Date: 201511
Local time of day: 0601-1200
Location: ZZZ.Airport, US
Flight conditions: VMC
Light: Night
Aircraft: DC-9 30
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Cargo / Freight / Delivery
Flight phase: Initial Approach
Airspace: Class C ZZZ
ATC / advisory: Tower ZZZ
Aircraft component: Indicating and Warning - Landing Gear
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Distraction; Troubleshooting; Situational Awareness; Confusion
Second reporter function: First Officer; Pilot Not Flying
Anomaly: Aircraft Equipment Problem Less Severe
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Overcame Equipment Problem; General Maintenance Action
Contributing factors: Human Factors; Manuals; Procedure; Aircraft; Company Policy
Primary problem: Aircraft

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.


Second narrative: During final approach under night VMC conditions; it was observed that the left main gear down lock light was not illuminated. A go-around was then conducted. I was the flying pilot during the event and was instructed by the PIC to fly a normal pattern while he handled the situation. A light bulb was changed for the left main gear down lock light; and after the gear was selected down; a normal indication for all three gear down lock lights was observed. A normal landing was then conducted.During approach on the return leg; the same observation was made during final approach. As before; a go around was conducted. During this event; I was the non-flying pilot. I told the tower that we were executing a go-around and asked what altitude we should maintain. We were told to and maintained 2;500 and then switched to Departure where we asked for vectors while we worked out the situation. The light bulb was changed again; and I then opened up 'Gear Handle Down--Abnormal Indication' checklist. As we worked our way through the checklist; it was observed that the left main gear down lock light had illuminated indicating a normal condition. We reviewed our way through the checklist until we reached End of Procedure on the checklist.Upon reaching the ramp; the aircraft discrepancy was entered into the logbook.


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