Date: 201511
Local time of day: 0001-0600
Location: ZKC.ARTCC, KS
Flight conditions: Mixed
Weather elements / visibility: Turbulence; Windshear
Light: Dusk
Aircraft: Large Transport; Low Wing; 2 Turbojet Eng
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Descent
Airspace: Class A ZKC
ATC / advisory: Center ZKC
Reporter organization: Air Carrier
Reporter function: First Officer; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Workload; Distraction; Situational Awareness
Second reporter function: Captain; Pilot Not Flying
Anomaly: Deviation - Speed All Types; Flight Deck / Cabin / Aircraft Event Illness / Injury; Inflight Event / Encounter Weather / Turbulence
Detector: Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Provided Assistance; Flight Crew Took Evasive Action; Flight Crew FLC Overrode Automation; General Physical Injury / Incapacitation; General Maintenance Action
Contributing factors: Procedure; Human Factors; Weather
Primary problem: Weather

Narrative: We were enroute.  We had flown earlier that day and knew there was some turbulence in the area.  Looking at the weather packet; we only noted an area of Moderate along our route from FL340 to FL390.  The Captain asked the Flight Attendants to remain seated until he called them enroute.  During the climb out; we did experience some Light Turbulence; and asked ATC about the rides.  They said that FL 390 and above had been reported as 'good ride.'We leveled off at FL390 and did have a 'good ride' (only small areas of light chop.) The Captain told the Flight Attendants they could start their service.  The flight continued to be mostly smooth so the Captain turned the seatbelt sign off.  From that point on; we did not received any adverse ride reports.  We had the radar on for most of the flight and were showing no weather ahead.  We were given an initial decent to FL350 with a crossing restriction of 11;000 feet at KAYLA.  The Captain turned the seatbelt sign ON for the descent.  We never leveled off at FL350 because we were already high for a crossing restriction. We were asked to keep the speed up.  With the huge tailwind; I transitioned 310 knots to maintain the flight path. The descent was not turbulent and we had not heard any other aircraft reporting adverse rides.  At approximately FL210; we encountered Moderate Turbulence and the airspeed quickly jumped up 20 knots and showing trend into Vmo.  I pulled back on the yolk to stop the descent; the autothrottles were already at idle.  The Captain asked Flight Attendants to take their seats over the PA.  The airspeed responded just under 340 knots.  It seemed we were in the clear and it had been small sheer zone.  We then encountered Severe Turbulence at approximately FL 200. The airspeed instantly increased from around 310 to 340-350. I immediately pulled back on yolk to slow plane down and thrust lever were still at idle.  The airspeed was slow to respond.  I eventually got it slowed to 280 and we continued descent. The plane was continuously jolted and banked abruptly.  The whole encounter lasted about three to four minutes. At approximately FL 180 we were clear of the severe turbulence.  The Flight Attendant called the Captain and told him the aft Flight Attendant injured her ankle trying to get to her seat.  The Captain reported the Severe Turbulence to ATC and Dispatch. We were given relief on the crossing restriction. The rest of the flight was uneventful. The paramedics met us at gate and attended to the injured Flight Attendant. There were no other injuries report to us.


Second narrative: Enroute with the Flight Attendants seated until approximately 15 minutes into the cruise flight due to turbulence. Approximately 30 minutes later; we were given a descent. Prior to the Top of Descent; I turned the seatbelt sign on as it had previously been off due to smooth rides. The descent was smooth until 21;000 feet where we encountered Moderate Turbulence. I immediately made a PA and told the Flight Attendants to take their seats. As we passed 20;000 feet we encountered Severe Turbulence with large fluctuations in airspeed and attitude. The Severe Turbulence lasted approximately two to three minutes and did not stop until we left 18;000 feet. Shortly after; the A Flight Attendant informed us that Flight Attendant had been hurt and would require medical assistance upon our arrival at the gate. I informed Operations and they complied with our request. We had no indications from Air Traffic Control or our weather packet that would indicate such ride conditions existed. Furthermore; we had no indication visually or on our weather radar of Severe Turbulence. I reported our encounter to Air Traffic Control as well as our Dispatcher through ACARS for any and all aircraft in the area. We landed without further incident and the injured Flight Attendant was removed by medical personnel. No other injuries were reported to us and after we discussed the situation; we handed the aircraft off to Company Maintenance to perform the necessary inspections.Pilot reports can always help in seating Passengers/Crew Members ahead of time before adverse conditions encountered.


Synopsis: An Air Carrier crew descending into a Mid Continent airport encountered un-forecast severe turbulence which resulted in a Flight Attendant injury before the attendants had an opportunity to be seated.