On July 31, 2000, about 2215 central daylight time (cdt), a Boeing 737-322, N313UA, operated as United Airlines flight #531 to Chicago, Illinois, piloted by an airline transport pilot-rated captain and copilot, sustained a serious in-flight injury to one passenger, during an evasive maneuver, following a Traffic Alert and Collision Avoidance System (TCAS) warning, approximately 20 minutes prior to landing. The 14 CFR Part 121 scheduled domestic passenger flight was operating on an IFR flight plan. Visual meteorological conditions prevailed at the time of the accident. The 2 flight crewmembers, 3 flight attendants, and remaining 112 passengers were uninjured. The flight originated from Boston General Edward Lawrence Logan International Airport, near Boston, Massachusetts, at 2116 eastern daylight time and landed at Chicago O'Hare International Airport at 2235 cdt. In his written statement, the captain stated, "In reference to the flight from Boston to Chicago I do recall a several hour delay leaving Boston. The delay was due to a radar outage and the unimatic computer system going down. However, I have no recollection of any unusual events that occurred during the remainder of the flight." In his written statement, the copilot stated, "Passing approximately 15,000 feet on the arrival into Chicago everything was normal, I was the Pilot Flying (PF), the autopilot was engaged, the seat belt sign was on, and we were clear of clouds. However, shortly thereafter while continuing our descent into Chicago, the Traffic Alert and Collision Avoidance System (TCAS) alerted us to a potential conflict with an aural 'TRAFFIC, TRAFFIC' warning, the EHSI [Electronic Horizontal Situation Indicator] displayed a visual warning as well. The captain and I both immediately scanned outside in an attempt to visually acquire the traffic, though we were clear of clouds it was a dark evening and neither of us could spot the traffic. Almost immediately after the initial Traffic Advisory (TA), a subsequent 'CLIMB, CLIMB, CLIMB' aural warning was issued at approximately 14,000 feet as well as visual warnings on the EHSI and the EADI [Electronic Attitude Director Indicator]. I immediately disengaged the autopilot and began an initial pitch up to place the airplane symbol on the outside of the red avoidance area on the EADI. At this point the Captain took control of the airplane and increased the rate of rotation. The TCAS Resolution Advisory 'RA' commanded a pitch attitude of approximately 5 degrees nose up. Within one minute the TCAS 'CLEAR OF CONFLICT' aural warning was issued and the captain reestablished a descent while I notified ATC [Air Traffic Control] of the RA and our resumption of the descent. ATC advised us that there was no target on their radar. The captain then gave control of the aircraft back to me and I reengaged the autopilot. The captain contacted the flight attendants to ascertain the condition of the passengers and cabin crew. He was informed that one passenger hurt his ankle during the avoidance maneuver, but was not in need of immediate medical assistance. The passenger subsequently requested a flight attendant coordinate for a supervisor and a wheel chair to meet the aircraft upon arrival at the gate. After an uneventful landing, I contacted ramp control and requested a supervisor and wheel chair meet the aircraft." A flight attendant stated, "I was in the galley in the back (737) - seat belt sign was off - 2 gentleman were visiting with us - standing - the seat belt sign was turned on. We requested they be seated, [and] severe turbulence hit. I grabbed a hand hold in galley, but still fell. One gentleman was knocked to the ground, hitting, I believe his knee. He was sitting in the last row of coach seat D. He was very nice and kept saying he was fine. We had a CSR speak to him upon landing. He was complaining of some pain." A United Airlines weather report summary stated, "Although the area was under the influence of low pressure, very little active weather was present. The available data does not support the presence of clear air turbulence. The only possibility of turbulence was associated with a few scattered showers along the route. Based on the ASD track/radar overlay, the flight passed near or over some of these showers in two locations - the southern tip of Lake Huron, and the eastern shore of Lake Michigan." The doctor who treated the passenger stated, "Radiographs were take[n] 01 August 2000 and were significant for a left fibular fracture."