Date: 201701
Local time of day: 0601-1200
Location: ZZZ.Airport, US
Flight conditions: IMC
Weather elements / visibility: Rain
Aircraft: B737 Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Final Approach
Airspace: Class B ZZZ
ATC / advisory: Tower ZZZ
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Workload; Time Pressure; Distraction; Situational Awareness
Anomaly: Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Unstabilized Approach
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew FLC complied w / Automation / Advisory
Contributing factors: Human Factors
Primary problem: Human Factors

Narrative: It would never happen to me! It's hard to know where to start. [Decades] of flying between the two of us and nothing like this before. It was an early morning; but we both got good sleep the night before. We were more busy than normal with turbulence; icing; off arrival vectors and altitudes; snow; braking action reports; last minute ATIS change and a practice CAT III approach. But; nothing that was too abnormal.We were not distracted by anything other than our duties. We were in true sterile mode. However; that was enough to distract us. On the approach checklist we got hung up on a discussion about the navigation displays. Shooting a CAT III for the first time in a while; we were talking about the single channel display on the FMA. Assigned 170 knots till FAF. Spacing looked good. Critical area was clear. New ATIS just came in. At the same time [company] (two aircraft in front of us) missed the exit on the runway. Tower told them the next exit was closed and to exit at the end. The first we had heard that exit was closed. Then; [company] reported braking action as fair; that was also the first report of anything less than good. 1;500 AGL CAT III self-test and flare armed. Finally; navigation displays from the approach checklist made more sense. More chatter on the radio; Tower reporting braking action as poor repeated 3 times by Tower; [company] corrected him to fair; etc. 1;000 AGL set missed approach; cleared to land.Then it happened! 800 AGL; landing gear warning horn; three red lights!Our first reaction was to fix the problem. Gear down; landing checklist; flaps on schedule. Very busy few seconds. We got everything done; on speed; and had another several seconds of stable approach to touch down. We were also VFR out of around 1;000 AGL.It wasn't until we were discussing the situation at the next departure that it occurred to either of us; that we more than likely did not meet the stable approach criteria bottom of 500 feet. It's not that we thought about it and ignored it. We simply were too busy fixing the problem. If we missed the window at 500; it wasn't by much. In hindsight; probably stable on speed by 400-350 feet. The aircraft was trending in the correct direction; we touched down on speed; in the landing zone; and made the first turn off. But that's not the point.Even when you think you are doing everything correctly; there is still room for errors to slip into your flight.




Synopsis: B737 Captain reported extending the landing gear below 1;000 feet AGL during an instrument approach; which violated published stabilized approach criteria.