On November 20, 2000, about 1600 eastern standard time, an Aerospatiale ATR-72-212, N630AS, operated by Atlantic Southeast Airlines (ASA) Inc., flight CAA4276, operating as a regularly scheduled domestic air carrier flight, had an in flight encounter with turbulence. Visual meteorological conditions prevailed at the time, and an IFR flight plan was filed for the 14 CFR Part 121 flight. The airplane was not damaged. The airline transport-rated pilot/captain, commercial-rated first officer (FO) pilot, 1 flight attendant and 36 passengers reported no injuries. One flight attendant reported serious injuries. The flight departed from Atlanta, Georgia, at 1500. At an altitude of 10,000 feet msl, while maneuvering for landing, the flight encountered an area of moderate turbulence. The seat belt sign was "ON" and the flight attendants were performing pre-landing duties. One of the flight attendants broke a bone during the turbulence event. The captain stated, "...I was not informed about [the] injury sustained to flight attendant...until I walked to the back of the aircraft after arriving at the gate." There was a SIGMET [significant metrological], and an AIRMET (Airmen Meteorological Information Network) calling for a large area of moderate turbulence below 12,000 feet, and according to the company the captain was in possession of these reports. According to the injured flight attendant's (F/A-A) written report, "...my original Captain informed me that crew scheduling was putting him on another flight and we would be getting a reserve captain instead. Everything at the time was very rushed... F/A-B was not briefed at all [by the captain] about how the...flight will go. I [F/A-A] was briefed 'only' we would be a little rough getting out of Atlanta, but once we got up everything would be ok...bound for Ashville, NC, [about] 3/4 of the way through the smooth flight...[F/A-B] was close to row 8abcd...to ask the passenger to fasten her seat belt... F/A-A was near the service door in the galley area...no warning from the cockpit at all of rough air/turbulence...[F/A-B] and I...[were] knocked to the floor to a sitting position, the turbulence didn't stop...[I] was next to a jump seat and reach to pull it down, but was unable to do so. I...was knocked to the floor over and over...up into the service door and the passenger door; my head hit the steps and ended up on the floor flat down. I tried to sit up to look at the [other] flight attendant...she was in a passenger seat...and she was strapped in...she continued to look at me in the back, I just said stay in your seat and I landed on my tailbone really hard. I tried to scoot away and up the isle, but realized I was injured at that point...we were approaching landing, and F/A-B crawled back to me, and strapped in my jump seat and held on to me because I couldn't move so I stayed on the floor...no bells were given when the aircraft was descending...the only 4 bells I heard was at landing time. Also no announcements from [the] cockpit was made when descending through 10,000 feet to the passengers because we were in 0.75 moderate turbulence at the time." F/A-B stated, "...none of the crew had any warning about the rough air/turbulence...[F/A-A] and I were both knocked to the floor to a sitting position. The turbulence didn't stop...[F/A-A] was injured, and stayed put with turbulence still bumping her around...after the initial 4 tones I went back to the F/A jump seat...we landed with [F/A-A] on the floor." SIGMET November 2 valid until 1615, and AIRMET Tango update 3, calling for a large area of moderate turbulence below 12,000 feet, were current and applicable to the flight's descent into Ashville. SIGMET November 3 stated, "OCNL SEV TURB BLW 120 DUE TO STG MID AND LOW LVL FLOW ACRS MTNUS TERRAIN. RPT BY ACFT INVOF AVL. CONDS CONTG BYD 2115Z." "Occasional sever turbulence below 12,000 due to strong mid and low level flows across mountainous terrain. Reported by aircraft in the vicinity of Ashville. Conditions continuing beyond 1615." AIRMET Tango update 3 stated, "OCNL MOD TURB BLW FL180. CONDS SPRDG EWD ACRS AREA...CONTG BYD 21Z THRU 03Z." " Occasional moderate turbulence below 18,000. Conditions spreading eastward across area...continuing beyond 1600 thru 2200." The company verified that both the SIGMET and AIRMET were part of the captain's weather briefing, and that he had them both for the flight before departure from Atlanta. According to the airline's [ASA], Standard Practice, Page 14, paragraph 35, Flight Crew Briefings: "The Captain is designated by FAR's and the Company as directly responsible for the aircraft and crew during operation of the aircraft. As such the Captain is...the team leader. The most effective means for the Captain to build team effort is with effective briefings with the entire crew (team) prior to their first flight together. The following items should be included in a Captain's briefing...F. Weather along route, including forecast of turbulence."