United Airlines flight 918 encountered clear air turbulence during cruise flight, while en route to the London Heathrow International Airport (LHR), London, United Kingdom resulting in one flight attendant (FA) sustaining a serious injury. The flight originated at Dulles International Airport (IAD), Dulles, Virginia. The flight crew reported that their preflight weather briefing revealed no significant areas of weather or turbulence over their planned route to LHR. During their preflight checks an open status message “PASS ADDRESS SYSTEM” displayed on their engine indication and crew alerting system (EICAS) display and maintenance was called. A technician responded who reinitialized the entire passenger address (PA) system and cleared the status message. The flight crew reported that prior to the JOBOC intersection, there was minimal light turbulence, and the seatbelt sign had been on and off sporadically as required. They also indicated that no turbulence was identified in their immediate vicinity from their inflight weather resources. About 80 nm east of JOBOC, the flight encountered very light turbulence and the first officer (FO), who was the pilot flying, made the PA announcement: “Flight Attendants take your seats.” In a postaccident statement, the captain reported that while in the forward lavatory, the flight encountered very light turbulence and he heard through the lavatory wall, the FO make the PA announcement: “Flight Attendants take your seats.” However, because he did not hear the PA announcement over the lavatory speaker he didn’t know if the rest of the crew or passengers heard this announcement. The in-flight service manager (ISM), who was in the cockpit at the time also heard the FO make the announcement “Flight Attendants take your jumpseats.” As the captain attempted to take his seat (left seat) the flight encountered a moderate turbulence spike that made his knees buckle and threw him back down into the 1st observer’s seat. He indicated that he was able to flash the seatbelt sign on and off three times rapidly. The flight crew then executed all the steps of the TURBULENCE IMMEDIATE ACTION GUIDE. The turbulence ended as abruptly as it had begun. The captain then made a PA announcement “Flight attendants check in.” The ISM advised the captain that all the flight attendants had been knocked to the floor and one was seriously injured. The flight crew contacted dispatch and advised of the clear air turbulence encounter. The dispatcher had no reports of turbulence from any other United flights in the area. According to the cabin crew, they did not hear the PA announcement “Flight Attendants take your seats” and therefore they were completing their normal duties when, without warning, the airplane encountered turbulence. One FA, who was delivering meals at the time of the turbulence encounter, was thrown down, impacted the floor, and was injured. According to the ISM, shortly after the encounter, the captain came on the PA and said “Flight Attendants, Check In.” The cabin crew heard that announcement and the ISM reported to the flight crew that one FA was on the floor and was unable to get up. Upon being notified of the injured FA, the flight crew declared a medical emergency and requested paramedics meet the aircraft at the gate in LHR. The injured FA was moved to an unoccupied passenger seat, given ice, and was assisted by an onboard medical doctor for the rest of the flight. After landing the FA was transported to a local hospital where she was diagnosed with a fractured left fibula. A postaccident functional check of the passenger address system revealed no malfunctions or anomalies with the system.