIGHTS WERE INOP: 2 WINDOW HEAT, L SIDE OF PITOT HEAT/STATIC, AND 3 'B' SYS HYD LOW PRESSURE/OVERHEAT (GUESSED WE HAD AN ELECTRIC BUSS FAILURE). WE DID INITIAL PART OF IRREGULAR CHKLIST 'LNDG GEAR UNSAFE INDICATION DURING EXTENSION' XYZ WHICH DIRECTED CHKING VIEW-PORT WINDOWS TO CONFIRM GEAR -- NOSE WAS DOWN AND LOCKED, R SIDE MAIN WAS ESPECIALLY TOO DIRTY TO CONFIRM ANYTHING, AND I WAS NOT CERTAIN OF L MAIN GEAR (ALTHOUGH WHAT I COULD SEE LOOKED MECHANICALLY OKAY). PROC XYZ NEXT DIRECTED A TWR FLY-BY. I CALLED FLT ATTENDANT PURSER TO COCKPIT AND ADVISED OF SIT AND MADE ANNOUNCEMENT TO PAX. FO ATTEMPTED RADIO CALL TO COMPANY DISPATCH, BUT WAS UNABLE. I THEN DIRECTED FO TO CALL MAINT TO GET FURTHER INFO ON ELECTRIC BUSS/CIRCUIT BREAKERS RELATED TO LNDG GEAR LIGHTS. AFTER DISCUSSION WE WERE GIVEN RADIO PATCH WITH MAINT WITH WHOM I FURTHER DISCUSSED SIT AND WE AGREED THAT WITH TWR OBSERVATION OF LNDG GEAR WAS DOWN, WE WOULD PLAN NORMAL LNDG. WE DID FLY BY AND THEY CONFIRMED OUR GEAR WAS DOWN. I ADVISED FLT ATTENDANTS (CABIN ADVISORY -- PREPARATION FOR EVAC NOT NECESSARY, BUT BE ON ALERT) AND PAX OF PLANNED NORMAL LNDG. ATC INFORMED US THAT COMPANY DISPATCHER HAD REQUESTED EMER VEHICLES FOR OUR LNDG. WE COMPLETED PROC XYZ AND THE OTHER REGULAR CHKLISTS AND LANDED NORMALLY ON RWY 35R AT DEN. AFTER SMOOTH TOUCHDOWN AND ROLLOUT I STOPPED AIRPLANE NEAR EMER VEHICLE, THEY NOTICED NO OBVIOUS PROBS SO WE TAXIED TO OUR GATE FOR DEPLANING. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE EXACT COMPONENT FAILURE IS UNKNOWN AS MAINT MADE NO RPT TO THE CREW. THE RPTR SAID NO PREVIOUS HISTORY OF LNDG GEAR INDICATION PROBS WERE NOTED IN A LOGBOOK REVIEW.

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.