Date: 201601
Local time of day: 0601-1200
Location: FLL.Airport, FL
Flight conditions: IMC
Weather elements / visibility: 3
Light: Daylight
Aircraft: B737 Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Initial Approach
Airspace: Class C FLL
ATC / advisory: TRACON MIA
Reporter organization: Air Carrier
Reporter function: Pilot Not Flying; Captain
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Situational Awareness; Confusion; Communication Breakdown
Second reporter function: Pilot Flying; First Officer
Anomaly: ATC Issue All Types; Conflict Airborne Conflict
Detector: Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Issued New Clearance; Flight Crew Executed Go Around / Missed Approach
Contributing factors: Airport; Airspace Structure; Human Factors
Primary problem: Airspace Structure

Narrative: The previous Controller had told us to expect an ILS to 10L. We briefed the approach for 10 left and also loaded the FMC for Runway 10 Left. We were south of the airport on a downwind leg when we were handed off to the Final Approach Controller. A small band of cumulonimbus was passing through the area. The line of storms was running north and south about even with the field and a little to the west. One Controller was working both runways and radio congestion was very busy because aircraft had started asking for deviations around the weather. We got the vector to final to join the Localizer late; and when he gave it to us neither of us were sure that he said one zero left; we both thought he might have said 10R. We tried to break in the radio chatter to clarify our clearance; but it was such rapid fire we could not get word in edgewise. About this time I noticed traffic 900 feet below us and slightly behind us that looked like they were tracking the 10 Left Localizer; so I called for a go-around and the First Officer executed a missed approach. We finally got a radio call in and told Approach Control we were going missed. He was so busy; he told us to contact the Tower for missed approach instructions. We then had an uneventful go-around and return to land on 10L.


Second narrative: Approach initially assigned us 10R for the expected runway. We queried and were told 10L. The next Controller started our right downwind to start our factor for down when10L. It was super congested; and when he finally gave us a turn to final we thought that someone else took our clearance. When we verified we were told to turn tight to join the 10L Localizer and descend to 3000 feet. When established inbound; we noticed an aircraft 1000 feet below us. After a few seconds; it appeared as if he was on final to the same runway. The Captain asked ATC to verify clearance to 10L. The Controller then gave us a right turn to join 10R. We called the field. He cleared us the visual. I immediately recognized that we were well over 1200 feet high on short final. I called go-around. When the Captain told the same Controller; he sent us to Tower who quickly sent us to Departure. We executed the missed approach and then were vectored back around for an uneventful landing to 10L.


Synopsis: B737 flight crew reported executing a missed approach due to confusion over which runway was assigned to his aircraft. The confusion was attributed to ATC workload and communication issues exacerbated by radio congestion.