Narrative:

Freighter configured dc-9 aircraft are equipped with a proximity switch type alternate gear indicating system. The system is activated by a red guarded switch. Normally; the system is off. A test of the system is performed by the crew during the acceptance check: the red guard is lifted; the switch is switched to on; and two green lights should be observed. This test does not consider a false positive. On several occasions during flight with the landing gear up; I have turned the system on and observed the system to be indicating one of the landing gear in the down position. These false positive events have eroded my confidence in this system.while on final approach; in night visual meteorological conditions; control tower closed; the left main landing gear down-lock light did not illuminate. I was the pilot in command and acting as pilot monitoring. I instructed the first officer to go-around; climb to pattern altitude and fly a full traffic pattern circuit.I considered that the abnormal procedure might bring us to a landing on the possible false positive of the alternate landing gear indicating system. I also considered that there was a possibility that the bulbs in the light were burned out. The abnormal procedure; gear handle down - abnormal indication; would not consider these conditions and it would ultimately take us to a landing on the indications of the alternate landing gear indicating system. I changed the light bulbs in the left main gear light. Then I put the gear down; and observed normal landing gear down indications. The subsequent landing was uneventful.on the next leg and on final approach; after the landing gear was extended the left main landing gear down-lock light did not illuminate. I elected to proceed as I had previously. While on vectors; I replaced the light bulbs. The indication was not corrected. Then; we began the procedure gear handle down - abnormal indication and had completed step 1; when it was observed that the landing gear was indicating normally. The subsequent landing was uneventful.step 1 of the procedure calls all hydraulic pumps... On; high. Per the descent procedure; the pumps were already high/on/checked. The average temperature aloft was colder than previous days. Also; although we had begun the abnormal procedure; we had not effected any change to cause the left main gear light to illuminate. Therefore; I reasoned the problem to be in the landing gear down lock indicating switch.

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Original NASA ASRS Text

Title: DC-9 flight crew overcame a left main gear indicating problem.

Narrative: Freighter configured DC-9 aircraft are equipped with a proximity switch type alternate gear indicating system. The system is activated by a red guarded switch. Normally; the system is off. A test of the system is performed by the crew during the Acceptance Check: The red guard is lifted; the switch is switched to ON; and two green lights should be observed. This test does not consider a false positive. On several occasions during flight with the landing gear up; I have turned the system on and observed the system to be indicating one of the landing gear in the down position. These false positive events have eroded my confidence in this system.While on final approach; in night visual meteorological conditions; control tower closed; the left main landing gear down-lock light did not illuminate. I was the Pilot in Command and acting as pilot monitoring. I instructed the first officer to go-around; climb to pattern altitude and fly a full traffic pattern circuit.I considered that the abnormal procedure might bring us to a landing on the possible false positive of the alternate landing gear indicating system. I also considered that there was a possibility that the bulbs in the light were burned out. The abnormal procedure; Gear Handle Down - Abnormal Indication; would not consider these conditions and it would ultimately take us to a landing on the indications of the alternate landing gear indicating system. I changed the light bulbs in the Left Main Gear light. Then I put the gear down; and observed normal landing gear down indications. The subsequent landing was uneventful.On the next leg and on final approach; after the landing gear was extended the left main landing gear down-lock light did not illuminate. I elected to proceed as I had previously. While on vectors; I replaced the light bulbs. The indication was not corrected. Then; we began the procedure Gear Handle Down - Abnormal Indication and had completed step 1; when it was observed that the landing gear was indicating normally. The subsequent landing was uneventful.Step 1 of the procedure calls All Hydraulic Pumps... On; High. Per the descent procedure; the pumps were already High/On/Checked. The average temperature aloft was colder than previous days. Also; although we had begun the abnormal procedure; we had not effected any change to cause the left main gear light to illuminate. Therefore; I reasoned the problem to be in the landing gear down lock indicating switch.

Data retrieved from NASA's ASRS site 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.