Incident Overview
The captain reported that while descending in instrument meteorological conditions (IMC), the traffic collision avoidance system (TCAS) issued a "traffic, traffic, traffic" alert. The TCAS display indicated a target 700 feet below and 15 miles ahead of the flight. No resolution advisory was generated by the system.
Crew Actions
Unable to visually acquire the traffic, the captain instructed the first officer to stop the descent. The first officer disconnected the autopilot and executed a rapid level-off maneuver. During the leveling, a flight attendant fell and sustained a fractured leg.
Post-Event Findings
Subsequent review of the TCAS data and coordination with air traffic control revealed that the conflicting traffic was at flight level 260, not flight level 270 as originally displayed. Additionally, a review of the airline's operations manual indicated that deviation from an ATC clearance was authorized only in response to a resolution advisory, which had not been issued in this instance.