Date: 202107
Local time of day: 1801-2400
Location: ZZZ.Airport, US
Flight conditions: Mixed
Weather elements / visibility: Rain
Light: Night
Aircraft: B737 Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Taxi
ATC / advisory: Ground ZZZ
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Instrument; Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP)
Human factors: Situational Awareness; Time Pressure
Second reporter function: First Officer; Pilot Not Flying
Anomaly: Deviation / Discrepancy - Procedural Published Material / Policy; Ground Event / Encounter Weather / Turbulence; Ground Event / Encounter Loss Of Aircraft Control
Detector: Person Flight Crew
When detected: Taxi
Result: Flight Crew Overcame Equipment Problem
Contributing factors: Weather; Human Factors; Procedure
Primary problem: Weather

Narrative: I was taxiing on ZZZ Taxiway XX from the north; having landed on XXL. A rain shower had just passed through the airport. I was taxiing at 15 kt. on the straightaway; brought the thrust levers to idle; and began braking to slow down for the 1st turn. The 1st turn is about a 30-degree angle. When I started the turn; we were at about 12 kt. and slowing; [and] then began hydroplaning; I had no nose wheel steering or braking. I continued to turn the nose wheel to the right hoping to gain traction and [to also] keep the airplane on the taxiway. After that turn; the airplane gained traction and the nose wheel steering was able to steer the plane to the right. However; I still did not have braking available. I was able to steer the plane back to the centerline and [to] stay on the taxiway.


Second narrative: We were taxiing in from landing on Runway XXL in ZZZ. During the approach there was moderate rainfall. Our taxi instructions were to taxi via Taxiway XY; Taxiway XZ; Taxiway XA; Taxiway XX; hold short of Runway XXC at Taxiway XX. As we turned onto Taxiway XX from Taxiway XA; the moderate rain had started to subside. The Captain was taxiing slower than usual due to the contaminated taxiway conditions - standing water. [At] the first bend on Taxiway XX as it turns from a south to a southwest heading; the aircraft almost completely lost traction. The nose wheel understeered and the aircraft continued straight ahead even as the taxiway turned. The Captain applied braking and the main tires also lost traction immediately. I also applied the brakes on the First Officer's side because I thought we were going to depart the taxiway. The aircraft skidded approximately 15 feet and nearly departed the prepared surface. At the last second; the aircraft slowed enough to allow the nose wheel to regain some effectiveness. The Captain was able to steer the aircraft back toward the centerline. There was a short; but secondary loss of traction [and] as we overshot the taxiway centerline; the aircraft was still in a partial skid.I have never seen an aircraft moving that slowly lose traction on a wet taxiway. I estimate the aircraft was moving at 15 kt. when the turn began. Both the Captain and I agree that this was something neither of us expected or have ever seen in our combined experience on the 737. I do believe any prudent pilot would have taken the turn at the same speed considering the conditions. It's my feeling that there was some issue with drainage or that [that] stretch of pavement is unusually slick during rainy conditions. I notified the Ground Controller of the event and reported braking action as fair to poor. I asked them to inform other aircraft of the slippery conditions on Taxiway XX between Runway XXL and XXC.


Synopsis: B737 flight crew reported hydroplaning and poor braking action while performing a slow turn during taxi but were able to safely regain aircraft control.